Rectifier Diode

If an N-channel MOSFET synchronous rectifier is not

used, a Schottky rectifier is needed. The MAX1640/

MAX1641’s high switching frequency demands a high-

speed 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

ture 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 synchro-

nous 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 4). The controller takes over fast

charge, pulse-trickle charge, charge termination, and

other smart functions. By monitoring the output voltage

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).

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

MAX1640

BATT

Figure 4. Microcontroller Battery Charger