RT9611A/B
D1
turned on. From Figure 1, the body diode “D2” had been
L
d1
s1
turned on before high side MOSFETs turned on.
V
V
OUT
IN
dV
12
I
= C
= C
gd1
(3)
Cgs1
gd1
gd1
Cgd1
dt
t
r1
Cgd2
Cgs2
d2
Igs1
Before the low side MOSFET is turned on, the Cgd2 have
been charged to VIN. Thus, as Cgd2 reverses its polarity
and g2 is charged up to 12V, the required current is :
Igd1
Ig1
Ig2 Igd2
Igs2
g1
g2
D2
dV
dt
+
Vi 12
s2
I
= C
= C
gd2
(4)
gd2
gd2
t
r2
GND
It is helpful to calculate these currents in a typical case.
Assume a synchronous rectified buck converter, input
voltage VIN = 12V, Vg1 = Vg2 = 12V. The high side
MOSFET is PHB83N03LT whose Ciss = 1660pF,
Crss = 380pF, and tr = 14ns. The low side MOSFET is
PHB95N03LT whose Ciss = 2200pF, Crss = 500pF and tr =
30ns, from the equation (1) and (2) we can obtain :
V
g1
V
+12V
PHASE
t
t
V
g2
12V
-12
1660 x 10 x 12
(5)
(6)
I
=
=
= 1.428 (A)
= 0.88 (A)
gs1
-9
14 x 10
-12
2200 x 10 x 12
I
gs2
Figure1. Equivalent Circuit andAssociated Waveforms
-9
30 x 10
from equation. (3) and (4)
In Figure 1, the current Ig1 and Ig2 are required to move the
gate up to 12V. The operation consists of charging Cgd1
,
380 x 10-12 x 12
Cgd2 , Cgs1 and Cgs2. Cgs1 and Cgs2 are the capacitors from
gate to source of the high side and the low side power
MOSFETs, respectively. In general data sheets, the Cgs1
and Cgs2 are referred as “Ciss” which are the input
capacitors. Cgd1 and Cgd2 are the capacitors from gate to
drain of the high side and the low side power MOSFETs,
respectively and referred to the data sheets as “Crss” the
reverse transfer capacitance. For example, tr1 and tr2 are
the rising time of the high side and the low side power
Igd1
=
=
= 0.326 (A)
(7)
(8)
14 x 10-9
500 x 10-12 x 12+12
(
)
Igd2
= 0.4 (A)
30 x 10-9
the total current required from the gate driving source can
be calculated as following equations :
Ig1 = Igs1 + Igd1 = 1.428 + 0.326 = 1.754 (A)
(9)
(
Ig2 = Igs2 + Igd2 = 0.88 + 0.4 = 1.28 (A)
(10)
(
)
MOSFETs respectively, the required current Igs1 and Igs2
,
By a similar calculation, we can also get the sink current
required from the turned off MOSFET.
are shown as below :
dV
C
x 12
gs1
g1
(1)
(2)
I
= C
=
gs1
gs1
dt
dV
t
r1
Select the Bootstrap Capacitor
C
x 12
g2
gs1
Figure 2 shows part of the bootstrap circuit of the RT9611A/
B. The VCB (the voltage difference between BOOT and
PHASE on RT9611A/B) provides a voltage to the gate of
the high side power MOSFET. This supply needs to be
ensured that the MOSFET can be driven. For this, the
I
= C
=
gs2
gs1
dt
t
r2
Before driving the gate of the high side MOSFET up to
12V (or 5V), the low side MOSFET has to be off; and the
high side MOSFET is turned off before the low side is
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©
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
10
DS9611A/B-03 June 2012
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