July 2000
ML4771
*
High Current Boost Regulator
GENERAL DESCRIPTION
The ML4771 is a continuous conduction boost regulator
designed for DC to DC conversion in multiple cell battery
powered systems. Continuous conduction allows the
regulator to maximize output current for a given inductor.
The maximum switching frequency can exceed 200kHz,
allowing the use of small, low cost inductors. The ML4771
is capable of start-up with input voltages as low as 1.8V,
and the output voltage can be set anywhere between 3.0V
and 5.5V by an external resistor divider connected to the
SENSE pin.
An integrated synchronous rectifier eliminates the need
for an external Schottky diode and provides a lower
forward voltage drop, resulting in higher conversion
efficiency. In addition, low quiescent battery current and
variable frequency operation result in high efficiency even
at light loads. The ML4771 requires a minimum number of
external components to build a very small regulator circuit
capable of achieving conversion efficiencies exceeding
85%.
FEATURES
s
Guaranteed full load start-up and operation at
1.8V input
s
Continuous conduction mode for high output current
s
Very low supply current (20A output referenced) for
micropower operation
s
Pulse Frequency Modulation and Internal Synchronous
Rectification for high efficiency
s
Maximum switching frequency > 200kHz
s
Minimum external components
s
Low ON resistance internal switching FETs
s
Adjustable output voltage (3.0V to 5.5V)
BLOCK DIAGRAM
1
VL2
5
VOUT
SENSE
R1
R2
COUT
VOUT
*CFB
6
VIN
2
GND
VBAT
*CIN
VL1
L1
1
8
PWR GND
3
BOOST
CONTROL
4
(* Indicates Part is End Of Life as of July 1, 2000)
2
ML4771
PIN CONFIGURATION
PIN DESCRIPTION
ML4771
8-Pin SOIC (S08)
1
2
3
4
8
7
6
5
VL1
VIN
GND
SENSE
PWR GND
NC
VL2
VOUT
TOP VIEW
PIN
NAME
FUNCTION
1
V
L1
Boost inductor connection
2
V
IN
Battery input voltage
3
GND
Ground
4
SENSE
Programming pin for setting the output
voltage
PIN
NAME
FUNCTION
5
V
OUT
Output of the boost regulator
6
V
L2
Boost inductor connection
7
NC
No connection
8
PWR GND Return for the NMOS output transistor
3
ML4771
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
OUT
........................................................................... 7V
Voltage on any other pin ..... GND 0.3V to V
OUT
+ 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
ML4771CS ................................................. 0C to 70C
ML4771ES ............................................. 20C to 70C
V
IN
Operating Range
ML4771CS .................................... 1.8V to V
OUT
0.2V
ML4771ES .................................... 2.0V to V
OUT
0.2V
V
OUT
Operating Range ................................. 3.0V to 5.5V
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, V
IN
= Operating Voltage Range, T
A
= Operating Temperature Range (Note 1).
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
SUPPLY
I
IN
V
IN
Current
V
IN
= V
OUT
0.2V
2
5
A
I
OUT(Q)
V
OUT
Quiescent Current
25
35
A
I
L(Q)
V
L
Quiescent Current
1
A
PFM REGULATOR
I
L
Peak Current
1.2
1.4
1.7
A
V
SENSE
SENSE Comparator Threshold Voltage
2.52
2.57
2.62
V
V
OUT
Output Voltage
See Figure 1, I
OUT
= 0
4.95
5.05
5.15
V
Load Regulation
See Figure 1,
4.85
4.95
5.15
V
V
IN
= 2.4V, I
OUT
= 220mA
Note 1:
Limits are guaranteed by 100% testing, sampling or correlation with worst case test conditions.
4
ML4771
Figure 1. Application Test Circuit.
Figure 3. Inductor Current and Voltage Waveforms.
Figure 2. PFM Regulator Block Diagram.
ML4771
IOUT
200F
100F
VIN
20H
(Sumida CD75)
VL1
VIN
GND
SENSE
PWR GND
NC
VL2
VOUT
268k
259k
VIN
VL2
5
VOUT
4
SENSE
6
+
VIN
IL
2
2.57V
START-UP
3
GND
VL1
1
SYNCHRONOUS
RECTIFIER
CONTROL
BOOST
CONTROL
+
+
8
PWR GND
A1
A2
A3
R1
R2
Q1
Q2
VOUT
RSENSE
ISET
Q1 ON
Q2 OFF
Q1 OFF
Q2 ON
IL
VL2
IL(MAX)
ISET
VOUT
0
0
5
ML4771
FUNCTIONAL DESCRIPTION
The ML4771 combines a unique form of current mode
control with a synchronous rectifier to create a boost
converter that can deliver high currents while maintaining
high efficiency. Current mode control allows the use of a
very small, high frequency inductor and output capacitor.
Synchronous rectification replaces the conventional
external Schottky diode with an on-chip PMOS FET to
reduce losses and eliminate an external component. Also
included on-chip are an NMOS switch and current sense
resistor, further reducing the number of external
components, which makes the ML4771 very easy to use.
REGULATOR OPERATION
The ML4771 is a variable frequency, current mode
switching regulator. Its unique control scheme converts
efficiently over more than three decades of load current.
A block diagram of the boost converter is shown in Figure 2.
Error amp A3 converts deviations in the desired output
voltage to a small current, I
SET
. The inductor current is
measured through a 50m
W resistor which is amplified by
A1. The boost control block matches the average inductor
current to a multiple of the I
SET
current by switching Q1
on and off. The peak inductor current is limited by the
controller to about 1.5A.
At light loads, I
SET
will momentarily reach zero after an
inductor discharge cycle , causing Q1 to stop switching.
Depending on the load, this idle time can extend to tenths
of seconds. While the circuit is not switching, only 20A
of supply current is drawn from the output. This allows the
part to remain efficient even when the load current drops
below 200A.
Amplifier A2 and the PMOS transistor Q2 work together to
form a low drop diode. When transistor Q1 turns off, the
current flowing in the inductor causes pin 6 to go high. As
the voltage on V
L2
rises above V
OUT
, amplifier A2 allows
the PMOS transistor Q2 to turn on. In discontinuous
operation, (where I
L
always returns to zero), A2 uses the
resistive drop across the PMOS switch Q2 to sense zero
inductor current and turns the PMOS switch off. In
continuous operation, the PMOS turn off is independent of
A2, and is determined by the boost control circuitry.
Typical inductor current and voltage waveforms are shown
in Figure 3.
DESIGN CONSIDERATIONS
OUTPUT CURRENT CAPABILITY
The maximum current available at the output of the
regulator is related to the maximum inductor current by
the ratio of the input to output voltage and the full load
efficiency. The maximum inductor current is
approximately 1.25A and the full load efficiency may be
as low as 70%. The maximum output current can be
determined by using the typical performance curves
shown in Figures 4 and 5, or by calculation using the
following equation:
I
V
V
A
OUT MAX
IN MIN
OUT
(
)
(
)
.
.
=
125
0 7
(1)
INDUCTOR SELECTION
The ML4771 is able to operate over a wide range of
inductor values. A value of 10H is a good choice, but any
value between 5H and 33H is acceptable. As the
inductor value is changed the control circuitry will
automatically adjust to keep the inductor current under
control. Choosing an inductance value of less than 10H
will reduce the component's footprint, but the efficiency
and maximum output current may drop.
It is important to use an inductor that is rated to handle 1.5A
peak currents without saturating. Also look for an inductor
with low winding resistance. A good rule of thumb is to
allow 5 to 10m
W of resistance for each H of inductance.
The final selection of the inductor will be based on trade-
offs between size, cost and efficiency. Inductor tolerance,
core and copper loss will vary with the type of inductor
selected and should be evaluated with a ML4771 under
worst case conditions to determine its suitability.
Several manufacturers supply standard inductance values
in surface mount packages:
Coilcraft
(847) 639-6400
Coiltronics
(561) 241-7876
Dale
(605) 665-9301
Sumida
(847) 956-0666
6
ML4771
Figure 7. Sample PC Board Layout
OUTPUT CAPACITOR
The output capacitor filters the pulses of current from the
switching regulator. Since the switching frequency will
vary with inductance, the minimum output capacitance
required to reduce the output ripple to an acceptable level
will be a function of the inductor used. Therefore, to
maintain an output voltage with less than 100mV of ripple
at full load current, use the following equation:
C
L
V
OUT
OUT
=
44
(2)
The output capacitor's Equivalent Series Resistance (ESR)
and Equivalent Series Inductance (ESL), also contribute to
the ripple. Just after the NMOS transistor, Q1, turns off, the
current in the output capacitor ramps quickly to between
0.5A and 1.5A. This fast change in current through the
capacitor's ESL causes a high frequency (5ns) spike to
appear on the output. After the ESL spike settles, the output
still has a ripple component equal to the inductor
discharge current times the ESR. To minimize these effects,
choose an output capacitor with less than 10nH of ESL
and 100m
W of ESR.
Suitable tantalum capacitors can be obtained from the
following vendors:
AVX
(207) 282-5111
Kemet
(846) 963-6300
Sprague
(207) 324-4140
DESIGN CONSIDERATIONS
(Continued)
Figure 5. Efficiency vs. I
OUT
Using the Circuit of Figure 8
Figure 4. I
OUT
vs. VIN Using the Circuit of Figure 8
Figure 6. No Load Input Current vs. V
IN
1000
800
600
400
200
0
I OUT
(mA)
VIN (V)
1.5
2.5
3.5
5.5
4.5
VOUT = 3.0V
VOUT = 5.5V
90
80
70
60
EFFICIENCY (%)
IOUT (mA)
1
10
100
1000
VOUT = 3.0V
VOUT = 5.5V
VIN = 2.4V
160
120
80
40
0
I IN
(A)
VIN (V)
1.5
2.5
3.5
5.5
4.5
VOUT = 3.0V
VOUT = 5.5V
7
ML4771
INPUT CAPACITOR
Due to the high input current drawn at startup and
possibly during operation, it is recommended to decouple
the input with a capacitor with a value of 47F to 100F.
This filtering prevents the input ripple from affecting the
ML4771 control circuitry, and also improves the efficiency
by reducing the I squared R losses during the charge cycle
of the inductor. Again, a low ESR capacitor (such as
tantalum) is recommended.
It is also recommended that low source impedance
batteries be used. Otherwise, the voltage drop across the
source impedance during high input current situations will
cause the ML4771 to fail to start-up or to operate
unreliably. In general, for two cell applications the source
impedance should be less than 200m
W, which means that
small alkaline cells should be avoided.
SETTING THE OUTPUT VOLTAGE
The adjustable output of the ML4771 requires an external
feedback resistor divider to set V
OUT
. The output voltage
can be determined from the following equation:
V
R
R
R
OUT
=
+
257
1
2
2
.
1
6
(3)
where R1 and R2 are connected as shown in Figure 2. The
value of R2 should be 250k
W or less to minimize bias
current errors. Choose an appropriate value for R2 and
calculate R1.
LAYOUT
Good layout practices will ensure the proper operation of
the ML4771. Some layout guidelines follow:
Use adequate ground and power traces or planes
Keep components as close as possible to the ML4771
Use short trace lengths from the inductor to the V
L1
and
V
L2
pins and from the output capacitor to the V
OUT
pin
Use a single point ground for the ML4771 ground pin,
and the input and output capacitors
Separate the ground for the converter circuitry from the
ground of the load circuitry and connect at a single
point
A sample layout is shown in Figure 7.
DESIGN CONSIDERATIONS
(Continued)
DESIGN EXAMPLE
In order to design a boost converter using the ML4871, it
is necessary to define a few parameters. For this example,
assume that V
IN
= 3.0V to 3.6V, V
OUT
= 5.0V, and
I
OUT(MAX)
= 500mA.
First, it must be determined whether the ML4871 is
capable of delivering the output current. This is done using
Equation 1:
I
V
V
A
A
OUT MAX
(
)
.
.
.
.
.
=
=
125
30
50
0 7
0 53
Next, select an inductor. As previously mentioned, the
recommended inductance is 10H. Make sure that the
peak current rating of the inductor is at least 1.5A, and
that the DC resistance of the inductor is in the range of 50
to 100m
W.
Then, the value of the output capacitor is determined
using Equation 2:
C
H
V
F
OUT
=
=
44 10
5 0
88
m
m
.
The closest standard value would be a 100F capacitor
with an ESR rating of 100m
W. If such a low ESR value
cannot be found, two 47F capacitors in parallel could
also be used.
Finally, the values of R1 and R2 are calculated using
equation 3, assuming that R2 = 250k
W:
R
k
k
k
1
5 0
257
250
250
236
=
-
=
.
.
W
W
W
The complete circuit is shown in Figure 8. As mentioned
previously, the use of an input supply bypass capacitor is
highly recommended.
Figure 8. Typical Application Circuit
ML4771
100F
100F
VIN
10H
(Sumida CD75)
VL1
VIN
GND
SENSE
PWR GND
NC
VL2
VOUT
250k
236k
VOUT
8
ML4771
ORDERING INFORMATION
PART NUMBER
TEMPERATURE RANGE
PACKAGE
ML4771CS (End Of Life)
0C to 70C
8-Pin SOIC (S08)
ML4771ES (Obsolete)
20C to 70C
8-Pin SOIC (S08)
PHYSICAL DIMENSIONS
inches (millimeters)
2092 Concourse Drive
San Jose, CA 95131
Tel: 408/433-5200
Fax: 408/432-0295
www.microlinear.com
SEATING PLANE
0.148 - 0.158
(3.76 - 4.01)
PIN 1 ID
0.228 - 0.244
(5.79 - 6.20)
0.189 - 0.199
(4.80 - 5.06)
0.012 - 0.020
(0.30 - 0.51)
0.050 BSC
(1.27 BSC)
0.015 - 0.035
(0.38 - 0.89)
0.059 - 0.069
(1.49 - 1.75)
0.004 - 0.010
(0.10 - 0.26)
0.055 - 0.061
(1.40 - 1.55)
8
0.006 - 0.010
(0.15 - 0.26)
0 - 8
1
0.017 - 0.027
(0.43 - 0.69)
(4 PLACES)
Package: S08
8-Pin SOIC
Micro Linear 2000. is a registered trademark of Micro Linear Corporation. All other trademarks are the property of their
respective owners.
Products described herein may be covered by one or more of the following U.S. patents: 4,897,611; 4,964,026; 5,027,116;
5,281,862; 5,283,483; 5,418,502; 5,508,570; 5,510,727; 5,523,940; 5,546,017; 5,559,470; 5,565,761; 5,592,128; 5,594,376;
5,652,479; 5,661,427; 5,663,874; 5,672,959; 5,689,167; 5,714,897; 5,717,798; 5,742,151; 5,747,977; 5,754,012; 5,757,174;
5,767,653; 5,777,514; 5,793,168; 5,798,635; 5,804,950; 5,808,455; 5,811,999; 5,818,207; 5,818,669; 5,825,165; 5,825,223;
5,838,723; 5.844,378; 5,844,941. Japan: 2,598,946; 2,619,299; 2,704,176; 2,821,714. Other patents are pending.
DS4771-01
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