L6699D datasheet(33/38 Pages) STMICROELECTRONICS

33 / 38 page

L6699

Bootstrap section

Doc ID 022835 Rev 2

33/38

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, a slight RDS(on) rise is not an issue. However, it is wise to check

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

driver:

Equation 14

where Qg is the gate charge of the external Power MOSFET, RDS(on) is the ON-resistance of

the bootstrap DMOS (150

Ω, 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 and with a deadtime of 300 ns:

Equation 15

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 RDS(on) of the internal DMOS. In this case it is also

recommended to add a small resistor (in the ten

Ω) in series to the diode to prevent the

bootstrap capacitor from being charged to a voltage exceeding the absolute maximum rating

(18 V), in case of significant negative spikes on the half bridge midpoint when the high-side

MOSFET turns off.

Figure 23.

Bootstrap supply: standard

circuit

Figure 24.

Bootstrap supply: internal

bootstrap synchronous diode

)

(

arg

)

(

arg

Drop

150

Drop

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