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MAX17598/MAX17599

Current-Mode PWM Controllers

n-Channel MOSFET Gate Driver

The NDRV output drives an external n-channel MOSFET.

NDRV can source/sink in excess of 900mA /1500mA

peak current. Therefore, select a MOSFET that yields

acceptable conduction and switching losses.

p-Channel MOSFET Gate Driver

The AUXDRV output drives an external p-channel

MOSFET with the aid of a level shifter, as shown in the

Typical Application Circuits. AUXDRV can source/sink in

excess of 300mA/600mA peak current. Therefore, select

a MOSFET that yields acceptable conduction and switch-

ing losses. The external PMOSFET used must be able to

withstand the maximum clamp voltage.

Dead Time

Dead time between the main and AUX output edges allow

ZVS to occur, minimizing switching losses and improving

and trailing edges of the main and AUX outputs as shown

in Figure 7. Connect a resistor between DT and GND to

DT

DT

10k

R

(t

)

25ns

W

=

×

Oscillator/Switching Frequency

The ICs’ switching frequency is programmable between

RT and GND. Use the following formula to determine the

10

RT

SW

1 10

R

f

×

=

Peak-Current-Limit

source of the n-channel MOSFET and PGND, sets the

current limit. The source end of the current-sense resis-

tor connects to the CS pin of the MAX17598/MAX17599.

The signal thus obtained is used by the devices,

both for current-mode control and peak-current limiting

purposes. The current-limit comparator has a voltage trip

compensation applied to stabilize the converter. The

CS

PRI_PEAK

300mV

R

1.2 I

=

×

of the transformer, which also flows through the main

n-channel MOSFET. When the voltage produced by this

current (through the current-sense resistor) exceeds the

current-limit comparator threshold, the MOSFET driver

(NDRV) terminates the current on-cycle within 40ns (typ).

The devices implement 70ns of internal leading-edge

blanking to ignore leading-edge current spikes encoun-

tered in practice due to parasitics. Use a small RC

network for additional filtering of the leading-edge spike

on the sense waveform when needed. Set the corner

frequency of the RC filter network at 5 to 10 times the

switching frequency.

For a given peak-current-limit setting, the runaway cur-

rent limit is typically 20% higher. The peak current-limit-

triggered hiccup operation is disabled until the end of

soft-start, while the runaway current-limit-triggered hiccup

operation is always enabled.

Negative Peak Current Limit

The MAX17598/MAX17599 protect against excessive

negative currents through the clamp switch, primary of

the transformer and the clamp capacitor under dynamic

operating conditions where the converter is not in steady

state. The devices limit negative current by monitoring

and the p-Channel FET is on. The typical negative-cur-

rent-limit threshold is set at -100mV (1/3 of the positive-

peak-current-limit threshold).

Figure 7. Dead Time Between AUXDRV and NDRV

AUXDRV

NDRV