MB39C308 Datasheet, PDF (34/60 Pages) Fujitsu Component Limited. – 6ch DC/DC Converter IC for LPIA Platform VR

MB39C308
• Bootstrap Capacitor Selection
Although the default bootstrap capacitor (the capacitor between CB and LX) is 0.1μF, this may need to be adjusted
if the FET used on CH1 and CH2 have a large Qg. The bootstrap capacitor needs to be able to charge sufficiently
to drive the gate of the High-side FET. As a rough guide, select a capacitor with a minimum value of capacitance
that is able to accumulate approximately 10 times the charge of the Qg of the High-side FET.
Qg
CCB ≥ 10 ×
VCB
CCB : Bootstrap capacitance [F]
Qg : High-side SWFET gate charge [C]
VCB : CB voltage (4.3 V)
• VB Capacitor Selection
Although the default VB capacitor is 1 μF, this may need to be adjusted if the FET used on CH1 and CH2 have
a large Qg. The VB capacitor needs to be able to charge sufficiently to drive the gate of the High-side FET. As
a rough guide, select a capacitor with a minimum value of capacitance that is able to accumulate approximately
50 times the charge of the Qg of the High-side FET.
( ^{CVB ≥ 50 ×}
QgH12 + 9.3 × 10^-9
VCB
QgL12 + 23 × 10^-9
+
VVB
CVB : VB capacitance [F]
QgH12 : Total gate charge of High-side FET for CH1 and CH2 [C] (Total when Vgs = 4.3 V)
QgL12 : Total gate charge of Low-side FET for CH1 and CH2 [C] (Total when Vgs = 5 V)
VVB : VB voltage (5 V)
VCB : CB voltage (4.3 V)
34
DS04-27261-6E