Synchronizable, High-Frequency Current- and

Voltage-Mode PWM Controllers for Isolated Supplies

8

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Pin Description

PIN

MAX8540

MAX8541

NAME

FUNCTION

1

1

REF

5V Reference Voltage Output. Bypass to ground with a 1µF capacitor. REF is pulled to GND

during shutdown.

22

OV

System Input Overvoltage Monitoring. The OV threshold is 3.021V with 130mV hysteresis.

Connect to the center of a resistor-divider from the system input to GND to set the overvoltage

trip point (see the Overvoltage Threshold section).

33

UV

System Input Undervoltage Monitoring and Feed-Forward Input. The UV threshold is 1.25V with

120mV hysteresis. Connect to the center of a resistor-divider from the system input to GND to

set the undervoltage trip point (see the Undervoltage Threshold section). This input can be used

as a disable input by pulling it below 1.2V. UV remains active during shutdown.

44

FREQ/

SYNC

Programmable Frequency and Sync Input. Connect a resistor in parallel with RC (10k

Ω, 0.01µF) from

FREQ/SYNC to GND to set the switching frequency or apply an external clock signal through a diode

to synchronize to an external frequency.

55

SS

Soft-Start Capacitor Connection. Connect a capacitor from SS to GND to set the soft-start time period.

6

6

MAXDTY

Programmable Maximum Duty-Cycle Input. Connect a resistor from MAXDTY to GND to set the

maximum duty cycle for minimum system input voltages. The maximum duty cycle is inversely

proportional to the system input voltage. The higher the input voltage, the less the maximum

duty cycle. The maximum duty cycle is internally limited to 80% at any switching frequency.

MAXDTY is high impedance during shutdown. Bypass MAXDTY with a 2200pF capacitor.

77

EN

Active-Low Enable Input. Drive EN to GND to enable the IC. Drive high to place the IC in

shutdown mode.

8

—

SCOMP

Programmable Slope Compensation Input. Connect a resistor from SCOMP to GND to set the

slope compensation.

—

8

PRAMP

Programmable Voltage-Mode Control-Ramp Input. Connect a resistor from PRAMP to GND to

set the magnitude of the voltage ramp.

9

9

OPTO

Isolated Feedback Input. Connect to an optocoupler for an isolated power converter or connect to the

output of a voltage-error amplifier for a nonisolated power converter for a feedback-error signal.

10

10

ILIM

Current-Limit Threshold Input. Connect to the center of a resistor-divider from REF to GND to set

the current-limit threshold voltage.

11

11

SKTOFF

Current-Limit Off-Time Input. Connect a capacitor from SKTOFF to GND to set the current-limit off-time.

Pull SKTOFF to REF through a 10k

Ω resistor to latch off the IC after an overcurrent event.

12

12

CS

Current-Sense Input. Provides current-sense feedback for cycle-by-cycle current limit and is

also the current-mode input for the MAX8540. Connect a current-sense resistor from CS to GND.

13

13

SKTON

Current-Limit On-Time Input. Connect a capacitor to ground to set the current-limit on-time.

14

14

GND

Ground

15

15

DRV

Gate Drive for External N-Channel Power MOSFET. Connect to the gate of an external MOSFET

for low-power applications. Connect to the input of an external gate driver for high-power

applications.

16

16

ceramic capacitor to GND.