Ω. The ramp-voltage magnitude is

independent of frequency. The range of values for

Ω to 42kΩ.

Soft-Start

The soft-start feature allows converters built using the

MAX8540/MAX8541 to apply power to the load in a

controllable soft ramp, thus reducing startup surges

and stresses. It also determines power-up sequencing

when several converters are used.

Upon power turn-on, the soft-start pin acts as a current

sink to discharge any capacitance connected to it. Once

the converter output voltage to ramp up. Full output volt-

age is reached in approximately 440ms/µF.

Current Limit

The MAX8540/MAX8541 utilize two current-limit

schemes: cycle-by-cycle current limit and short-circuit

current limit. Set the current-limit threshold using a

resistor-divider from REF to GND with ILIM connected to

the center. The current-limit threshold is determined as:

where R5 is the resistor from ILIM to GND and R6 is the

resistor from REF to ILIM. Use 240k

Ω for R16 and vary

R23 to change the threshold.

The CS signal provides feedback on the current ramp

through the main external MOSFET. The voltage on CS

is monitored by the IC. The cycle-by-cycle current limit

abbreviates the on-time of the external MOSFET in the

event that the voltage at CS is greater than the thresh-

old voltage set by ILIM.

The current-limit feature protects against a hard short

or overcurrent fault at the output by one of two selected

protection modes: by latching off the output, or pulsing

the output to reduce the average output current

(hiccup mode).

To select latched mode, connect SKTOFF to REF. In

this mode, if the hard short exists for the time period set

by the capacitance at SKTON, the output is latched off.

To unlatch the output, toggle EN or cycle the input

To select hiccup mode, connect capacitors to SKTON

and SKTOFF to program the hiccup mode on- and off-

times. When a cycle-by-cycle event is detected, the IC

charges the capacitor at SKTON. The capacitor contin-

ues to charge as long as the CS voltage is greater than

the ILIM threshold voltage. Once the voltage on SKTON

reaches its threshold voltage, the MAX8540/MAX8541

begin skipping switching cycles for a time determined

by the capacitance connected to SKTOFF. Once this

time period has elapsed, the IC begins to switch for the

time period set by the capacitance connected to

SKTON. This process continues until the output short is

removed. See the SKTON and SKTOFF section for

details on setting the hiccup-mode periods.

Connect SKTOFF to REF and SKTON to GND to disable

the latched-mode and hiccup-mode protection, and

operate continuously in cycle-by-cycle current-limit.

PMW Comparator

The PWM comparator of the MAX8540 transforms the

comparing the opto feedback-error voltage with a

summed voltage. The summed voltage is the sum of

the programmable slope compensation and the cur-

rent-sense voltages. When the summed voltage

exceeds the opto feedback error voltage, the gate-

drive logic turns off the external MOSFET.

The PWM comparator in the MAX8541 compares

turns off the external MOSFET.

SKTON and SKTOFF

allowed before the short-circuit current limit initiates.

Once the CS voltage exceeds the ILIM threshold, the

capacitor at SKTON begins to charge. The capacitor

continues to charge until the SKTON threshold voltage

is reached or the overcurrent event is removed. This

feature allows for the higher currents required during

allow sufficient time for startup. The required capaci-

tance at SKTON is determined as:

The capacitance at SKTOFF determines the time period

that the external MOSFET is turned off during an over-

current event. Once the SKTON time period is exceeded,

determined as:

V

R

RR

V

ILIM

REF

=

+

×

5

56

Synchronizable, High-Frequency Current- and

Voltage-Mode PWM Controllers for Isolated Supplies

______________________________________________________________________________________________________

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