_______________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
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