MAX8541EEE datasheet(10/16 Pages) MAXIM

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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,

plus the VF of the external diode. Alternatively, a 1.5V

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

input voltage (VIN(MIN)) connecting a resistor from

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:

where DMAX is the desired maximum duty cycle.

The range of valid resistor values for RMAXDTY is from

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

GND (RSCOMP). The value of RSCOMP is determined as

follows.

For applications using a synchronous rectifier in the

output, set the slope compensation equal to the nega-

tive slope of the output inductor. RSCOMP is equal to:

where dVRAMP/dt = 2.5V(fS), RSUM = 25k

Ω, N is the

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

inductance, VOUT is the output voltage, fS is the switch-

ing frequency, and RCS is the current-sense resistance.

For applications where a diode is used in the output

instead of the synchronous rectifier, the slope compen-

sation resistor is then equal to:

where dVRAMP/dt = 2.5V(fS),RSUM = 25k

Ω, N is the turns

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

tance, VOUT is the output voltage, VF is the diode voltage,

fS is the switching frequency, and RCS is the current-

sense resistance.

Voltage-Ramp Amplitude (MAX8541)

The MAX8541 is a voltage-mode device and features

adjustable voltage ramp. Connect a resistor from

PRAMP to GND (RPRAMP) to set the voltage-ramp

magnitude, VM.

PRAMP

= 125

Ω

SCF

SCOMP

RAMP

SUM

=×

SCF

OUT

SCF

SCOMP

RAMP

SUM

=×

SCF

OUT

MAXDTY

MAX

××

)

97 6

Synchronizable, High-Frequency Current- and

Voltage-Mode PWM Controllers for Isolated Supplies

______________________________________________________________________________________

VIN

36V TO 76V

MAX8541

Figure 1. A Method to Set UV and OV Thresholds

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