400EAGAB2 datasheet(34/47 Pages) ONSEMI

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NCP12400

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Clamped Driver

The supply voltage for the NCP12400 can be as high as

36 V, but most of the MOSFETs that will be connected to the

DRV pin cannot accept more than 20 V on their gate. The

driver pin is therefore safely clamped below 16 V. This

driver has a typical capability of 500 mA for source current

and 800 mA for sink current.

Current−Mode Control With Slope Compensation and

Soft−Start

NCP12400 is a current−mode controller, which means

that the FB voltage sets the peak current flowing in the

transformer primary inductance and the MOSFET. This is

done through a PWM comparator: the current is sensed

across a resistor and the resulting voltage is applied to the CS

pin. It is applied to one input of the PWM comparator

through a 250 ns LEB block. On the other input the FB

voltage subducted by offset typically 0.8 V and divided by

4 sets the threshold: when the voltage ramp reaches this

threshold, the output driver is turned off. The maximum

value for the current sense is 0.7 V, and it is set by a dedicated

comparator.

Each time the controller is starting, i.e. the controller was

off and starts – or restarts – when VCC reaches VCC(on), a

soft−start is applied: the current sense set−point is increased

by 32 discrete steps from 0 (the minimum level can be higher

than 0 because of the LEB and propagation delay) until it

reaches VILIM (after a duration of tSSTART), or until the FB

loop imposes a setpoint lower than the one imposed by the

soft−start (the 2 comparators outputs are OR’ed).

During the soft−start the oscillator frequency increase

from the minimum switching frequency to the maximum

switching frequency following the ramp applied to current

sense set−point.

Figure 64. Soft−Start Feature

Time

CS Setpoint

Soft−start ramp

VFB takes

over soft −start

tSSTART

Time

VFB

VILIM

KFB x VILIM

Time

OSC frequency

Soft−start ramp

tSSTART

VILIM

fSW

tSSTART

fSW,min

Time

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