July 2000
ML4866
*
3.3V Output DCDC Step-Down Converter
1
GENERAL DESCRIPTION
The ML4866 is a high efficiency pulse width modulated
(PWM) buck regulator designed for use in 5V systems or
portable equipment that need a compact, efficienct 3.3V
supply. It has a switching frequency of 120kHz and uses
synchronous rectification to optimize power conversion
efficiency. Unlike other solutions, the ML4866 requires no
external diodes or FETs.
The ML4866 can provide up to 500mA of output current,
and operates over an input voltage range of 3.5V to 6.5V
(3 to 4 cells or a 5 VDC supply). A complete switched
mode power converter can be quickly and easily
implemented with few external components. Thanks to a
built-in autoburst mode, power conversion efficiency of
this DCDC converter can exceed 90% over more than 2
decades of output load current.
Stability and fast loop response are provided by current
programming and a current sense circuit. The ML4866
also has a SHDN pin for use in systems which have power
management control. Undervoltage lockout and soft start
are also built in.
BLOCK DIAGRAM
FEATURES
s
High power conversion efficiency over 2 decades of
load current
s
No external FETs or diodes; minimum external
components
s
3.5V to 6.5V input voltage range
s
Significantly extends battery life over linear regulator
based solutions
s
Micropower operation
s
Low shutdown mode quiescent current
VREF
VIN
5
SHDN
6
VREF
3
BURST
4
COMP
2
GND
8
VOUT
1
VL
7
+
REFERENCE
UVLO/
SHUTDOWN
CURRENT
SENSE
+
ERROR
AMPLIFIER
BUCK
CONTROL
OSC
SLOPE
COMPENSATION
BURST
(* Indicates Part is End Of Life as of July 1, 2000)
2
ML4866
PIN DESCRIPTION
PIN CONFIGURATION
PIN
NAME
FUNCTION
1
V
OUT
Regulated 3.3V output
2
COMP
Connection point for an external
compensation network
3
V
REF
1.25V reference output
4
BURST
This pin controls when the control
circuit switches between PWM and
PFM modes of operation
ML4866
8-Pin SOIC (S08)
1
2
3
4
8
7
6
5
VOUT
COMP
VREF
BURST
GND
VL
SHDN
VIN
TOP VIEW
PIN
NAME
FUNCTION
5
V
IN
Input voltage
6
SHDN
Pulling this pin low shuts down the
regulator
7
V
L
Buck inductor connection
8
GND
Ground
3
ML4866
ABSOLUTE MAXIMUM RATINGS
Absolute maximum ratings are those values beyond which
the device could be permanently damaged. Absolute
maximum ratings are stress ratings only and functional
device operation is not implied.
V
IN ...................................................................................................
7V
Voltage on any other pin ......... GND - 0.3V to V
IN
+ 0.3V
Peak Switch Current (I
PEAK
) ......................................... 2A
Average Switch Current (I
AVG
) ..................................... 1A
Junction Temperature .............................................. 150C
Storage Temperature Range ....................... -65C to 150C
Lead Temperature (Soldering 10 Sec.) ..................... 260C
Thermal Resistance (
q
JA
) .................................... 160C/W
OPERATING CONDITIONS
Temperature Range
ML4866CS ................................................. 0C to 70C
ML4866ES .............................................. -20C to 70C
ML4866IS ............................................... -40C to 85C
V
IN
Operating Range ................................... 3.5V to 6.5V
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, V
IN
= 5V, L = 50H, C
OUT
= 100F, R
COMP
= 390k
W, C
COMP
= 15nF,
T
A
= Operating Temperature Range (Note 1)
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
REFERENCE
V
REF
Output Voltage
0 < I(V
REF
) < 5A, I
OUT
= 0mA
1.22
1.25
1.27
V
PWM REGULATOR
f
OSC
Oscillator Initial Accuracy
I
OUT
= 200mA, T
A
= 25C
100
115
165
kHz
Oscillator Total Variation
Line and Temp
90
130
185
kHz
Soft Start V
IN
to V
OUT
Delay
3
5
ms
BURST Burst Mode Threshold
250
400
mV
BURST PWM Mode Threshold
500
850
mV
BURST Bias Current
35
A
Output Voltage
I
OUT
= 200mA
3.2
3.3
3.4
V
I
OUT
= 20mA, BURST = 0V
3.28
3.38
3.48
V
Line Regulation
V
IN
= 4V to 6.5V, T
A
= 25C
2
%
Load Regulation
I
OUT
= 100mA to 500mA,
2.5
%
T
A
= 25C
I
OUT
= 5mA to 100mA,
2.5
%
BURST = 0V, T
A
= 25C
Temperature Stability
T
A
= -40C to 85C
1
%
Total Variation
Line, Load, Temp
5
%
SHUTDOWN
UVLO Startup Threshold
3.2
3.5
V
UVLO Shutdown Threshold
2.9
3.1
V
SHDN Threshold
2
V
SHDN Bias Current
5
A
4
ML4866
FUNCTIONAL DESCRIPTION
The ML4866 is a current-mode, step-down (buck)
converter designed to keep the buck inductor current in
the continuous conduction mode (CCM). Current-mode
operation provides faster output response to input voltage
and output current changes along with cycle-by-cycle
current limiting. CCM inductor current is preferred when
the highest conversion efficiencies are required.
For high efficiencies at low output current, the ML4866
contains an autoburst function which automatically
switches from pulse width modulation (PWM) to pulsed
frequency modulation (PFM) operation when the output
current drops below 100mA. Selection of either mode is
possible by applying the correct logic level signal to the
BURST pin. When operating in PWM mode, loop
compensation of the ML4866 is simplified due to its
transconductance type error amplifier.
An under voltage lockout (UVLO) circuit within the
ML4866 enables the converter when the input voltage
is greater than 3.25V and disables it when the input
voltage is below 3.10V. The IC can also be disabled
externally by applying a logic low signal to the SHDN
pin. When disabled, the ML4866 draws less than 20A of
current.
The internal 1.25V bandgap reference is made available
via the V
REF
pin, and may be used for general
applications requiring less than 10A of current. For
proper operation, this pin must always be bypassed to
GND with a 100nF capacitor.
BURST MODE
Burst (PFM) mode is a method of regulating the output
voltage by applying a variable frequency modulation
technique to the buck inductor. This method maintains
higher efficiencies at light loads than if PWM were used.
If BURST is left open, the ML4866 switches from PWM
mode to PFM mode when the output current falls below
100mA. When the output voltage falls out of regulation
while in PFM mode, the internal buck switch turns on and
ramps the inductor current up to 300mA. The buck switch
then turns off and the synchronous switch turns on,
ramping the inductor current down to 0mA. This action is
repeated until the output voltage returns to its nominal
setting and begins again when the output drops below its
nominal setting. The rate or frequency at which this
"bursting" occurs is directly proportional to the output
current. When the average output current rises above
130mA, the ML4866 returns to PWM operation.
For applications having a load current range of less than
100mA and greater than 130mA, the BURST pin should be
left open and bypassed to ground with a 15nF or larger
capacitor. It is possible to tailor an application for the
highest possible efficiency by externally forcing the
ML4866 into either control mode. Applying a logic low
level to BURST forces the IC into PFM mode. Conversely,
a logic high places it in PWM mode. Care should be
taken to avoid reducing the efficiency by placing the
controller in the least efficient mode for a given output
current.
ELECTRICAL CHARACTERISTICS
(Continued)
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
SUPPLY
I
IN
V
IN
Current
I
OUT
= 0mA, BURST = 5V
400
500
A
I
OUT
= 0mA, BURST = 0V
120
220
A
SHDN = 0V
20
35
A
Note 1: Limits are guaranteed by 100% testing, sampling, or correlation with worst case test conditions.
Figure 1. Inductor Current
I
V
OUT
L
V
IN
V
OUT
L
5
ML4866
DESIGN CONSIDERATIONS
INDUCTOR SELECTION
Figure 1 shows the inductor current in a step-down
converter operating in CCM. Note that the inductor
current does not reach zero during each switching cycle.
This is unlike discontinuous conduction mode (DCM)
where the inductor current is allowed to reach zero. CCM
operation generally results in lower peak to peak output
ripple voltage and higher circuit efficiencies because of
lower peak and RMS currents in the switching FETs and
buck inductor. The minimum value of inductance required
for CCM operation with a 6.5V input and a load range of
100mA to 500mA is:
L
V
V
V
V
I
f
OUT
IN MAX
OUT
IN MAX
OUT MIN
SW
>
-
(
)
(
)
(
)
(
)
2
(1)
L
V
V
V
V
mA
kHz
H
>
-
>
3 3
6 5
33
2 6 5
100
120
68
.
( .
.
)
.
m
To guarantee reliable operation, the peak inductor current
must be between 80% and 85% of its maximum rated
value. This value is the sum of the inductor peak to peak
current and the maximum output current:
I
V
V
V
V
f
L
L P P
OUT MAX
IN MIN
OUT MAX
IN MIN
SW MIN
(
)
(
)
(
)
(
)
(
)
(
)
(
)
=
-
2
(2)
I
V
V
V
V
kHz
H
mA
L P P
(
)
.
( .
.
)
.
=
-
m
=
2 3 465
4 0
3 465
4 0
90
100
103
I
I
V
V
V
V
f
L
L PEAK
OUT MAX
OUT MAX
IN MIN
OUT MAX
IN MIN
SW
(
)
(
)
(
)
(
)
(
)
(
)
(
)
=
+
-
(3)
I
I
V
V
V
V
kHz
H
mA
L PEAK
OUT MAX
(
)
(
)
.
( .
.
)
.
=
+
-
m
=
3 465
4 0
3 465
4 0
120
100
550
For the highest efficiency, inductor core and copper losses
must be minimized. Good high frequency core material
such as Kool-Mu, ferrite or Molyperm are popular choices
for this converter. Disregarding physical size
requirements, the lowest loss inductor will generally be
the one with the highest peak current rating.
Figure 2 displays the efficiency of the ML4866 under
various input voltage and output current conditions. These
results were obtained using a Coiltronics CTX100-4
inductor having the following specifications:
Nominal Inductance - 100H
Peak Current Rating - 950mA
DC Resistance - 175m
W
A partial listing of inductor manufacturers with standard
parts which meet the criteria for use with the ML4866 is
given below.
Coiltronics
(561) 241-7876
Dale
(605) 665-9301
Coilcraft
(847) 639-6400
XFMRS, Inc (317) 834-1066
Sumida
(847) 956-0666
CAPACITOR SELECTION
A typical digital system requires a peak to peak output
ripple voltage of no greater than 1% to 3% of the nominal
output voltage. In a step-down converter, the largest
contributor to ripple voltage is almost always the product
of the inductor peak-to-peak current times the output
capacitor's equivalent series resistance. To select the
correct capacitor, first calculate the minimum
capacitance value required:
C
V
V
V
V
V
L
f
OUT
OUT
IN MAX
OUT
P P MAX
IN MAX
SW
>
-
-
(
)
(
)
(
)
(
)
8
2
(4)
C
V
V
mV
V
H
kHz
F
OUT
>
-
m
>
33
6 5
33
8 33
6 5
100
120
4 27
2
.
( .
.
)
.
.
m
Next, calculate the maximum permissible ESR of the
output capacitor:
ESR
<
<
( .
)
( . )
.
0 033
0 1
0 33
W
(5)
When limited space is available, tantalum capacitors are
the best choice. Electrolytic capacitors can be used and
will be less expensive, but the ESR for low capacitance
values as needed here will be much higher than for the
same value tantalum. Table 2 lists the ESR values for a
number of general purpose tantalum capacitors which are
widely available from a number of sources. A 47F
capacitor was chosen for the design example.
Figure 2. Efficiency vs. Input Voltage
EFFICIENCY (%)
INPUT VOLTAGE (V)
3.5
4.5
6.0
4.0
5.0
6.5
5.5
100
98
96
94
92
90
88
86
IOUT = 500mA
IOUT = 100mA
IOUT = 10mA
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