The MAX8540/MAX8541 also synchronize with an

external oscillator. Drive FREQ/SYNC with a square

wave through a series diode with a positive pulse width

of at least 200ns and a minimum pulse amplitude of 3V,

level-shifted external clock can be applied without the

series diode. The maximum duty cycle of the external

signal allowed is 55%. The MAX8540/MAX8541 syn-

chronize to frequencies between 200kHz and 1MHz;

however, the signal must be within

±30% of the fre-

quency set by the external resistor at FREQ/SYNC.

Maximum Duty Cycle

Set the maximum duty cycle at the minimum system

MAXDTY to GND. The maximum duty cycle is inversely

proportional to the voltage at UV. As the voltage on UV

increases, the duty cycle decreases. The maximum

duty cycle is internally limited to 80% at all switching

frequencies. The MAXDTY resistor is determined as:

24.3k

Ω to 130kΩ.

N-Channel MOSFET Driver

The DRV output drives an N-channel MOSFET in low-

power applications. In high-power applications, the gate

driver internal to the MAX8540/MAX8541 may not be

capable of driving the external MOSFET efficiently and

an external gate driver may be required. In this situation,

connect DRV to the input of the external gate driver.

Slope Compensation (MAX8540)

The MAX8540 is a current-mode device and requires

slope compensation for proper operation. To provide

slope compensation, connect a resistor from SCOMP to

follows.

For applications using a synchronous rectifier in the

output, set the slope compensation equal to the nega-

Ω, N is the

turns ratio of the primary to secondary, L1 is the output

For applications where a diode is used in the output

instead of the synchronous rectifier, the slope compen-

sation resistor is then equal to:

Ω, N is the turns

ratio of the primary to secondary, L1 is the output induc-

sense resistance.

Voltage-Ramp Amplitude (MAX8541)

The MAX8541 is a voltage-mode device and features

adjustable voltage ramp. Connect a resistor from

VV

k

R

M

PRAMP

= 125

25

.

Ω

R

dV

dt

R

SCF

SCOMP

RAMP

SUM

=×

2

SCF

N

VV

L

R

OUT

F

CS

=

+

1

R

dV

dt

R

SCF

SCOMP

RAMP

SUM

=×

2

SCF

N

V

L

R

OUT

CS

=

1

R

D

MAXDTY

MAX

=

××

(.

)

97 6

10

60

3

Synchronizable, High-Frequency Current- and

Voltage-Mode PWM Controllers for Isolated Supplies

10

______________________________________________________________________________________

C1

C2

C3

RA

RB

RC

36V TO 76V

MAX8541

OV

UV

Figure 1. A Method to Set UV and OV Thresholds