L6599AF datasheet(26/32 Pages) STMICROELECTRONICS

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Application information

L6599AF

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DocID028175 Rev 3

This concern applies to converters designed with a high resonance frequency (indicatively,

> 150 kHz), so that they run at high frequency also at full load. Otherwise, the converter runs

at high frequency at light load, where the current flowing in the MOSFETs of the half bridge

leg is low, so that, generally, an R(DS)ON rise is not an issue. However, it is wise to check this

point anyway and the following equation is useful to compute the drop on the bootstrap

driver:

Equation 13

where Qg is the gate charge of the external power MOSFET, R(DS)ON is the on-resistance of

the bootstrap DMOS (150 W, typ.) and Tcharge is the ON-time of the bootstrap driver, which

equals about half the switching period minus the deadtime TD. For example, using

a MOSFET with a total gate charge of 30 nC, the drop on the bootstrap driver is about 3 V at

a switching frequency of 200 kHz:

Equation 14

If a significant drop on the bootstrap driver is an issue, an external ultra-fast diode can be

used, therefore saving the drop on the R(DS)ON of the internal DMOS.

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arg

Drop

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150

Drop

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L6599AF Datasheet(PDF) 26 Page - STMicroelectronics