Datasheet

Block diagram description L6598
14/23 DocID6554 Rev 8
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
amplitude of the driving signal, can significantly increase the R
DSON
of the external power
MOS (and so the dissipation).
To be considered that in resonant power supplies the current which flows in the power MOS
decreases increasing the switching frequency and generally the increases of R
DSON
is not
a problem because power dissipation is negligible. Equation 8 is useful to compute the drop
on the bootstrap driver:
Equation 8
where Q
g is the gate charge of the external power MOS, Rdson is the on-resistance of the
bootstrap DMOS, and T
charge is the time in which the bootstrap driver remains on (about the
semi-period of the switching frequency minus the deadtime). The typical resistance value of
the bootstrap DMOS is 150 . 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
the drop on the R
DSON
of the DMOS.
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
connected to R
fmin
pin to adjust the oscillation frequency.
V
drop
I
ch earg
R
dson
V
diode
V
drop
+
Q
g
T
ch earg
------------------- R
dson
V
diode
+==
V
drop
30nC
2.23s
------------------ 150 0.6V 2.6V+=