STCMB1 datasheet(49/59 Pages) STMICROELECTRONICS

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STCMB1

Application information

The bootstrap structure introduces a voltage drop while recharging CBOOT (e.g.: when the

low-side driver is on), which increases with the operating frequency and with the size of the

external power MOS. It is the sum of the drop across the R(DS)ON and the forward drop

across the series diode. At low frequency this drop is very small and can be neglected but,

as the operating frequency increases, it must be taken into account. In fact, the drop

reduces the amplitude of the driving signal and can significantly increase the R(DS)ON of the

external high-side MOSFET and then its conductive loss.

Figure 30. Bootstrap supply: a) standard circuit; b) internal bootstrap synchronous

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

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

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

half-bridge leg is low, so that, generally, a slight R(DS)ON rise is not an issue. However, it is

wise to check this point anyway and Equation 30 is useful to estimate the drop on the

bootstrap driver.

Equation 30

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

the bootstrap DMOS (230

, typ.) and Tcharge is the on-time of the bootstrap driver, which

equals about a half 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 31

A typical range of the bootstrap capacitor (Cboot) is from 100 to 470 nF.

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

used, thus saving the drop on the R(DS)ON of the internal DMOS. In this case it is also

recommended to add a small resistor (in the ten

) in series to the diode to prevent the

D S

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c h a r g e

D S

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c h a r g e

D r o p

3 . 7

0 . 6

2 3 0

1 0

0 . 3

1 0

2 . 5

1 0

3 0

D r o p

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