Rainbow Electronics MAX1641 User Manual

General Description

________________

The MAX1640/MAX1641 CMOS, adjustable-output,
switch-mode current sources operate from a +5.5V to
+26V input, and are ideal for microprocessor-controlled
battery chargers. Charging current, maximum output
voltage, and pulse-trickle charge are programmed with
external resistors. Programming the off-time modifies
the switching frequency, suppressing undesirable har­monics in noise-sensitive circuits. The MAX1640’s high­side current sensing allows the load to connect directly
to ground, eliminating ground-potential errors. The
MAX1641 incorporates a low-side current sense.

The MAX1640/MAX1641 step-down pulse-width-modu­lation (PWM) controllers use an external P-channel
MOSFET switch and an optional, external N-channel
MOSFET synchronous rectifier for increased efficiency.
An internal low-dropout linear regulator provides power
for the internal reference and circuitry as well as the
gate drive for the N-channel synchronous rectifier.

The MAX1640/MAX1641 are available in space-saving,
16-pin narrow QSOP packages.

________________________Applications

Battery-Powered Equipment

Laptop, Notebook, and Palmtop Computers

Handy Terminals

Portable Consumer Products

Cordless Phones

Cellular Phones

PCS Phones

Backup Battery Charger

____________________________Features

♦ 95% Efficiency
♦ +5.5V to +26V Input Supply Range
♦ 2V to 24V Adjustable-Output Voltage Range
♦ 100% Maximum Duty Cycle (Low Dropout)
♦ Up to 500kHz PWM Operation
♦ Optional Synchronous Rectifier
♦ 16-Pin QSOP Package
♦ Current-Sense Accuracy: 2% (MAX1641),

5.3% (MAX1640)

MAX1640/MAX1641

Adjustable-Output, Switch-Mode

Current Sources with Synchronous Rectifier

________________________________________________________________

Maxim Integrated Products

1

Pin Configuration

19-1245; Rev 1; 1/02

PART

MAX1640C/D

MAX1640EEE
MAX1641C/D

0°C to +70°C

-40°C to +85°C

0°C to +70°C

TEMP. RANGE PIN-PACKAGE

Dice*

16 QSOP

Dice*

__________________

______________________

Ordering Information

*

Dice are specified at T

A

= +25°C, DC parameters only.

MAX1641EEE -40°C to +85°C 16 QSOP

_____________

Typical Operating Circuit

EVALUATION KIT

AVAILABLE

For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.

TOP VIEW

LDOL IN

1

TOFF

2

D1

3

MAX1640

4

D0

CC

REF

SET

TERM

5

6

7

8

MAX1641

16

15

14

13

12

11

10

9

LDOH

PDRV

NDRV

PGND

CS+

CS-

GND

V

= +5.5V TO +26V

IN

IN LDOH

D0

D1

R

TOFF

TOFF

REF

SET

MAX1640

CC

LDOLGND

PDRV

NDRV

PGND

CS+

CS-

TERM

OUT

P

QSOP

MAX1640/MAX1641

Adjustable-Output, Switch-Mode
Current Source with Synchronous Rectifier

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(V

IN

= +12V, V

OUT

= 6V, Circuit of Figure 2, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.

Note 1: This ratio is generated by a 1:8 clock divider and is not an error source for current calculations.

IN to GND...............................................................-0.3V to +28V

LDOH to IN...............................................................+0.3V to -6V

LDOL to GND...........................................................-0.3V to +6V

PDRV to GND .............................. (V

LDOH

- 0.3V) to (VIN+ 0.3V)

NDRV to GND.........................................-0.3V to (V

LDOL

+ 0.3V)

TOFF, REF, SET, TERM, CC to GND ......-0.3V to (V

LDOL

+ 0.3V)

D0, D1 to GND .........................................................-0.3V to +6V

CS+, CS- to GND ...................................................-0.3V to +28V

PGND to GND.....................................................................±0.3V

Continuous Power Dissipation (TA = +70°C)

QSOP (derate 8.30mW/°C above +70°C)................... 667mW

Operating Temperature Range

MAX164_EEE...................................................-40°C to +85°C

Storage Temperature Range.............................-65°C to +150°C

Lead Temperature (soldering, 10sec)............................ +300°C

I

REF

= 0 to 50µA

D0 = D1 = low

D0 = D1 = low (off mode)

D0 or D1 = high

V

OUT

= 2V to 24V

VIN= V

OUT

+ 0.5V to 26V

MAX1641

MAX1641

MAX1640

VIN= 5.5V to 26V, I

LOAD

= 0 to 20mA

VIN= 5.5V to 26V, I

LOAD

= 0 to 20mA

MAX1640

CONDITIONS

µA1V

SET

Input Current

mV4 10Reference Load Regulation

V1.96 2.00 2.04V

REF

Reference Voltage

V4.05 4.20 4.35V

LDOL

Undervoltage Lockout

µA1Output Current in Off Mode

500

mA2 4

Quiescent VINSupply Current

0.1

%/V

0.1 0.4

Output Current Compliance

%/V0.03Current-Sense Line Regulation

34 37.5 41

mV

36 42 48

147 150 153

mV

142 150 158

Full-Scale Current-Sense
Threshold

V

VIN- VIN- VIN-

5.5 5.0 4.5

V

LDOH

Linear-Regulator Output
Voltage, VINReferenced

V4.5 5.0 5.5V

LDOL

Linear-Regulator Output
Voltage, Ground Referenced

UNITSMIN TYP MAXSYMBOLPARAMETER

V5.5 26V

IN

Input Voltage Range

FET Drive Output Resistance PFET and NFET drive 12 Ω
Off-Time Range 1 10 µs
Off-Time Accuracy R

TOFF

= 62kΩ 1.7 2.2 2.7 µs

Pulse-Trickle Mode Duty-Cycle
Period

D0 = low, D1 = high, R

TOFF

= 100kΩ 27 33 40 ms

Pulse-Trickle Mode Duty Cycle
(Note 1)

D0 = low, D1 = high, R

TOFF

= 100kΩ 12.5 %

Quarter-Scale Current-Sense
Threshold

MAX1640
MAX1641

µA

MAX1640/MAX1641

Adjustable-Output, Switch-Mode

Current Source with Synchronous Rectifier

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS

(V

IN

= +12V, V

OUT

= 6V, Circuit of Figure 2, TA = -40°C to +85°C, unless otherwise noted.)

I

REF

= 0 to 50µA

V

OUT

= 2V to 24V (MAX1640)

D0 = D1 = low

D0 or D1 = high

FET Drive Output Resistance

MAX1641

MAX1640

VIN= 5.5V to 26V,
I

LOAD

= 0 to 20mA

VIN= 5.5V to 26V,
I

LOAD

= 0 to 20mA

MAX1640

CONDITIONS

µA1V

SET

Input Current

mV10Reference Load Regulation

V1.94 2.06V

REF

Reference Voltage

V4.0 4.4V

LDOL

Undervoltage Lockout

µA1Output Current in Off Mode

mA4Quiescent VINSupply Current

%/V0.4Output Current Compliance

mV

34 48

Quarter-Scale Current-Sense
Threshold

12 Ω

Off-Time Range

V5.5 26V

IN

Input Voltage Range

1.5 8 µs

Off-Time Accuracy R

TOFF

= 62kΩ 1.5 2.5 µs

146 154

Pulse-Trickle Mode Duty-Cycle
Period

mV

141 159

Full-Scale Current-Sense
Threshold

V

VIN- VIN-

5.5 4.5

V

LDOH

Linear-Regulator Output
Voltage, VINReferenced

D0 = low, D1 = high, R

TOFF

= 50kΩ 25 42 ms

V4.5 5.5V

LDOL

Linear-Regulator Output
Voltage, Ground Referenced

UNITSMIN TYP MAXSYMBOLPARAMETER

PWM Maximum Duty Cycle 100 %
Input Low Voltage V

IL

D0, D1 0.8 V

Input High Voltage V

IH

D0, D1 2.4 V

Input Leakage Current I

IN

D0, D1 ±1 µA

33 42MAX1641

ELECTRICAL CHARACTERISTICS (continued)

(V

IN

= +12V, V

OUT

= 6V, Circuit of Figure 2, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)

CONDITIONS UNITSMIN TYP MAXSYMBOLPARAMETER

PWM Maximum Duty Cycle 100 %
Input Low Voltage V

IL

D0, D1 0.8 V

Input High Voltage V

IH

D0, D1 2.4 V

Input Leakage Current I

IN

D0, D1 ±1 µA

MAX1640/MAX1641

Adjustable-Output, Switch-Mode
Current Source with Synchronous Rectifier

4 _______________________________________________________________________________________

0.45

0.51

0.49

0.47

0.53

0.55

0.57

0.59

0.61

0.63

0.65

4 128 16 20 24 28

OFF-MODE SUPPLY CURRENT

(NO-LOAD)

MAX1640/41-TOC07

INPUT VOLTAGE (V)

OFF-MODE SUPPLY CURRENT (mA)

TA = -40°C

TA = +25°C

TA = +85°C

10,000

1

0 50 100 150 200 250 300 350 400

SWITCHING FREQUENCY vs. R

TOFF

MAX1640/41 TOC 08

T

OFF

(kΩ)

SWITCHING FREQUENCY (kHz)

10

100

1000

V

OUT

= +3V

V

OUT

= +6V

__________________________________________Typical Operating Characteristics

(Circuit of Figure 2, TA = +25°C, unless otherwise noted.)

40

60

50

80

70

90

100

2 6 8 104 12 14 16 18 20 22 24

EFFICIENCY vs. OUTPUT VOLTAGE

MAX1640/41-TOC01

OUTPUT VOLTAGE (V)

EFFICIENCY (%)

VIN = 12V

VIN = 18V

VIN = 26V

1.460

1.470

1.480

1.490

1.500

1.510

4 128 16 20 24 28

MAX1640

OUTPUT CURRENT vs. INPUT VOLTAGE

MAX1640/41 TOC02

INPUT VOLTAGE (V)

OUTPUT CURRENT (A)

TA = -40°C

(V

OUT

= 4V)

TA = +25°C

TA = +85°C

1.450

1.470

1.460

1.490

1.480

1.500

1.510

2 6 8 104 12 14 16 18 20 22 24

MAX1640

OUTPUT CURRENT vs. OUTPUT VOLTAGE

MAX1640/41-TOC03

OUTPUT VOLTAGE (V)

OUTPUT CURRENT (A)

TA = -40°C

TA = +85°C

TA = +25°C

1.450

1.475

1.500

1.525

1.550

4 128 16 20 24 28

MAX1641

OUTPUT CURRENT vs. INPUT VOLTAGE

MAX1640/41 TOC04

INPUT VOLTAGE (V)

OUTPUT CURRENT (A)

TA = -40°C

(V

OUT

= 4V)

TA = +25°C

TA = +85°C

1.420

1.460

1.440

1.500

1.480

1.540

1.520

1.560

2 6 8 104 12 14 16 18 20 22 24

MAX1641

OUTPUT CURRENT vs. OUTPUT VOLTAGE

MAX1640/41-TOC05

V

OUT

(V)

OUTPUT CURRENT (A)

TA = -40°C

TA = +25°C

TA = +85°C

1.5

1.9

1.7

2.1

2.3

2.5

2.7

2.9

4 128 16 20 24 28

QUIESCENT CURRENT

vs. INPUT VOLTAGE (NO-LOAD)

MAX1640/41-TOC06

INPUT VOLTAGE (V)

QUIESCENT CURRENT (mA)

TA = -40°C

TA = +25°C

TA = +85°C

A: OUTPUT CURRENT, D1 = D0 = 1 1A/div

V

LOAD

= 3V

B: INPUT VOLTAGE, 10V/div

LINE-TRANSIENT RESPONSE

A

B

0A

0V

2ms/div

MAX1640/41 TOC 09

MAX1640/MAX1641

Adjustable-Output, Switch-Mode

Current Source with Synchronous Rectifier

_______________________________________________________________________________________ 5

____________________________Typical Operating Characteristics (continued)

(Circuit of Figure 2, TA = +25°C, unless otherwise noted.)

A: OUTPUT CURRENT, D0 = D1 = 0 1A/div

V

IN

= 12V, V

SET

= 1V, R

LOAD

= 4Ω, NO OUTPUT CAPACITOR

B: LOAD VOLTAGE, AC coupled, 500mV/div

CURRENT-MODE CHANGE RESPONSE TIME

A

B

2ms/div

0A

0V

MAX1640/41 TOC 10

B

A

EXITING OFF MODE

MAX1640/41 TOC11

A: D0 = D1 = 1 2V/div

V

IN

= 12V, R

LOAD

= 4Ω

B: OUTPUT CURRENT, 0.5A/div

20µs/div

______________________________________________________________Pin Description

NAME FUNCTION

1 LDOL

Internal, Ground-Referenced Low-Dropout Linear Regulator Output.
Bypass with a 0.1µF capacitor in parallel with a 4.7µF capacitor to GND.

2 TOFF

Off-Time Select Input. A resistor (R

TOFF

) connected from this pin to GND programs the off-time for the hys-

teretic PWM step-down converter. This resistor also sets the period in duty-cycle mode. See

Duty-Cycle

Mode and Programming the Off-Time.

PIN

3, 4 D1, D0 Digital Inputs. Select mode of operation (Table 1).

5 CC Constant-Current Loop Compensation Input. Bypass with a 0.01µF capacitor to GND.

8 TERM

Maximum Output Voltage Termination Input. When V

TERM

exceeds the reference voltage, the comparator

resets the internal PWM latch, shutting off the external P-channel FET.

7 SET

Current Select Input. Program the desired current level by applying a voltage at SET between 0V and V

REF

,

(I = V

SET

/ 13.3R

SENSE

). See Figure 3.

6 REF Reference Voltage Output (V

REF

= 2V). Bypass with a 0.1µF capacitor to GND.

13 NDRV Gate Drive for an Optional N-Channel FET Synchronous Rectifier

12 PGND High-Current Ground Return for the Output Drivers

11 CS+ Positive Current-Sense Comparator Input

10 CS- Negative Current-Sense Comparator Input

9 GND Ground

14 PDRV Gate Drive for the P-Channel FET

15 LDOH

Internal, Input-Referenced Low-Dropout Linear Regulator Output.
Bypass with a 0.33µF capacitor to IN.

16 IN Power-Supply Input. Input of the internal, low-dropout linear regulators.

MAX1640/MAX1641

Adjustable-Output, Switch-Mode
Current Source with Synchronous Rectifier

6 _______________________________________________________________________________________

MODE

CONTROL

CC TOFFD0, D1

TERM

REF

SET

CS-

CS+

REG

PDRV

NDRV

PGND

IN

LDOL

LDOH

A1

A2

Gm

MUX

B

A

SEL

MAX1640
MAX1641

Figure 1. MAX1640/MAX1641 Functional Diagram

MAX1640/MAX1641

Adjustable-Output, Switch-Mode

Current Source with Synchronous Rectifier

_______________________________________________________________________________________ 7

_______________Detailed Description

The MAX1640/MAX1641 switch-mode current sources
utilize a hysteretic, current-mode, step-down pulse­width-modulation (PWM) topology with constant off­time. Internal comparators control the switching
mechanism. These comparators monitor the current
through a sense resistor (R

SENSE

) and the voltage at
TERM. When inductor current reaches the current limit
[(V

CS+

- V

CS-

) / R

SENSE

], the P-channel FET turns off
and the N-channel FET synchronous rectifier turns on.
Inductor energy is delivered to the load as the current
ramps down. This ramp rate depends on R

TOFF

and
inductor values. When off-time expires, the P-channel
FET turns back on and the N-channel FET turns off.

Two digital inputs, D0 and D1, select between four pos­sible current levels (Table 1). In pulse-trickle mode, the

part operates for 12.5% of the period set by R

TOFF

,
resulting in a lower current for pulse-trickle charging.
Figure 1 is the MAX1640/MAX1641 functional diagram.
Figure 2 shows the standard application circuits.

Charge Mode: Programming the

Output Currents

The sense resistor, R

SENSE

, sets two charging current
levels. Choose between these two levels by holding
D0 high, and toggling D1 either high or low (Table 1).
The fast-charge current level equals VCS/ R

SENSE

where VCSis the full-scale current-sense voltage of
150mV. Alternatively, calculate this current by V

REF

/

(13.3R

SENSE

). The top-off current equals V

SET

/

(13.3R

SENSE

). A resistor-divider from REF to GND pro-

grams the voltage at SET (Figure 3).

P

N

0.1µF

4.7µF

47µF

0.33µF

0.01µF

0.1µF

R1

R

TOFF

R2

SET

REF

TOFF

D1

D0

LDOL

PDRV

NDRV

PGND

CS+

CS-

TERM

GNDCC

R3

47µH

V

OUT

BATT

1/2 IR7309

1/2 IR7309

R4

100mΩ

LDOHIN

MAX1641

Figure 2a. Standard Application Circuit Figure 2b. Standard Application Circuit

47µF

0.33µF

4.7µF

0.1µF

R1

R2

0.1µF

R

TOFF

0.01µF

LDOL

D0
D1

TOFF

REF

SET

LDOHIN

MAX1640

GNDCC

PDRV

NDRV

PGND

CS+

CS-

TERM

1/2 IR7309

R3

R4

1/2 IR7309

N

P

47µH

100mΩ

V

OUT

BATT

MAX1640/MAX1641

The voltage at SET is given by:

R1 = R2 (V

REF

/ V

SET

-1 ); 10kΩ < R2 < 300kΩ

where V

REF

= 2V and V

SET

is proportional to the

desired output current level.

The MAX1640/MAX1641 are specified for V

SET

between 0V and V

REF

. For V

SET

> V

REF

, output current
increases linearly (with reduced accuracy) until it
clamps at V

SET

≈ 4V.

Pulse-Trickle Mode: Selecting the

Pulse-Trickle Current

Pulling D0 low and D1 high selects pulse-trickle mode.
This current equals V

SET

/ (13.3R

SENSE

) and remains

on for 12.5% of the period set by R

TOFF

. Pulse-trickle
current maintains full charge across the battery and
can slowly charge a cold battery before fast charging
commences.

Off Mode: Turning Off the Output Current

Pulling D0 and D1 low turns off the P-channel FET and
hence the output current flow. This mode also controls
end of charge and protects the battery against exces­sive temperatures.

Setting the Maximum Output

Voltage Level

The maximum output voltage should be programmed to
a level higher than the output/battery voltage (I

LOAD

x

R

LOAD

). An external resistor-divider between the output
and ground (Figure 4) sets the voltage at TERM. Once
the voltage at TERM exceeds the reference, the internal
comparator turns off the P-channel FET, terminating
current flow. Select R4 in the 10kΩ to 500kΩ range.
R3 is given by:

R3 = R4 (V

OUT

/ V

TERM

) -1

PERIOD = 3.2 x 10 x R (sec)

-7
TOFF

Adjustable-Output, Switch-Mode
Current Source with Synchronous Rectifier

8 _______________________________________________________________________________________

D1 DO MODE

OUTPUT CURRENT (A)

0 0 OFF 0
0 1 Top-Off V

SET

/ (13.3R

SENSE

)

1 0 Pulse-Trickle

V

SET

/ (13.3R

SENSE

)

12.5% duty cycle

1 1 Fast Charge V

REF

/ (13.3R

SENSE

)

Table 1. Selecting Output Current Levels

MAX1640
MAX1641

REF

SET

R1

R2

Figure 3. Adjusting the Output Current Level

MAX1640

CS+

TERM

CS-

R3

R

SENSE

BATT

L

R4

Figure 4a. Setting the Maximum Output Voltage Level

MAX1641

CS+

TERM

CS-

R3

BATT

R4

R

SENSE

L

Figure 4b. Setting the Maximum Output Voltage Level

where V

TERM

= 2V and V

OUT

is the desired output

voltage.

Programming the Off-Time

When programming the off-time, consider such factors
as maximum inductor current ripple, maximum output
voltage, inductor value, and inductor current rating. The
output current ripple is less than the inductor current rip­ple and depends heavily on the output capacitor’s size.

Perform the following steps to program the off-time:

1) Select the maximum output current ripple. IR(A)

2) Select the maximum output voltage. V

OUT

(MAX)(V)

3) Calculate the inductor value range as follows:
L

MIN

= (V

OUTMAX

x 1µs) / I

R

L

MAX

= (V

OUTMAX

x 10µs) / I

R

4) Select an inductor value in this range.

5) Calculate t

OFF

as follows:

6) Program t

OFF

by selecting R

TOFF

from:

R

TOFF

= (29.3 x 109) x t

OFF

7) Calculate the switching frequency by:

fs = 1 / (tON+ t

OFF

)

where tON= (IRx L) / (VIN- V

OUT

) and IR= (V

OUT

x

t

OFF

) / L. L is the inductor value, VINis the input volt-

age, V

OUT

is the output voltage, and IRis the output

peak-to-peak current ripple.

Note that R

TOFF

sets both the off-time and the pulse-

trickle charge period.

Reference

The on-chip reference is laser trimmed for a precise 2V
at REF. REF can source no more than 50µA. Bypass
REF with a 0.1µF capacitor to ground.

Constant-Current Loop: AC Loop

Compensation

The constant-current loop’s output is brought out at CC.
To reduce noise due to variations in switching currents,
bypass CC with a 1nF to 100nF capacitor to ground. A
large capacitor value maintains a constant average out­put current but slows the loop response to changes in
switching current. A small capacitor value speeds up
the loop response to changes in switching current,

generating increased ripple at the output. Select C

CC

to optimize the ripple vs. loop response.

Synchronous Rectification

Synchronous rectification reduces conduction losses in
the rectifier by shunting the Schottky diode with a low­resistance MOSFET switch. In turn, efficiency increases
by about 3% to 5% at heavy loads. To prevent cross­conduction or “shoot-through,” the synchronous rectifier
turns on shortly after the P-channel power MOSFET

turns off. The synchronous rectifier remains off for 90%
of the off-time. In low-cost designs, the synchronous
rectifier FET may be replaced by a Schottky diode.

Component Selection

External Switching Transistors

The MAX1640/MAX1641 drive an enhancement-mode
P-channel MOSFET and a synchronous-rectifier N­channel MOSFET (Table 2).

When selecting a P-channel FET, some important para­meters to consider are on-resistance (r

DS(ON)

), maxi­mum drain-to-source voltage (VDSmax), maximum
gate-to-source voltage (VGSmax), and minimum
threshold voltage (V

TH

min).

In high-current applications, MOSFET package power
dissipation often becomes a dominant design factor.
I2R power losses are the greatest heat contributor for
both high-side and low-side MOSFETs. Switching loss­es affect the upper MOSFET only (P-channel), since the
Schottky rectifier or the N-FET body diode clamps the
switching node before the synchronous rectifier turns on.

Rectifier Diode

If an N-channel MOSFET synchronous rectifier is not
used, a Schottky rectifier is needed. The MAX1640/

t =

L x I

V

OFF

R

OUTMAX

MAX1640/MAX1641

Adjustable-Output, Switch-Mode

Current Source with Synchronous Rectifier

_______________________________________________________________________________________ 9

Table 2. Component Manufacturers

COMPONENT MANUFACTURER

MOSFETs

Sense Resistor

Capacitors

International Rectifier

Dale

AVX

Sumida
Coilcraft

Inductor

Coiltronics

Siliconix S14539DY

IRF7309

WSL-2010 series

TPS series

MBAR5340t3

CDRH125 series
D03316P series
UP2 series

IRC LR2010-01 series

Sprague 595D series

Nihon NSQ03A04

Rectifier

IN5817-IN5822

Motorola

MAX1640/MAX1641

MAX1641’s high switching frequency demands a high­speed rectifier (Table 2). Schottky diodes such as the
1N5817–1N5822 are recommended. Make sure the
Schottky diode’s average current rating exceeds the
peak current limit and that its breakdown voltage
exceeds the output voltage (V

OUT

). For high-tempera­ture applications, Schottky diodes may be inadequate
due to their high leakage current; high-speed silicon
diodes such as the MUR105 or EC11FS1 can be used
instead. At heavy loads and high temperatures, the
benefits of a Schottky diode’s low forward voltage may
outweigh the disadvantage of high leakage current. If
the application uses an N-channel MOSFET synchro­nous rectifier, a parallel Schottky diode is usually
unnecessary except with very high charge current (> 3
amps). Best efficiency is achieved with both an
N-channel MOSFET and a Schottky diode.

Inductor Value

Refer to the section

Programming the Off-Time

to select

the proper inductor value. There is a trade-off between

inductor value, off-time, output current ripple, and
switching frequency.

__________Applications Information

All-Purpose Microcontroller Battery

Charger: NiCd, NiMH

In applications where a microcontroller is available, the
MAX1640/MAX1641 can be used as a low-cost battery
charger (Figure 5). The controller takes over fast
charge, pulse-trickle charge, charge termination, and
other smart functions. By monitoring the output voltage
at V

OUT

, the controller initiates fast charge (set D0 and
D1 high), terminates fast charge and initiates top-off
(set D0 high and D1 low), enters trickle charge (set D0
low and D1 high), or shuts off and terminates current
flow (set D0 and D1 low).

Layout and Grounding

Due to high current levels and fast switching wave­forms, proper PC board layout is essential. High-cur­rent ground paths should be connected in a star

Adjustable-Output, Switch-Mode
Current Source with Synchronous Rectifier

10 ______________________________________________________________________________________

P

N

D1

D0

T

I/0

I/0

CH1

CH0

PDRV

NDRV

PGND

CS+

CS-

R3

R4

TERM

GND

LOW-SIDE IS SHORTED

DC IN

R

SENSE

MAX1640

BATT

Figure 5. Microcontroller Battery Charger

configuration to PGND. These traces should be wide to
reduce resistance and as short as possible to reduce
stray inductance. All low-current ground paths should
be connected to GND. Place the input bypass capaci­tor as close as possible to the IN pin. See MAX1640 EV
kit for layout example.

MAX1640/MAX1641

Adjustable-Output, Switch-Mode

Current Source with Synchronous Rectifier

___________________Chip Information

TRANSISTOR COUNT: 1233

QSOP.EPS

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 _____________________ 11

© 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

Package Information

(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages.)