34713 datasheet(17/21 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34713

TYPICAL APPLICATIONS

PROTECTION AND DIAGNOSTIC FEATURES

Equating the Pole 2 at Crossover Frequency to achieve a

faster response and a proper phase margin,

BOOTSTRAP CAPACITOR

The bootstrap capacitor is needed to supply the gate

voltage for the high side MOSFET. This N-Channel MOSFET

needs a voltage difference between its gate and source to be

able to turn on. The high side MOSFET source is the SW

node, so it is not ground and it is floating and moving in

voltage, so we cannot just apply a voltage directly to the gate

of the high side that is referenced to ground, we need a

voltage referenced to the SW node. That is why the bootstrap

capacitor is needed for. This capacitor charges during the

high side off time, since the low side will be on during that

time, so the SW node and the bottom of the bootstrap

capacitor will be connected to ground and the top of the

capacitor will be connected to a voltage source, so the

capacitor will charge up to that voltage source (say 5V). Now

when the low side MOSFET switches off and the high side

MOSFET switches on, the SW nodes rises up to Vin, and the

voltage on the boot pin will be Vcap + Vin. So the gate of the

high side will have Vcap across it and it will be able to stay

enhanced. A 0.1

µF capacitor is a good value for this

bootstrap element.

SOFT START SELECTION

Table 6 shows the voltage that should be applied to the

terminal ILIM to get the desired configuration of the soft start

timing.

⇒

Table 6. ILIM Table

Soft Start (ms)

Voltage Applied to ILIM

3.2

1.25 - 1.49

1.6

1.50 - 1.81

0.8

1.82 - 2.13

0.4

2.14 - 2.50

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