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_______________General Description
The MAX724/MAX726 are monolithic, bipolar, pulse-
width modulation (PWM), switch-mode DC-DC regula-
tors optimized for step-down applications. The
MAX724 is rated at 5A, and the MAX726 at 2A. Few
external components are needed for standard opera-
tion because the power switch, oscillator, and control
circuitry are all on-chip. Employing a classic buck
topology, these regulators perform high-current step-
down functions, but can also be configured as invert-
ers, negative boost converters, or flyback converters.
These regulators have excellent dynamic and transient
response characteristics, while featuring cycle-by-cycle
current limiting to protect against overcurrent faults and
short-circuit output faults. The MAX724/MAX726 also
have a wide 8V to 40V input range in the buck step-
down configuration. In inverting and boost configura-
tions, the input can be as low as 5V.
The MAX724/MAX726 are available in a 5-pin TO-220
package. The devices have a preset 100kHz oscillator
frequency and a preset current limit of 6.5A (MAX724)
or 2.6A (MAX726).
_______________________Applications
Distributed Power from High-Voltage Buses
High-Current, High-Voltage Step-Down Applications
High-Current Inverter
Negative Boost Converter
Multiple-Output Buck Converter
Isolated DC-DC Conversion
MAX724/MAX726
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
________________________________________________________________
Maxim Integrated Products
1
V
IN
V
SW
GND
V
C
FB
CASE IS CONNECTED TO GROUND.
STANDARD PACKAGE HAS STAGGERED LEADS.
CONTACT FACTORY FOR STRAIGHT LEADS.
5-PIN TO-220
FRONT VIEW
MAX724
MAX726
1
2
3
4
5
__________________Pin Configuration
V
IN
V
C
V
SW
220
F
GND
2.7k
0.01
F
OUTPUT
5V AT 5A
FB
50
H
MBR745
2.21k
2.8k
470
F
INPUT
8V TO 40V
MAX724
5A STEP-DOWN CONVERTER
__________Typical Operating Circuit
Call toll free 1-800-998-8800 for free samples or literature.
19-0107; Rev 3; 9/95
___________________________Features
o
Input Range: Up to 40V
o
5A On-Chip Power Switch (MAX724)
2A On-Chip Power Switch (MAX726)
o
Adjustable Output: 2.5V to 35V
o
100kHz Switching Frequency
o
Excellent Dynamic Characteristics
o
Few External Components
o
8.5mA Quiescent Current
o
TO-220 Package
______________Ordering Information
PART
TEMP. RANGE
PIN-PACKAGE
MAX724
CCK
0C to +70C
5 TO-220
MAX724ECK
-40C to +85C
5 TO-220
MAX726
CCK
0C to +70C
5 TO-220
MAX726ECK
-40C to +85C
5 TO-220
8V
V
IN
40V
MAX724/MAX726
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
2
_______________________________________________________________________________________
Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45V
Switch Voltage with Respect to Input Voltage. . . . . . . . . . . . . . . . 50V
Switch Voltage with Respect to Ground Pin (V
SW
Negative)
(Note 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35V
Feedback Pin Voltage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V, +10V
Operating Temperature Ranges
MAX72_CCK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0C to +70C
MAX72_ECK. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -40C to +85C
Junction Temperature Ranges
MAX72_CCK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0C to +125C
MAX72_ECK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - 40C to +125C
Storage Temperature Range . . . . . . . . . . . . . . . . . . . -65C to +160C
Lead Temperature (soldering, 10sec). . . . . . . . . . . . . . . . . . . . +300C
ELECTRICAL CHARACTERISTICS
(V
IN
= 25V, T
j
= T
MIN
to T
MAX
, unless otherwise noted.)
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Switch-On Voltage (Note 2)
MAX724
T
j
0C
1.85
T
j
< 0C
2.10
T
j
0C
T
j
< 0C
2.50
MAX726
1.2
1.7
Switch-Off Leakage
MAX724
T
j
= +25C
5
300
A
T
j
= +25C
10
500
ABSOLUTE MAXIMUM RATINGS
I
SW
= 1A
I
SW
= 5A
I
SW
= 0.5A
I
SW
= 2A
V
IN
25V, V
SW
= 0V
V
IN
= 40V, V
SW
= 0V
V
IN
25V, V
SW
= 0V
V
IN
= 40V, V
SW
= 0V
MAX726
T
j
= +25C
150
T
j
= +25C
250
2.30
V
V
FB
= 2.5V, V
IN
40V
8.5
11
Supply Current (Note 3)
mA
85
90
Maximum Duty Cycle
%
T
j
= +25C
90
100
110
T
j
= +25C
20
Switching Frequency
kHz
V
FB
= grounded through 2k
(Note 5)
T
j
+125C
85
120
MAX724
5.5
6.5
8.5
Switch-Current Limit (Note 5)
A
MAX726
2.0
2.6
3.2
0.03
0.1
Switching Frequency Line Regulation
%/V
Input Supply Voltage Range
8.0
40.0
V
T
j
0C
3.5
4.8
V
Normal Mode
7.3
8.0
Start-Up Mode (Note 4)
T
j
< 0C
3.5
5.0
Minimum Supply Voltage
V
MAX724/MAX726
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
_______________________________________________________________________________________
3
ELECTRICAL CHARACTERISTICS (continued)
(V
IN
= 25V, T
j
= T
MIN
to T
MAX
, unless otherwise noted.)
Note 1:
Do not exceed switch-to-input voltage limitation.
Note 2:
For switch currents between 1A and 5A (2A for MAX726), maximum switch-on voltage can be calculated via linear
interpolation.
Note 3:
By setting the feedback pin (FB) to 2.5V, the V
C
pin is forced to its low clamp level and the switch duty cycle is forced to
zero, approximating the zero load condition.
Note 4:
For proper regulation, total voltage from V
IN
to GND must be
8V after start-up.
Note 5:
To avoid extremely short switch-on times, the switch frequency is internally scaled down when V
FB
is less than 1.3V. Switch-
current limit is tested with V
FB
adjusted to give a 1s minimum switch-on time.
Note 6:
Guaranteed, not production tested.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Error-Amplifier Voltage Gain
1V
V
C
4V
T
j
= +25C
2000
V/V
Error-Amplifier Transconductance
T
j
= +25C
3000 5000 9000
mho
Error-Amplifier Source Current
V
FB
= 2V
T
j
= +25C
100
140
225
A
Reference Voltage Tolerance
VREF (nominal) = 2.21V
T
j
= +25C
0.5
1.5
%
Reference Voltage
V
C
= 2V
2.155 2.210 2.265
V
Feedback Pin Bias Current
V
FB
= VREF
0.5
2
A
Error-Amplifier Sink Current
V
FB
= 2.5V
T
j
= +25C
0.6
1.0
1.7
mA
T
j
= T
MIN
to T
MAX
-4
mV/C
VC Voltage at 0% Duty Cycle
T
j
= +25C
1.5
V
Reference Voltage Line Regulation
8V
V
IN
40V
0.005 0.02
%/V
1.0
2.5
MAX724
2.5
C/W
Thermal Resistance,
Junction to Case (Note 6)
All conditions of input voltage, output voltage,
temperature and load current
MAX726
4.0
__________________________________________Typical Operating Characteristics
110
50
0
6
MAX724
STEP-DOWN CONVERTER EFFICIENCY
vs. OUTPUT CURRENT
60
100
OUTPUT CURRENT (A)
EFFICIENCY (%)
4
80
70
1
3
5
90
2
CIRCUIT OF FIGURE 2
V
OUT
= 12V, V
IN
= 20V
V
OUT
= 5V, V
IN
= 15V
16
4
-40
100
SUPPLY CURRENT
vs. JUNCTION TEMPERATURE
6
14
JUNCTION TEMPERATURE (C)
SUPPLY CURRENT (mA)
50
10
8
-25
25
75
12
0
CIRCUIT OF FIGURE 2
V
IN
= 25V, V
OUT
= 5V
I
OUT
= 1mA
2
0
125
20
8
0
QUIESCENT SUPPLY CURRENT
vs. INPUT VOLTAGE
10
18
V
IN
INPUT VOLTAGE (V)
QUIESCENT SUPPLY CURRENT (mA)
40
14
12
10
30
16
20
6
4
V
C
= 1V
DEVICE NOT SWITCHING
2
0
MAX724/MAX726
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
4
_______________________________________________________________________________________
2.25
2.19
-40
100
REFERENCE VOLTAGE
vs. JUNCTION TEMPERATURE
2.20
2.24
JUNCTION TEMPERATURE (C)
REFERENCE VOLTAGE (V)
50
2.22
2.21
-25
25
75
2.23
0
2.18
2.17
125
3.0
0
6
SWITCH-ON VOLTAGE
vs. SWITCH CURRENT
0.5
2.5
SWITCH CURRENT (A)
SWITCH-ON VOLTAGE (V)
4
1.5
1.0
1
3
5
2.0
2
T
j
= +25C
MAX724
MAX726
120
90
-40
100
SWITCHING FREQUENCY
vs. JUNCTION TEMPERATURE
95
115
JUNCTION TEMPERATURE (C)
SWITCHING FREQUENCY (kHz)
50
105
100
-25
25
75
110
0
85
80
125
8000
1k
ERROR-AMPLIFIER PHASE AND g
M
3000
7000
FREQUENCY (Hz)
TRANSCONDUCTANCE (
mho)
10M
5000
4000
10k
1M
6000
100k
0
2000
1000
200
-50
150
PHASE (degrees)
50
0
100
-200
-100
-150
PHASE
g
M
160
0
SWITCHING FREQUENCY
vs. FEEDBACK PIN VOLTAGE
60
140
FB VOLTAGE (V)
SWITCHING FREQUENCY (kHz)
2.0
100
80
0.5
1.5
120
1.0
0
40
20
2.5
3.0
+125C
+25C
-40C
500
0
FEEDBACK PIN CURRENT
vs. FB VOLTAGE
0
400
FB VOLTAGE (V)
FB CURRENT (
A)
4
200
100
1
3
300
2
-300
-100
-200
5
6
-400
-500
7
8
9
10
START OF FREQUENCY
SHIFTING
8
-40
OUTPUT CURRENT LIMIT
vs. TEMPERATURE
3
7
JUNCTION TEMPERATURE (C)
OUTPUT CURRENT LIMIT (A)
50
5
4
-25
25
6
0
2
75
1
0
100
125
MAX724
MAX726
____________________________Typical Operating Characteristics (continued)
MAX724/MAX726
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
_______________________________________________________________________________________
5
______________________________________________________________Pin Description
_________________Detailed Description
The MAX724/MAX726 are complete, single-chip, pulse-
width modulation (PWM), step-down DC-DC converters
(Figure 1). All oscillator (100kHz), control, and current-
limit circuitry, including a 5A power switch (2A for
MAX726), are included on-chip. The oscillator turns on
the switch (V
SW
) at the beginning of each clock cycle.
The switch turns off at a point later in the clock cycle,
which is a function of the signal provided by the error
amplifier. The maximum switch duty cycle is approxi-
mately 93% at the MAX724/MAX726's 100kHz switch-
ing frequency.
Both the input (FB) and output (V
C
) of the error
amplifier are brought out to simplify compensation.
Most applications require only a single series RC
network connected from V
C
to ground. The error
amplifier is a transconductance amplifier with a g
M
of
approximately 5000
mho. When slewing, V
C
can
source about 140
A, and sink about 1.1mA. This
asymmetry helps minimize start-up overshoot by
allowing the amplifier output to slew more quickly in
the negative direction.
Current limiting is provided by the current-limit com-
parator. If the current-limit threshold is exceeded, the
switch cycle terminates within about 600ns. The cur-
rent-limit threshold is internally set to approximately
6.5A (2.6A for MAX726). V
SW
is a power NPN, internally
driven by the PWM controller circuitry. V
SW
can swing
35V below ground and is rated for 5A (2A for MAX726).
Basic Step-Down Application
Figure 2 shows the MAX724/MAX726 in a basic step-
down DC-DC converter. Typical MAX724 waveforms
are shown in Figure 3 for V
IN
= 20V, V
OUT
= 5V, L =
50
H, and I
OUT
= 3A and 0.16A. Two sets of wave-
forms are shown. One set shows high load current (3A)
where inductor current never falls to zero during the
switch "off-cycle" (continuous-conduction mode, CCM).
The second set of waveforms, at low output current
(0.16A), shows inductor current at zero during the latter
half of the switch off-cycle (discontinuous-conduction
mode, DCM). The transition from CCM to DCM occurs
at an output current (I
DCM
) that can be derived with the
following equation:
I
DCM
= (V
OUT
+ V
D
) [(V
IN
- V
SW
) - (V
OUT
+ V
D
)]
2 (V
IN
- V
SW
) f
OSC
L
where V
D
is the diode forward voltage drop, V
SW
is the
voltage drop across the switch, and f
OSC
= 100kHz. In
most applications, the distinction between CCM and
DCM is academic since actual performance differences
are minimal. All CCM designs can be expected to exhibit
DCM behavior at some level of reduced load current.
PIN
V
IN
supplies power to the MAX724/MAX726's internal circuitry and also connects to the collector. V
IN
must be
bypassed with a low-ESR capacitor, typically 200F or 220F.
Internal Power Switch Output. The
S
witch output can swing 35V below ground and is rated for 5A (MAX724), 2A
(MAX726).
Ground requires a short low-noise connection to ensure good load regulation. The internal reference is referred
to GND, so errors at this pin are multiplied by the error amplifier. See the
Applications Information section for
grounding details.
Error-Amplifier Output. A series RC network connected to this pin compensates the MAX724/MAX726. Output
swing is limited to about 5.8V in the positive direction and -0.7V in the negative direction. V
C
can also synchro-
nize the MAX724/MAX726 to an external clock. (See the
Applications Information section).
Feedback Input is the error amplifier's inverting input, and controls output voltage by adjusting switch duty cycle.
Input bias current is typically 0.5A when the error amplifier is balanced (I
OUT
= 0V). FB also aids current limiting
by reducing the oscillator frequency when the output voltage is low. (See the
Applications Information section.)
V
IN
5
V
SW
4
GND
3
V
C
2
FB
1
FUNCTION
NAME
MAX724/MAX726
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
6
_______________________________________________________________________________________
In DCM, ringing occurs at V
SW
in the latter part of the
switch off-cycle. This is due to the inductor resonating
with the parallel capacitance of the catch diode and the
V
SW
node. This ringing is harmless and does not
appear at the output. Furthermore, attempts to damp
this ringing by adding circuitry will reduce efficiency
and are not advised. No off-state ringing occurs in
CCM because the diode always conducts during the
switch-off time and consequently damps any reso-
nance at V
SW
.
_______________Component Selection
Table 1 lists component suppliers for inductors, capaci-
tors, and diodes appropriate for use with the
MAX724/MAX726. Be sure to observe specified ratings
for all components.
Table 1. Component Suppliers
Surface-Mount Components (for designs typically below 2A)
Inductors:
Sumida Electric - CDR125 Series
USA:
Phone (708) 956-0666
Japan: Phone 81-3607-5111
FAX 81-3607-5144
Coiltronics - CTX series
USA:
Phone (305) 781-8900
FAX (305) 782-4163
Capacitors:
Matsuo - 267 series
USA:
Phone (714) 969-2491
FAX (714) 960-6492
Japan: Phone 81-6337-6450
Sprague - 595D series
USA:
Phone (603) 224-1961
FAX (603) 224-1430
Diodes:
Motorola - MBRS series
USA:
Phone (602) 244-5303
FAX (602) 244-4015
Nihon - NSQ series
USA:
Phone (805) 867-2555
FAX (805) 867-2556
Japan: Phone 81-3-3494-7411
FAX 81-3-3494-7414
Through-Hole Components
Inductors:
Sumida - RCH-110 series
(see above for phone number)
Cadell-Burns - 7070, 7300, 6860, and 7200 series
USA:
Phone (516) 746-2310
FAX (516) 742-2416
Renco - various series
USA:
Phone (516) 586-5566
FAX (516) 586-5562
Coiltronics - various series
(see above for phone number)
Capacitors:
Nichicon - PL series low-ESR electrolytics
USA:
Phone (708) 843-7500
FAX (708) 843-2798
Japan: Phone 81-7-5231-8461
FAX 81-7-5256-4158
United Chemi-Con - LXF series
USA:
Phone (714) 255-9500
FAX (714) 255-9400
Sanyo - OS-CON low-ESR organic semiconductor
USA:
Phone (619) 661-6835
FAX (619) 661-1055
Japan: Phone 81-7-2070-6306
FAX 81-7-2070-1174
Diodes:
General Purpose - 1N5820-1N5825
Motorola - MBR and MBRD series
(see above for phone number)
L
50
H (MAX724)
100
H (MAX726)
D
MBR745
R1
2.8k
R2
2.2k
C1
470
F
OUTPUT
5V at 5A (MAX724)
5V at 2A (MAX726)
V
SW
FB
GND
V
IN
V
C
220
F
R3
2.7k
INPUT
8V TO 40V
C2
0.01
F
MAX724
MAX726
Figure 2. Basic Step-Down Converter
MAX724
PWM
LOGIC
CONTROL
CURRENT-LIMIT
COMPARATOR
ERROR
AMPLIFIER
100kHz
OSCILLATOR
2.21V
REF
FB
SWITCH
V
C
GND
V
SW
V
IN
INTERNAL
BIAS
Figure 1. MAX724 Block Diagram
MAX724/MAX726
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
_______________________________________________________________________________________
7
Inductor Selection
Although most MAX724 designs perform satisfactorily
with 50
H inductors (100H for the MAX726), the
MAX724/MAX726 are able to operate with values rang-
ing from 5
H to 200
H. In some cases, inductors other
than 50
H may be desired to minimize size (lower
inductance), or reduce ripple (higher inductance). In
any case, inductor current must at least be rated for the
desired output current.
In high-current applications, pay particular attention to
both the RMS and peak inductor ratings. The induc-
tor's peak current is limited by core saturation.
Exceeding the saturation limit actually reduces the
coil's inductance and energy storage ability, and
increases power loss. Inductor RMS current ratings
depend on heating effects in the coil windings.
The following equation calculates maximum output cur-
rent as a function of inductance and input conditions:
I
OUT
= I
SW
-
V
OUT
(V
IN
- V
OUT
)
2 f
OSC
V
IN
L
where I
SW
is the maximum switch current (5.5A for
MAX724), V
IN
is the maximum input voltage, V
OUT
is the
output voltage, and f
OSC
is the switching frequency.
For the MAX724 example in Figure 2, with L = 50
H
and V
IN
= 25V,
I
OUT
= 5.5A -
5V (25V - 5V)
= 5.1A
2 (10
5
Hz) 25V (50 x 10
-6
H)
Note that increasing or decreasing inductor value pro-
vides only small changes in maximum output current
(100
H = 5.3A, 20
H = 4.5A). The equation shows that
output current is mostly a function of the
MAX724/MAX726 current-limit value. Again, a 50
H
inductor works well in most applications and provides
5A with a wide range of input voltages.
Catch Diode
D1 provides a path for inductor current when V
SW
turns
off. Under normal load conditions, the average diode
current may only be a fraction of load current; but dur-
ing short-circuit or current-limit, diode current is higher.
Conservative design dictates that the diode average
current rating be 2 times the desired output current. If
operation with extended short-circuit or overload time is
expected, then the diode current rating must exceed
the current limit (6.5A = MAX724, 2.6A = MAX726), and
heat sinking may be necessary.
Under normal operating conditions (not shorted), power
dissipated in the diode P
D
is calculated by:
P
D
= I
OUT
(V
IN
- V
OUT
) V
D
V
IN
where V
D
is forward drop of the diode at a current
equal to I
OUT
. In nearly all circuits, Schottky diodes
provide the best performance and are recommended
due to their fast switching times and low forward voltage
drop. Standard power rectifiers such as the 1N4000
series are too slow for DC-DC conversion circuits and are
not
recommended.
Output Filter Capacitor
For most MAX724/MAX726 applications, a high-quality,
low-ESR, 470
F or 500
F output filter capacitor will suf-
fice. To reduce ripple, minimize capacitor lead length
and connect the capacitor directly to the GND pin.
Capacitor suppliers are listed in Table 1. Output ripple
is a function of inductor value and output capacitor
effective series resistance (ESR). In continuous-con-
duction mode:
V
CR(p-p)
=
ESR (V
OUT
) (1 - V
OUT
/V
IN
)
L f
OSC
It is interesting to note that input voltage (V
IN
), and not
load current, affects output ripple in CCM. This is
because only the DC, and not the peak-to-peak, induc-
tor current changes with load (see Figure 3).
In discontinuous-conduction mode, the equation is dif-
ferent because the peak-to-peak inductor current does
depend on load:
V
DR(p-p)
= ESR
2 I
OUT
V
OUT
(V
IN
- V
OUT
)
L f
OSC
V
IN
where output ripple is proportional to the square root of
load current. Refer to the earlier equation for I
DCM
to
determine where DCM occurs and hence when the
DCM ripple equation should be used.
Input Bypass Capacitor
An input capacitor (200
F or 220
F) is required for step-
down converters because the input current, rather than
being continuous (like output current), is a square wave.
For this reason the capacitor must have low ESR and a
ripple-current rating sufficiently large so that its ESR and
the AC input current do not conspire to overheat the
capacitor. In CCM, the capacitor's RMS ripple current is:
I
R(RMS)
= I
OUT
V
OUT
(V
IN
- V
OUT
)
V
IN
2
The power dissipated in the input capacitor is then P
C
:
P
C
= I
R(RMS)
2
(ESR)
MAX724/MAX726
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
8
_______________________________________________________________________________________
CONTINUOUS-CURRENT MODE (I
OUT
= 3A)
0
-0.5
V
D
I
P
= 3.4A
I
SW
0
0
I
P
= 3.4A
I
AVG
= I
OUT
= 3A
0
I
P
= 3.4A
I
AVG
= 2.1A
I
D
DISCONTINUOUS-CURRENT MODE
(I
OUT
= 0.16A)
V
SW
VOLTAGE (TO GND)
(ALSO DIODE VOLTAGE)
5V/div
I
P
= 0.5A
SWITCH CURRENT
1A/div
INDUCTOR CURRENT
1A/div
DIODE CURRENT
1A/div
I
L
Figure 3. MAX724 Step-Down Converter Waveforms with V
IN
= 20V, L = 50H (all waveforms 2s/div)
MAX724/MAX726
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
_______________________________________________________________________________________
9
Be sure that the selected capacitor can handle the ripple
current over the required temperature range. Also locate
the input capacitor very close to the MAX724/MAX726 and
use minimum length leads (surface-mount or radial
through-hole types). In most applications, ESR is more
important than actual capacitance value since electrolytic
capacitors are mostly resistive at the MAX724/MAX726's
100kHz switching frequency.
__________Applications Information
Setting Output Voltage
R1 and R2 set output voltage as follows:
R1 =
V
OUT
R2
2.21V
-R2
2.21V is the reference voltage, so setting R2 to 2.21k
(standard 1% resistor value) results in 1mA flowing
through R1 and R2 and simplifies the above equation.
Other values will also work for R2, but should not
exceed 4k
.
Synchronizing the Oscillator
The MAX724/MAX726 can be synchronized to an exter-
nal 110kHz to 160kHz source by pulsing the V
C
pin to
ground at the desired clock rate. This is conveniently
done with the collector of an external grounded-emitter
NPN transistor. V
C
should be pulled low for 300ns.
Doing this may have some impact on output regulation,
but the effect should be minimal for compensation
resistor values between 1k
and 4k
.
Power Dissipation
The MAX724/MAX726 draw about 7.5mA operating cur-
rent, which is largely independent of input voltage or
load current. They draw an additional 5mA during
switch on-time. Power dissipated in the internal V
SW
transistor is proportional to load current and depends
on both conduction losses (product of switch on-volt-
age and switch current) and dynamic switching losses
(due to switch rise and fall times). Total MAX724 power
dissipation can be calculated as follows:
P = V
IN
[7.5mA + 5mA (DC) + 2 I
OUT
t
SW
f
OSC
] + . . .
. . . DC [I
OUT
(1.8V) + 0.1
(I
OUT
)
2
]
DC = Duty Cycle =
V
OUT
+ 0.5V
V
IN
- 2V
t
SW
= Overlap Time = 50ns + (3ns/A) I
OUT
where t
SW
is "overlap" time. Switch dissipation is
momentarily high during overlap time because both cur-
rent and voltage appear across the switch at the same
time. t
SW
is approximately: [50ns + (3ns/A) (I
OUT
)] for
the MAX724.
Power dissipation in the MAX726 can be estimated in
exactly the same way as the MAX724, except that 1.1V
(and not 1.8V) is a more reasonable value for the nomi-
nal voltage drop across the on-board power switch.
Ground Connections
GND demands a short low-noise connection to ensure
good load regulation. Since the internal reference is
referred to GND, errors in the GND pin voltage get mul-
tiplied by the error amplifier and appear at the output.
If the MAX724/MAX726 GND pin is separated from the
negative side of the load, then high load return current
can generate significant error across a seemingly small
ground resistance. Single-point grounding is the most
effective way to eliminate these errors. A recommend-
ed ground arrangement is shown in Figure 4.
Overload Protection
The V
SW
current is internally limited to about 6.5A in the
MAX724 and 2.6A in the MAX726. In addition, another
feature of the MAX724/MAX726's overload protection
scheme is that the oscillator frequency is reduced
when the output voltage falls below approximately half
its regulated value. This is the case during short-circuit
and heavy overload conditions.
Since the minimum on-time for the switch is about
0.6
s, frequency reduction during overload ensures
that switch duty cycle can fall to a low enough value to
maintain control of output current. At the normal
100kHz switching frequency, an on-time as short as
R1
R2
FB
GND
HIGH CURRENT
RETURN PATH
NEGATIVE OUTPUT
NODE WHERE LOAD
REGULATION WILL
BE MEASURED
MAX724
MAX726
Figure 4. Recommended Ground Connection
MAX724/MAX726
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
10
______________________________________________________________________________________
0.2
s would be needed to provide a narrow enough
duty cycle that could control current when the output is
shorted. Since 0.6
s is too long (at 100kHz), the f
OSC
is lowered to 20kHz once FB (and hence the output)
drops below about 1.3V (see Frequency vs. V
FB
Voltage
graph in the
Typical Operating Characteristics). This
way, the MAX724/MAX726's 0.6
s minimum t
ON
allows
a sufficiently small duty cycle (at the reduced f
OSC
) so
that current can still be limited.
Compensation Network
A series RC network connected from V
C
to ground
compensates the MAX724/MAX726. Compensation
R
C
values are shown in the applications circuits. R
C
and C
C
shape error-amplifier gain as follows: At DC,
R
C
and C
C
have no effect, so the error-amplifier's
gain is the product of its transconductance (approxi-
mately 5000
mhos) and an internal 400k
load
impedance (r
INT
) at V
C
. So at DC, A
V(DC)
= g
M
(r
INT
) =
approximately 2000
mhos. R
C
and C
C
then add a
low-frequency pole and a high-frequency zero, as
shown in Figure 5.
Output Overshoot
The MAX724/MAX726 error-amplifier design minimizes
overshoot, but precautions against overshoot should
still be exercised in sensitive applications. Worst-case
overshoot typically occurs when recovering from an
output short because V
C
slews down from its highest
voltage. This can be checked by simply shorting and
releasing the output.
Reduce objectional overshoot by increasing the com-
pensation resistor (to 3k
or 4k
) at V
C
. This allows
the error-amplifier output, V
C
, to move more rapidly in
the negative direction. In some cases, loop stability
may suffer with a high-value compensation resistor. An
option, then, is to add output filter capacitance, which
reduces short-circuit recovery overshoot by limiting out-
put rise time. Lowering the compensation capacitor to
below 0.05
F may also help by allowing V
C
to slew fur-
ther before the output rises too far.
Optional Output Filters
Though not shown in the application circuits in Figures
2, 7, and 8, additional filtering can easily be added to
reduce output ripple to levels below 2%. It is more
effective to add an LC type filter rather than additional
output capacitance alone. A small-value inductor (2
H
to 10
H) and between 47
F and 220
F of filter capaci-
tance should suffice (Figure 6). Although the inductor
does not need to be of high quality (it is not switching),
it must still be rated for the full load current.
When an LC filter is added, do not move the connection
of the feedback resistor to the LC output. It should be left
connected to the main output filter capacitor (C1 in Figure
2). If the feedback connection is moved to the LC filter
point, the added phase shift may impact stability.
L
F
C
F
TO LOAD
FEEDBACK RESISTOR
MAIN FILTER CAP
Figure 6. Optional LC Output Filter
FREQUENCY
GAIN
90 PHASE SHIFT
f
POLE
= 1/[2
(400k
)]C
C
-A
V(MID)
= g
M
/ (2
f C
C
)
f
ZERO
= 1 / (2
R
C
C
C
)
A
V(HI)
= g
M
R
C
A
V(DC)
= g
M
(400k
)
2000
Figure 5. Error-Amplifier Gain as Set by R
C
and C
C
at V
C
Pin
___________________Typical Applications
Positive-to-Negative DC-DC Inverter
The MAX724/MAX726 can convert positive input volt-
ages to negative outputs if the sum of input and output
voltage is greater than 8V, and the minimum positive
supply is 4.5V. The connection in Figure 7 shows the
MAX724 generating -5V. The device's GND pin is con-
nected to the negative output, which allows the feed-
back divider, R3, and R4 to be connected normally. If
the GND pin were tied to circuit ground, a level shift
and inversion would be required to generate the proper
feedback signal.
Component values in Figure 8 are shown for input volt-
ages up to 35V and for a 1A output. If the maximum
input voltage is lower, a Schottky diode with lower
reverse breakdown than the MBR745 (D1) may be
used. If lower output current is needed, then the cur-
rent rating of both D1 and L1 may be reduced. In addi-
tion, if the minimum input voltage is higher than 4.5V,
then greater output current can be supplied.
R1, R2, and C4 provide compensation for low input
voltages, but R1 and R2 also figure in the output-volt-
age calculation because they are effectively connected
in parallel with R3. For larger negative outputs,
increase R1, R2, and R3 proportionally while maintain-
ing the following relationships. If V
IN
does not fall below
2V
OUT
, then R1, R2, and C4 can be omitted and only R3
and R4 set the output voltage.
R4 = 1.82k
R3 = |V
OUT
| - 2.37 (in k
)
R1 = 1.86 (R3)
R2 = 3.65 (R3)
Negative Boost DC-DC Converter
The MAX724/MAX726 can also work as a negative
boost converter (Figure 8) by tying the GND pin to the
negative output. This allows the regulator to operate
from input voltages as low as -4.5V. If the regulated
output is at least -8V, R1 and R2 set the output volt-
age as in a conventional connection, with R1 selected
from:
R1 =
V
OUT
R2
- R2
2.21
L1 must be a low value to maintain stability, but if V
IN
is
greater than -10V, L1 can be increased to 50
H. Since
this is a boost configuration, if the input voltage
exceeds the output voltage, D1 will pull the output more
negative and out of regulation. Also, if the output is
pulled toward ground, D1 will drag down the input sup-
ply. For this reason, this configuration is not short-cir-
cuit protected.
MAX724/MAX726
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
______________________________________________________________________________________
11
D1 - MOTOROLA MBR745
C1 - NICHICON UPL1C221MRH6
C2 - NICHICON UPL1A102MRH6
L1 - COILTRONICS CTX25-5-52
MAX724
V
IN
+4.5V TO +35V
220
H
50V
C1
L1
50
H
5A
R1
5.1k
R2
10k
R3
2.74k
R4
1.82k
C2
1000
F
10V
-5V
1A
C4
0.01
F
D1
C3
0.1
F
V
SW
V
IN
V
C
GND
FB
ALL RESISTORS HAVE 1% TOLERANCE
Figure 7. Positive-to-Negative DC-DC Inverter
MAX724
V
IN
R3
750
R2
2.21k
R1
12.7k
C1
1000
F
25V
D1
MBR735
L1
25
H
0.01
F
GND
V
C
V
SW
FB
C3
100
F
25V
1000pF
V
OUT
-15V
-V
IN
-4.5V TO -15V
C2
1
F
Figure 8. Negative Step-Up DC-DC Converter
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
12
__________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600
1995 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
MAX724/MAX726
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
________________________________________________________Package Information
E
J3
F
B
DIM
A
B
C1
D
E
e
F
H1
J1
J2
J3
L
L1
L2
P
Q
MIN
0.140
0.015
0.014
0.560
0.380
0.045
0.230
0.080
0.170
0.327
0.170
0.260
0.700
0.139
0.100
MAX
0.190
0.040
0.022
0.650
0.420
0.055
0.270
0.115
0.185
0.335
0.200
0.340
0.720
0.161
0.120
MIN
3.56
0.38
0.41
14.23
9.66
1.14
5.85
2.04
4.32
8.31
4.32
6.60
17.78
3.54
2.54
MAX
4.82
1.01
0.50
16.51
10.66
1.39
6.85
2.92
4.70
8.51
5.08
8.64
18.29
4.08
3.04
INCHES
MILLIMETERS
e
21-005-
5-PIN TO-220
(STAGGERED LEAD)
PACKAGE
1.70 BSC
0.067 BSC
P
H1
Q
D
A
L2
J1
L1
L
C1
J2
E
F
B
DIM
A
B
C1
D
E
e
F
H1
J1
L
P
Q
MIN
0.140
0.015
0.014
0.560
0.380
0.045
0.230
0.080
0.500
0.139
0.100
MAX
0.190
0.040
0.022
0.650
0.420
0.055
0.270
0.115
0.580
0.161
0.120
MIN
3.56
0.38
0.41
14.23
9.66
1.14
5.85
2.04
12.70
3.54
2.54
MAX
4.82
1.01
0.50
16.51
10.66
1.39
6.85
2.92
14.73
4.08
3.04
INCHES
MILLIMETERS
e
21-4737-
5-PIN TO-220
(STRAIGHT LEAD)
PACKAGE
1.70 BSC
0.067 BSC
P
H1
Q
D
A
J1
C1
L
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
12
__________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600
1995 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
CONTACT FACTORY FOR AVAILABILITY
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