L6599 datasheet(30/36 Pages) STMICROELECTRONICS

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Application information

L6599

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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 will

run at high frequency only at light load, where the current flowing in the MOSFETs of the

half-bridge leg is lower, 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:

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

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

equals about half the switching period minus the dead time TD. For example, using a

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

switching frequency of 200kHz:

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

used, thus saving the drop on the r(DS)ON of the internal DMOS.

Drop

arg r DS

()ON

arg

--------------------R

()ON

Drop

–

⋅

2.5

–

0.3

–

⋅

–

⋅

-------------------------------------------------------------150

0.6

2.7V

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