Block diagram description

L6598

14/24

Doc ID 6554 Rev 7

The bootstrap driver introduces a voltage drop during the recharging of the capacitor Cboot

(i.e. when the low side driver is on), which increases with the frequency and with the size of

threshold voltage. At low frequency this drop is very small and can be neglected. Anyway

increasing the frequency it must be taken in to account. In fact the drop, reducing the

MOS (and so the dissipation).

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

problem because power dissipation is negligible. The following equation is useful to

compute the drop on the bootstrap driver:

Equation 8

semi-period of the switching frequency minus the dead time). The typical resistance value of

the bootstrap DMOS is 150

Ω. For example using a power MOS 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. In

fact:

Equation 9

To summaries, 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

5.4

Op amp section

The integrated op amp is designed to offer low output impedance, wide band, high input

impedance and wide common mode range. It can be readily used to implement protection

features or a closed loop control. For this purpose the op amp output can be properly

V

drop

I

ch

e

V

diode

V

drop

→

+

Q

g

T

ch

e

arg

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

dson

V

diode

+

==

V

drop

30nC

2.23

μs

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

Ω 0.6V 2.6V

∼

+

=