Datasheet ML4870CS-3, ML4870CS-5, ML4870ES-3, ML4870ES-5 Datasheet (Micro Linear Corporation)

July 2000

PRELIMINARY

ML4870*

High Current Boost Regulator with Load Disconnect

GENERAL DESCRIPTION

The ML4870 is a continuous conduction boost regulator
designed for DC to DC conversion in multiple cell battery

FEATURES

■ Guaranteed full load start-up and operation at

1.8V input
power systems. Continuous conduction allows the
regulator to maximize output current for a given inductor.

■ Continuous conduction mode for high output current

The maximum switching frequency can exceed 200kHz,
allowing the use of small, low cost inductors. The ML4870
is capable of start-up with input voltages as low as 1.8V,

■ Pulse Frequency Modulation and internal synchronous

rectification for high efficiency
and is available in 5V and 3.3V output versions with an
output voltage accuracy of ±3%.

An integrated synchronous rectifier eliminates the need

■ Isolates the load from the input during shutdown

■ Minimum external components

for an external Schottky diode and provides a lower
forward voltage drop, resulting in higher conversion

■ Low ON resistance internal switching FETs

efficiency. In addtion, low quiescent current and variable
frequency operation result in high efficiency even at light

■ Low supply current

loads. The ML4870 requires only a few external
components to build a very small regulator capable of

■ 5V and 3.3V output versions

achieving conversion efficiencies approaching 85%.

The SHDN input allows the user to stop the regulator from
switching, and provides complete isolation of the load
from the battery. *Some Packages Are Obsolete

BLOCK DIAGRAM

V

IN

2

+

–

V

L1

SHDN

1

START-UP

CONTROL

BOOST

PWR GND

6

V

L2

SYNCHRONOUS

RECTIFIER
CONTROL

+

–

2.4V

8

GND

SHUTDOWN

CONTROL

+

–

3

SHDN

V

OUT

4

5

1

ML4870

PIN CONFIGURATION

ML4870

8-Pin SOIC (S08)

PIN DESCRIPTION

PIN NAME FUNCTION

1V

2V

L1

IN

3 GND Ground

Boost inductor connection

Battery input voltage

V

L1

V

IN

GND

SHDN

1

2

3

4

TOP VIEW

8

7

6

5

PIN NAME FUNCTION

5V

6V

PWR GND

NC

V

L2

V

OUT

OUT

L2

Boost regulator output

Boost inductor connection

7 NC No connection

4 SHDN Pulling this pin to VIN shuts down the

regulator, isolating the load from the
input

8 PWR GND Return for the NMOS output transistor

2

ML4870

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

ML4870CS-X .............................................. 0°C to 70°C

ML4870ES-X ........................................... -20°C to 70°C

VIN Operating Range .......................1.8V to V

OUT

- 0.2V

Junction Temperature ..............................................150°C

Storage Temperature Range ..................... –65°C to 150°C

Lead Temperature (Soldering, 10 sec) .....................260°C

Thermal Resistance (qJA).................................... 160°C/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

IN(Q)

I

OUT(Q)VOUT

PFM REGULATOR

VIN Quiescent Current VIN = V

VIN = SHDN = 2.4V, V

Quiescent Current SHDN = 0V 25 35 µA

V

= SHDN = 2.4V 14 20 µA

IN

V

OUT

- 0.2V, SHDN = 0V 3 6 µA

OUT

= 0V 0.3 1 µA

OUT

= V

OUT(NOM)

I

PEAKIL

V

OUT

SHUTDOWN

V

IL

V

IH

Note 1:

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

Peak Current 1.1 1.3 1.6 A

Output Voltage I

Load Regulation -3 Suffix, VIN = 2.4V, I

Input Low Voltage 0.5 V

Input High Voltage VIN - 0.5 V

Input Bias Current -100 100 nA

= 0 -3 Suffix 3.30 3.35 3.40 V

OUT

See Figure 1 -5 Suffix 4.95 5.05 5.15 V

= 400mA 3.20 3.25 3.40 V

OUT

-5 Suffix, VIN = 2.4V, I

= 220mA 4.85 4.95 5.15 V

OUT

3

ML4870

V

IN

100µF

27µH

(SUMIDA CD75)

ML4870

V

PWR GND

L1

V

IN

GND

SHDN

V

NC

V

OUT

L2

100µF

Figure 1. Application Test Circuit

I

L

I

OUT

Q1

PWR GND

6

V

L2

SYNCHRONOUS

RECTIFIER
CONTROL

8

A3

Q3

Q2

A2

+

–

2.4V

GND

+

–

3

SHUTDOWN

CONTROL

SHDN

V

OUT

4

V

OUT

5

1

V

L1

V

2

R

IN

SENSE

+

–

A1

SHDN

START-UP

CONTROL

BOOST

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

ML4870

FUNCTIONAL DESCRIPTION

The ML4870 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 P-channel
MOSFET to reduce losses, eliminate an external
component, and provide the means for load disconnect.
Also included on-chip are an N-channel MOSFET main
switch and current sense resistor.

REGULATOR OPERATION

The ML4870 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 including the key
external components is shown in Figure 2.

Error amp A3 converts deviations in the desired output
voltage to a small current, I
measured through a current sense resistor (R
is amplified by A1. The boost control block matches the
average inductor current to a multiple of the I
by switching Q1 on and off. The peak inductor current is
limited by the controller to about 1.3A.

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. When the circuit is not switching, only
25µA of supply current is drawn from the output. This
allows the part to remain efficient even when the load
current drops below 250µA.

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 VL2 to go high.
As the voltage on VL2 rises above V
allows 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 point is
independent of A2 and is determined by the boost
control circuitry.

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

SHUTDOWN

will momentarily reach zero after an

SET

. The inductor current is

SET

SENSE

SET

, amplifier A2

OUT

) which

current

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 conversion
efficiency. The maximum inductor current is limited by
the boost converter to about 1A. The conversion
efficiency is determined mainly by the internal switches
as well as the external components, but can be
estimated at about 80%. The maximum output current
can be estimated by using the typical performance
curves shown in Figures 4 and 5, or by calculation using
the following equations:

V

I

OUT V

I

OUT V

Since the maximum output current is based on when the
inductor current goes into current limit, it is not
recommended to operate the ML4870 at the maximum
output current continuously. Applications that have high
transient load currents should be evaluated under worst
case conditions to determine suitability.

INDUCTOR SELECTION

The ML4870 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 changes, 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 1µH 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 ML4870 under
worst case conditions to determine its suitability.

Several manufacturers supply standard inductance values
in surface mount packages:

0972

()

5

(. )

33



..




V



.

081




IN MIN

()

V

5

IN MIN

()

V

.

33



-






-




0144=

.

0144=

A

A

(1)

(2)

The ML4870 output can be shut down by pulling the
SHDN pin high (to VIN). When SHDN is high, the
regulator stops switching, the control circuitry is powered
down, and the body diode of the PMOS synchronous
rectifier is disconnected from the output. By switching
Q1, Q2, and Q3 off, the load is isolated from the input.
This allows the output voltage to be independent of the
input while in shutdown.

Coilcraft [phone]

Coiltronics [phone]

Dale [phone]

Sumida [phone]

5

ML4870

DEISGN 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

(3)

The output capacitor’s Equivalent Series Resistance (ESR)
and Equivalent Series Inductance (ESL), also contribute to
the ripple. Just after the Q1 turns off, the current in the

1000

800

600

(mA)

OUT

400

I

V

OUT

= 3.3V

V

OUT

= 5V

output capacitor ramps quickly to between 0.5A and

1.3A. 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 less than
100mW of ESR.

Suitable tantalum capacitors can be obtained from the
following vendors:

AVX [phone]

Kemet [phone]

Sprague [phone]

90

V

= 3.3V

OUT

V

= 5V

80

EFFICIENCY (%)

70

OUT

200

0

Figure 4. I

350

300

250

200

(nA)

IN

150

I

100

50

0

1.0 2.0 3.0 5.0

VIN (V)

vs. VIN Using the Circuit of Figure 8

OUT

1.0 3.0 5.0

VIN (V)

4.0

7.0

VIN = 2.4V

60

1 10 100 1000

Figure 5. Efficiency vs. I

80

60

40

(µA)

IN

V

I

20

0

1.0 2.0 3.0 5.0

OUT

= 3.3V

I

(mA)

OUT

Using the Circuit of Figure 8

OUT

V

= 5V

OUT

4.0

VIN (V)

Figure 6. Input Leakage vs. VIN in Shutdown Figure 7. No Load Input Current vs. V

6

IN

ML4870

DEISGN 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
ML4870 control circuitry, and also improves the
efficiency by reducing the I2R 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 ML4870 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.

SHUTDOWN

The input levels of the SHDN pin are CMOS compatible.
To guarantee proper operation, SHDN must be pulled to
within 0.5V of GND or VIN to prevent excessive power
dissipation and possible oscillations. A graph of input
leakage current while in shudown is shown in Figure 6.

LAYOUT

10µH

(SUMIDA CD75)

ML4870

V

PWR GND

V

IN

100µF

L1

V

IN

GND

SHDN

V

NC

V

OUT

L2

100µF

V

OUT

Figure 8. Design Example Schematic Diagram

DESIGN EXAMPLE

In order to design a boost converter using the ML4870,
it is necessary to define the values of a few parameters.
For this example, we have assumed that VIN = 3.0V to

3.6V, V

First, it must be determined whether the ML4870 is
capable of delivering the output current. This is done
using Equation 1:

= 5.0V, and I

OUT

OUT(MAX)

= 400mA

Good layout practices will ensure the proper operation of
the ML4870. Some layout guidelines follow:

• Use adequate ground and power traces or planes

• Keep components as close as possible to the ML4870

• Use short trace lengths from the inductor to the V

VL2 pins and from the output capacitor to the V

• Use a single point ground for the ML4870 PWR GND

pin and the input and output capacitors, and connect
the GND pin to PWR GND using a separate trace

• Separate the ground for the converter circuitry from the

ground of the load circuitry and connect at a single
point

L1

OUT

and

pin

V

.

30



I

OUT MAX()

Next, select an inductor:

As previously mentioned, it is 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 3:

C

OUT

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 8. As mentioned
previously, the use of an input supply bypass capacitor is
strongly recommended.

44 10

=

.



50

.

H

m

V



.=

-=0 972




50

=

88

0144 439




V

.

m

F

mA

7

ML4870

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)

0.004 - 0.010
(0.10 - 0.26)

8-Pin SOIC

0.228 - 0.244
(5.79 - 6.20)

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

ML4870CS-3 3.3V 0°C to 70°C 8-Pin SOIC (S08)
ML4870CS-5 5V 0°C to 70°C 8-Pin SOIC (S08)

ML4870ES-3 (Obsolete) 3.3V -20°C to 70°C 8-Pin SOIC (S08)

ML4870ES-5(Obsolete) 5V -20°C to 70°C 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.

8

DS4870-01

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San Jose, CA 95131

Tel: [phone]

Fax: [phone]

www.microlinear.com

11/24/97 Printed in U.S.A.