Page 1
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
■ Guaranteed full load start-up and operation at
1.8V input
■ Continuous conduction mode for high output current
■ Very low supply current (20µA output referenced) for
micropower operation
■ Pulse Frequency Modulation and Internal Synchronous
Rectification for high efficiency
■ Maximum switching frequency > 200kHz
■ Minimum external components
■ Low ON resistance internal switching FETs
■ Adjustable output voltage (3.0V to 5.5V)
(* Indicates Part is End Of Life as of July 1, 2000)
BLOCK DIAGRAM
V
2
*C
IN
V
BAT
GND
3
L1
1
V
L1
IN
BOOST
CONTROL
PWR GND
6
V
L2
V
OUT
5
R1
SENSE
4
R2
8
V
OUT
C
OUT
*C
FB
1
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ML4771
PIN CONFIGURATION
ML4771
8-Pin SOIC (S08)
V
L1
V
IN
GND
SENSE
PIN DESCRIPTION
PIN NAME FUNCTION
1V
2V
L1
IN
3 GND Ground
4 SENSE Programming pin for setting the output
Boost inductor connection
Battery input voltage
voltage
1
2
3
4
TOP VIEW
8
7
6
5
PIN NAME FUNCTION
5V
6V
PWR GND
NC
V
L2
V
OUT
OUT
L2
Output of the boost regulator
Boost inductor connection
7 NC No connection
8 PWR GND Return for the NMOS output transistor
2
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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
........................................................................... 7V
OUT
Voltage on any other pin ..... GND – 0.3V to V
Peak Switch Current (I
Average Switch Current (I
)..........................................2A
PEAK
) ..................................... 1A
AVG
OUT
+ 0.3V
OPERATING CONDITIONS
Temperature Range
ML4771CS................................................. 0ºC to 70ºC
ML4771ES ............................................. –20ºC to 70ºC
VIN Operating Range
ML4771CS....................................1.8V to V
ML4771ES .................................... 2.0V to V
V
Operating Range .................................3.0V to 5.5V
OUT
OUT
OUT
– 0.2V
– 0.2V
Junction Temperature .............................................. 150ºC
Storage Temperature Range...................... –65ºC to 150ºC
Lead Temperature (Soldering 10 sec) .......................260ºC
Thermal Resistance (qJA) .................................... 160ºC/W
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, VIN = Operating Voltage Range, TA = Operating Temperature Range (Note 1).
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
SUPPLY
I
I
OUT(Q)VOUT
I
L(Q)
PFM REGULATOR
V
SENSE
V
OUT
Note 1: Limits are guaranteed by 100% testing, sampling or correlation with worst case test conditions.
VIN Current VIN = V
IN
Quiescent Current 25 35 µA
VL Quiescent Current 1µA
IL Peak Current 1.2 1.4 1.7 A
SENSE Comparator Threshold Voltage 2.52 2.57 2.62 V
Output Voltage See Figure 1, I
Load Regulation See Figure 1, 4.85 4.95 5.15 V
VIN = 2.4V, I
– 0.2V 2 5 µA
OUT
= 0 4.95 5.05 5.15 V
OUT
= 220mA
OUT
3
Page 4
ML4771
20µH
(Sumida CD75)
ML4771
V
PWR GND
L1
V
IN
100µF
268kΩ
V
IN
GND
SENSE
259kΩ
V
OUT
NC
200µF
I
OUT
V
IN
V
L2
Figure 1. Application Test Circuit.
I
L
Q1
PWR GND
6
V
L2
A3
Q2
R1
R2
V
OUT
V
A2
+
–
+
–
2.57V
OUT
5
SENSE
4
SYNCHRONOUS
RECTIFIER
CONTROL
I
SET
8
1
V
A1
GND
L1
START-UP
BOOST
CONTROL
3
V
IN
2
R
SENSE
+
–
Figure 2. PFM Regulator Block Diagram.
I
L(MAX)
I
I
SET
L
0
V
OUT
V
L2
0
Q1 ON
Q2 OFF
Q1 OFF
Q2 ON
Figure 3. Inductor Current and Voltage Waveforms.
4
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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
measured through a 50mW resistor which is amplified by
A1. The boost control block matches the average inductor
current to a multiple of the I
on and off. The peak inductor current is limited by the
controller to about 1.5A.
At light loads, I
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 20µA
of supply current is drawn from the output. This allows the
part to remain efficient even when the load current drops
below 200µA.
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 VL2 rises above V
the PMOS transistor Q2 to turn on. In discontinuous
operation, (where IL 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.
will momentarily reach zero after an
SET
. The inductor current is
SET
current by switching Q1
SET
, amplifier A2 allows
OUT
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:
V
I
OUT MAX
()
INDUCTOR SELECTION
The ML4771 is able to operate over a wide range of
inductor values. A value of 10µH is a good choice, but any
value between 5µH and 33µH 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 10µH
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 10mW of resistance for each µH of inductance.
The final selection of the inductor will be based on tradeoffs 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
..=
IN MIN
()
V
OUT
A
125 07
(1)
Typical inductor current and voltage waveforms are shown
in Figure 3.
Sumida (847) 956-0666
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ML4771
DESIGN CONSIDERATIONS (Continued)
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:
L
C
OUT
44
=
V
OUT
(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
1000
800
V
= 3.0V
600
(mA)
OUT
400
I
OUT
V
OUT
= 5.5V
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 100mW of ESR.
Suitable tantalum capacitors can be obtained from the
following vendors:
AVX (207) 282-5111
Kemet (846) 963-6300
Sprague (207) 324-4140
90
V
= 3.0V
OUT
80
V
= 5.5V
OUT
EFFICIENCY (%)
70
200
0
1.5 2.5 3.5 5.5
VIN (V)
vs. VIN Using the Circuit of Figure 8
OUT
160
V
= 5.5V
OUT
V
= 3.0V
OUT
1.5 2.5 3.5 5.5
VIN (V)
(µA)
IN
I
120
80
40
0
4.5
4.5
VIN = 2.4V
60
1 10 100 1000
Figure 5. Efficiency vs. I
OUT
I
OUT
Using the Circuit of Figure 8Figure 4. I
(mA)
Figure 6. No Load Input Current vs. V
IN
Figure 7. Sample PC Board Layout
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Page 7
DESIGN CONSIDERATIONS (Continued) DESIGN EXAMPLE
ML4771
100µF
100µF
V
IN
10µH
(Sumida CD75)
V
L1
V
IN
GND
SENSE
PWR GND
NC
V
L2
V
OUT
250kΩ
236kΩ
V
OUT
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 47µF to 100µF.
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 200mW, 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
can be determined from the following equation:
RR
+
16
V
=
257
OUT
where R1 and R2 are connected as shown in Figure 2. The
value of R2 should be 250kW or less to minimize bias
current errors. Choose an appropriate value for R2 and
calculate R1.
.
12
R
2
. The output voltage
OUT
(3)
LAYOUT
Good layout practices will ensure the proper operation of
the ML4771. Some layout guidelines follow:
In order to design a boost converter using the ML4871, it
is necessary to define a few parameters. For this example,
assume that VIN = 3.0V to 3.6V, V
I
OUT(MAX)
First, it must be determined whether the ML4871 is
capable of delivering the output current. This is done using
Equation 1:
Next, select an inductor. As previously mentioned, the
recommended inductance is 10µH. 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 100mW.
Then, the value of the output capacitor is determined
using Equation 2:
The closest standard value would be a 100µF capacitor
with an ESR rating of 100mW. If such a low ESR value
cannot be found, two 47µF capacitors in parallel could
also be used.
Finally, the values of R1 and R2 are calculated using
equation 3, assuming that R2 = 250kW:
The complete circuit is shown in Figure 8. As mentioned
previously, the use of an input supply bypass capacitor is
highly recommended.
= 500mA.
V
.
30
I
OUT MAX()
C
OUT
Rkkk
1
44 10
=
50
50
.
257
.
.
V
.
250 250 236=
m
H
WWW
=125
V
.
50
m
F
=
88
-=
= 5.0V, and
OUT
AA
..=
07 053
• 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 VL1 and
VL2 pins and from the output capacitor to the V
• 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.
OUT
pin
Figure 8. Typical Application Circuit
7
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ML4771
PHYSICAL DIMENSIONS inches (millimeters)
0.189 - 0.199
(4.80 - 5.06)
8
Package: S08
8-Pin SOIC
0.017 - 0.027
(0.43 - 0.69)
(4 PLACES)
0.055 - 0.061
(1.40 - 1.55)
1
0.012 - 0.020
(0.30 - 0.51)
ORDERING INFORMATION
PART NUMBER TEMPERATURE RANGE PACKAGE
ML4771CS (End Of Life) 0ºC to 70ºC 8-Pin SOIC (S08)
ML4771ES (Obsolete) –20ºC to 70ºC 8-Pin SOIC (S08)
PIN 1 ID
0.050 BSC
(1.27 BSC)
0.059 - 0.069
SEATING PLANE
0.148 - 0.158
(3.76 - 4.01)
(1.49 - 1.75)
0.228 - 0.244
(5.79 - 6.20)
0.004 - 0.010
(0.10 - 0.26)
0º - 8º
0.015 - 0.035
(0.38 - 0.89)
0.006 - 0.010
(0.15 - 0.26)
© 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.
2092 Concourse Drive
San Jose, CA 95131
Tel: 408/433-5200
DS4771-01
Fax: 408/432-0295
www.microlinear.com
8
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