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 rip-

ple and depends heavily on the output capacitor’s size.

Perform the following steps to program the off-time:

3) Calculate the inductor value range as follows:

4) Select an inductor value in this range.

7) Calculate the switching frequency by:

peak-to-peak current ripple.

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 out-

put 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,

to optimize the ripple vs. loop response.

Synchronous Rectification

Synchronous rectification reduces conduction losses in

the rectifier by shunting the Schottky diode with a low-

resistance MOSFET switch. In turn, efficiency increases

by about 3% to 5% at heavy loads. To prevent cross-

conduction 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 N-

channel MOSFET (Table 2).

When selecting a P-channel FET, some important para-

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 loss-

es 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.

t=

L x I

V

OFF

R

OUTMAX

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