Bootstrap driver

L6390

20/25

DocID14493 Rev 11

The following equation is useful to compute the drop on the bootstrap DMOS:

Equation 3

where Qgate is the gate charge of the external power MOSFET, Rdson is the on-resistance of

the bootstrap DMOS and Tcharge is the charging time of the bootstrap capacitor.

For example: using a power MOSFET with a total gate charge of 30 nC, the drop on the

bootstrap DMOS is about 1 V, if the Tcharge is 5 s. In fact:

Equation 4

Vdrop should be taken into account when the voltage drop on CBOOT is calculated: if this drop

is too high, or the circuit topology doesn’t allow a sufficient charging time, an external diode

can be used.

Figure 11. Bootstrap driver

Vdrop

Ich e

arg Rdson

Vdrop



Qgate

Tch e

arg

------------------Rdson

==

Vdrop

30nC

5

s

--------------- 120

 0.7V





=