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AN1673 APPLICATION NOTE

DEVICE OPERATION.

The device will start its function as the supply voltage reaches the UVLO threshold.

Before the supply threshold is O.K., both the external half bridge's Powermos are cut off thank to the low

impedance of the drivers. As the supply is valid, the circuit starts to run; the low side driver is active during

the first half period so that the boost capacitor will be fully charged.

frequency is set to the maximum value (Fmax) and will gradually decrease to the operative one during the

startup time.

The oscillator controls the power stage circuit by the Low and High side gate drivers, connected to the

trolled condition.

The significant current capability of both, the high and low side drivers (typically 450mA source and 250mA

sink), allows many different power MOS size to be driven maintaining fast switching transition. The internal

logic ensures a dead time between the gate turn off of the high/low side and the low/high side gate turn

on. This important feature allows to easily operating in zero voltage-switching mode, minimizing the tran-

sistor switching losses and the electromagnetic interference (see below in the soft switching section). The

built-in dead time is typical 300ns; the transition will be completed within this time.

The integrated Bootstrap function, allows avoiding the need of the external fast diode for charge the boot-

strap capacitor (required to supply the floating driver).

The internal operational amplifier can be useful in closed loop control or protection function.

SOFT START AND OSCILLATOR.

Figure 2. Soft start timing and Oscillator to drivers phase.

delivered to a trans conductive amplifier, as shown in fig. 2. Thus this voltage signal is converted in a grow-

quency during the soft start, is equal to:

[1]

D03IN1441

LVG

HVG

I

osc

Iƒ

min

Iƒ

start

gm V

CCS t

()

⋅

–

()

+

Iƒ

min

Iƒ

start

gm I

SS

⋅

C

SS

---------------------- t

–







+

==