Datasheet

DocID6554 Rev 8 13/23
L6598 Block diagram description
23
5.3 Bootstrap section
The supply of the high voltage section is obtained by means of a bootstrap circuitry. This
solution normally requires a high voltage fast recovery diode for charging the bootstrap
capacitor (Figure 13 - part a). In the device a patented integrated structure replaces this
external diode. It is released by means of a high voltage DMOS, driven synchronously with
the low side driver (LVG), with in series a diode, as shown in Figure 13 - part b.
Figure 13. Bootstrap driver
To drive the synchronized DMOS it is necessary a voltage higher than the supply voltage
Vs. This voltage is obtained by means of an internal charge pump (Figure 13 - part b).
The diode connected in series to the DMOS has been added to avoid undesirable turn on of
it. The introduction of the diode prevents any current can flow from the Vboot pin to the VS
one in case that the supply is quickly turned off when the internal capacitor of the pump is
not fully discharged.
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
the external power MOS. It is the sum of the drop across the R
DSON
and of the diode
Figure 12. Typ. (f
start
-f
min
) vs. R
fstar
at C
f
= 470 pF