DocID030317 Rev 2 13/18
L6494 Bootstrap driver
18
7 Bootstrap driver
A bootstrap circuitry is needed to supply the high voltage section. This function is usually
accomplished by a high voltage fast recovery diode (Figure 8). In the L6494 an integrated
structure replaces the external diode.
C
BOOT
selection and charging
To choose the proper C
BOOT
value the external MOS can be seen as an equivalent
capacitor. This capacitor C
EXT
is related to the MOS total gate charge:
Equation 1
The ratio between the capacitors C
EXT
and C
BOOT
is proportional to the cyclical voltage loss.
It has to be:
Equation 2
if Q
gate
is 30 nC and V
gate
is 10 V, C
EXT
is 3 nF. With C
BOOT
= 100 nF the drop is 300 mV.
If HVG has to be supplied for a long time, the C
BOOT
selection has also to take into account
the leakage and quiescent losses.
HVG steady-state consumption is lower than 120 A, so if HVG T
ON
is 5 ms, C
BOOT
has to
supply 0.6 C. This charge on a 1 F capacitor means a voltage drop of 0.6 V.
The internal bootstrap driver gives a great advantage: the external fast recovery diode can
be avoided (it usually has great leakage current).
This structure can work only if V
OUT
is close to SGND (or lower) and in the meanwhile the
LVG is on. The charging time (T
charge
) of the C
BOOT
is the time in which both conditions are
fulfilled and it has to be long enough to charge the capacitor.
The bootstrap driver introduces a voltage drop due to the DMOS R
DS(on)
(typical value:
175 ). At low frequency this drop can be neglected. Anyway, the rise of frequency has to
take into account.
The following equation is useful to compute the drop on the bootstrap DMOS:
Equation 3
where Q
gate
is the gate charge of the external power MOS, R
DS(on)
is the on resistance of
the bootstrap DMOS and T
charge
is the charging time of the bootstrap capacitor.
C
EXT
Q
gate
V
gate
--------------=
V
drop
I
ch earg
R
DS on
V
drop
Q
gate
T
ch earg
------------------ R
DS on
==
