E-L6598D013TR Datasheet, PDF (14/24 Pages) STMicroelectronics – High voltage resonant controller

Block diagram description
L6598
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
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. The following equation is useful to
compute the drop on the bootstrap driver:
Equation 8
V_drop
=
I_{ch arg e}R_dson + V_diode → V_drop
=
-------^Q-----^g-------
T_{ch arge}
R_ds
on
+
V_d
i
o
de
where Qg is the gate charge of the external power MOS, Rdson is the on resistance of the
bootstrap DMOS, and Tcharge is the time in which the bootstrap driver remains on (about the
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
V_drop = ₂--³--_.-₂-⁰---_3n----_μ^C---_s--150Ω + 0.6V ∼ 2.6V
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.
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Doc ID 6554 Rev 7