AN1673 APPLICATION NOTE

6/24

Figure 5. Bootstrap driver.

is obtained by means of an internal charge pump.

The diode connected in series to the DMOS has been added to avoid undesirable turn on of it. The intro-

is quickly turned off when the internal capacitor of the pump is not fully discharged.

low side driver is on), which increases with the frequency and with the size of the external power MOS. It

very small and can be neglected. Anyway increasing the frequency it must be taken in to account. In fact

power MOS (and so the dissipation).

To be considered that in resonant power supplies the current, which flows in the power, MOS decreases

er dissipation is negligible. The following equation is useful to compute the drop on the bootstrap driver:

[8]

switching frequency minus the dead time). The typical resistance value of the bootstrap DMOS is 150

Ohm. For example using a power MOS with a total gate charge of 30nC the drop on the bootstrap driver

is about 2.5V, at a switching frequency of 200kHz. In fact:

[9]

In practice, if a significant drop on the bootstrap driver (at high switching frequency when large power MOS

are used) represents a problem, an external diode can be used, avoiding the voltage reduction on the

ab

LVG

D02IN1390

V

drop

I

ch

e

arg

R

dson

V

diod

V

drop

Q

g

T

ch

e

arg

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

dson

V

diode

+

⋅⋅

⇒

+

⋅

=

V

drop

130nQ

2.3

µs

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

Ω 0.6V 2.6V

≅

+

⋅

=