Datasheet ML4871ES-3, ML4871ES-5, ML4871CS-5, ML4871CS-3 Datasheet (Micro Linear Corporation)

July 2000

FEATURING

Extended Commercial Temperature Range

-20˚C to 70˚C

for Portable Handheld Equipment

ML4871

High Current Boost Regulator

GENERAL DESCRIPTION

The ML4871 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 ML4871
is capable of start-up with input voltages as low as 1.8V
and is available in 5V and 3.3V output versions with an
output voltage accuracy of ±3%.

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 ML4871 requires only one inductor and two
capacitors to build a very small regulator circuit capable
of achieving conversion efficiencies approaching 90%.

The circuit also contains a RESET output which goes low
when the DETECT input drops below 1.25V.

BLOCK DIAGRAM

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

■ 5V and 3.3V output versions

V

L1

V

IN

2

+

–

1

DETECT

4

V

REF

START-UP

CONTROL

+

COMP

–

BOOST

7

RESET

PWR GND

6

V

L2

SYNCHRONOUS

RECTIFIER
CONTROL

+

–

1.25V

8

GND

V

OUT

+

–

3

5

1

ML4871

PIN CONFIGURATION

ML4871

8-Pin SOIC (S08)

V

L1

V

IN

GND

DETECT

PIN DESCRIPTION

NO. NAME FUNCTION

1V

2V

L1

IN

3 GND Ground

4 DETECT Pulling this pin below 1.25V causes

Boost inductor connection

Battery input voltage

the RESET pin to go low

1

2

3

4

TOP VIEW

8

7

6

5

5V

6V

PWR GND

RESET

V

L2

V

OUT

NO. NAME FUNCTION

OUT

L2

Boost regulator output

Boost inductor connection

7 RESET Output goes low when DETECT goes

below 1.25V

8 PWR GND Return for the NMOS output transistor

2

ML4871

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

ML4871CS-X ............................................. 0ºC to 70ºC

ML4871ES-X .......................................... –20ºC to 70ºC

VIN Operating Range

ML4871CS-X ................................ 1.8V to V

ML4871ES-X ................................. 2.0V to V

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

OUT

RESET

VIN Current VIN = V

IN

VL Quiescent Current 1µA

IL Peak Current 1.2 1.4 1.7 A

Output Voltage I

Load Regulation See Figure 1, -3 Suffix 3.20 3.25 3.40 V

COMPARATOR

DETECT Threshold 1.18 1.25 1.28 V

DETECT Hysteresis 25 35 45 mV

DETECT Bias Current –100 100 nA

RESET Output High Voltage I

RESET Output Low Voltage I

OUT

Quiescent Current DETECT = V

DETECT = 0V 25 35 µA

= 0 -3 Suffix 3.30 3.35 3.40 V

L(PEAK)

VIN = 2.4V, I

See Figure 1, -5 Suffix 4.85 4.95 5.15 V
VIN = 2.4V, I

= –200µA V

RESET

= 500µA 0.2 V

RESET

– 0.2V 2 5 µA

IN

OUT

OUT

£ 400mA

£ 220mA

-5 Suffix 4.95 5.05 5.15 V

– 0.2 V

OUT

30 40 µA

Note 1:

Limits are guaranteed by 100% testing, sampling, or correlation with worst case test conditions.

3

ML4871

V

IN

100µF

20µH

(Sumida CD75)

ML4871

V

PWR GND

L1

V

IN

GND

DETECT

RESET

V

V

OUT

L2

200µF

Figure 1. Application Test Circuit.

I

L

I

OUT

Q1

PWR GND

6

V

L2

A3

Q2

V

A2

+

–

+

–

1.25V

GND

OUT

3

V

OUT

5

SYNCHRONOUS

RECTIFIER
CONTROL

I

SET

8

1

V

L1

V

IN

2

R

SENSE

START-UP

+

–

A1

BOOST

CONTROL

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

ML4871

FUNCTIONAL DESCRIPTION

The ML4871 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 ML4871 very easy to use.

REGULATOR OPERATION

The ML4871 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 ML4871 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 ML4871 under
worst case conditions to determine its suitability.

Several manufacturers supply standard inductance values
in surface mount packages:

Coilcraft [phone]

Coiltronics [phone]

Dale [phone]



..=




IN MIN

()

V

OUT



A

125 07




(1)

Typical inductor current and voltage waveforms are shown
in Figure 3.

RESET

COMPARATOR

An additional comparator is provided to detect low VIN,
low V
want to sense. The inverting input of the comparator is
connected to the 1.25V reference, and the non-inverting
input is connected to the DETECT pin. The output of this
comparator is connected to the RESET pin of the device
and can swing from V

, or any other error condition that the user may

OUT

to ground.

OUT

Sumida [phone]

5

ML4871

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

1000

800

600

(mA)

OUT

400

I

200

V

OUT

= 3.3V

V

OUT

= 5V

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 100mW of ESR.

Suitable tantalum capacitors can be obtained from the
following vendors:

AVX [phone]

Kemet [phone]

Sprague [phone]

90

V

= 3.3V

OUT

80

EFFICIENCY (%)

70

V

= 5V

OUT

0

1.0 2.0 3.0 5.0

VIN (V)

vs. VIN Using the Circuit of Figure 8

OUT

4.0

90

60

(µA)

IN

I

V

30

0

1.0 2.0 3.0 5.0

OUT

V

OUT

= 3.3V

60

Figure 5. Efficiency vs. I

= 5V

4.0

VIN (V)

VIN = 2.4V

1 10 100 1000

I

(mA)

OUT

Using the Circuit of Figure 8Figure 4. I

OUT

Figure 6. No Load Input Current vs. V

IN

6

ML4871

V

IN

R

A

R

B

2

DETECT

4

RESET

V

REF

FROM

START-UP

CIRCUITRY

+
–

COMP

7

DESIGN CONSIDERATIONS

(Continued)

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
ML4871 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 ML4871 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.

BATTERY MONITORING

The condition of the batteries can be monitored using the
DETECT pin. For primary batteries, the comparator can be
used to signal that the batteries will soon need to be
replaced. For rechargeable batteries, the comparator can
be used to signal the start of a charging cycle.

LAYOUT

Good layout practices will ensure the proper operation of
the ML4871. Some layout guidelines follow:

• Use adequate ground and power traces or planes

• Keep components as close as possible to the ML4871

• 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 ML4871 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 8.

OUT

pin

For input voltages greater than the minimum operating
voltage, the RESET pin can be set to go low at a specified
battery voltage by connecting a resistor divider across the
battery stack and to the DETECT pin of the ML4871 as
shown in Figure 7. The low battery trip voltage is
determined by first choosing a minimum battery voltage,
V

, and then calculating the values of RA and RB:

IN(MIN)

RR

+

16

V

IN MIN

()

.=

125

AB

R

B

(3)

The values of RA and RB should be sufficiently large to
minimize the power dissipation in the divider. Also, use
care when selecting the low battery trip point. Too high a
trip voltage can lead to memory effects in the battery,
while too low a trip point can lead to reduced service life
or polarity reversal. Refer to the manufacturer’s data sheets
for more information on selecting and designing battery
systems.

Figure 7. Battery Monitoring Circuit

Figure 8. Sample PC Board Layout

7

ML4871

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 VIN = 3.0V to 3.6V, V
I

OUT(MAX)

= 500mA.

= 5.0V, and

OUT

First, it must be determined whether the ML4871 is
capable of delivering the output current. This is done using
Equation 1:

V

.

30



I

OUT MAX()

.



.

50

V

=125







AA

..=

07 053

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.

Finally, the value of the output capacitor is determined
using Equation 2:

C

OUT

50

.

m

V

=

m

F

=

88

H



44 10

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.

The complete circuit is shown in Figure 9. As mentioned
previously, the use of an input supply bypass capacitor is
highly recommended.

10µH

(Sumida CD75)

ML4871

V

PWR GND

V

IN

100µF

L1

V

IN

GND

DETECT

RESET

V

V

OUT

L2

100µF

V

OUT

Figure 9. Typical Application Circuit

I

OUT(MAX)

V

(V) V

IN

1.8 386.2 286.2

2.0 451.9 332.1

2.2 521.5 379.1

2.4 585.9 430.0

2.6 651.0 479.0

2.8 716.5 525.4

3.0 782.0 571.8

3.2 618.5

3.4 665.0

3.6 711.7

3.8 758.7

4.0 805.3

4.2 851.9

4.4 899.0

4.6 946.1

4.8 992.7

= 3.3V V

OUT

(mA)

OUT

Table 1. Typical I

= 5.0V

I

VIN = 2.4V, V

VIN = 2.4V, V

and Efficiency vs. V

OUT

(mA) EFFICIENCY PERCENTAGE

OUT

= 3.3V

OUT

1.0 82.0

2.0 84.4

5.0 87.0

10.0 87.6

20.0 87.9

50.0 88.3

100.0 88.6

200.0 88.2

586.0 65.1

= 5.0V

OUT

1.0 84.4

2.0 87.0

5.0 87.7

10.0 88.4

20.0 88.9

50.0 89.1

100.0 88.9

200.0 87.5

485.0 71.6

IN

8

ML4871

PHYSICAL DIMENSIONS

0.017 - 0.027
(0.43 - 0.69)

(4 PLACES)

0.055 - 0.061

(1.40 - 1.55)

0.012 - 0.020

inches (millimeters)

Package: S08

0.189 - 0.199
(4.80 - 5.06)

8

PIN 1 ID

1

0.050 BSC
(1.27 BSC)

(0.30 - 0.51)

SEATING PLANE

0.148 - 0.158
(3.76 - 4.01)

0.059 - 0.069
(1.49 - 1.75)

8-Pin SOIC

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)

ORDERING INFORMATION

PART NUMBER OUTPUT VOLTAGE TEMPERATURE RANGE PACKAGE

ML4871CS-3 3.3V 0ºC to 70ºC 8-Pin SOIC (S08)
ML4871CS-5 5.0V 0ºC to 70ºC 8-Pin SOIC (S08)

ML4871ES-3 3.3V –20ºC to 70ºC 8-Pin SOIC (S08)
ML4871ES-5 5.0V –20ºC to 70ºC 8-Pin SOIC (S08)

© Micro Linear 1997. 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. Japan: 2,598,946;
2,619,299; 2,704,176. Other patents are pending.

Micro Linear reserves the right to make changes to any product herein to improve reliability, function or design. Micro Linear does not assume any liability
arising out of the application or use of any product described herein, neither does it convey any license under its patent right nor the rights of others. The circuits
contained in this data sheet are offered as possible applications only. Micro Linear makes no warranties or representations as to whether the illustrated circuits
infringe any intellectual property rights of others, and will accept no responsibility or liability for use of any application herein. The customer is urged to consult
with appropriate legal counsel before deciding on a particular application.

DS4871-01

2092 Concourse Drive

San Jose, CA 95131

Tel: [phone]

Fax: [phone]

www.microlinear.com

9