RT9612AZQW 데이터 시트보기 (PDF) - Richtek Technology
부품명
상세내역
일치하는 목록
RT9612AZQW Datasheet PDF : 15 Pages
| |||
RT9612A/B
d1
VIN
Cgd1
D1
s1
Cgs1
L
VOUT
Igd1 Igs1
Ig1
g1 g2
Ig2 Igd2
Igs2
Cgd2
d2
D2
Cgs2 s2
Vg1
VPHASE +12V
GND
t
Vg2
12V
t
Figure 1. Equivalent Circuit and Associated Waveforms
In Figure 1, the current Ig1 and Ig2 are required to move the
gate up to 12V. The operation consists of charging Cgd1,
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
MOSFETs respectively, the required current Igs1 and Igs2,
are shown as below :
dVg1 Cgs1 x 12
Igs1 = Cgs1 dt =
tr1
(1)
dVg2 Cgs1 x 12
Igs2 = Cgs1 dt =
tr2
(2)
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 will be turned off before the
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
www.richtek.com
10
low side is turned on. From Figure 1, the body diode “ D2”
will be turned on before high side MOSFETs turn on.
Igd1
= Cgd1
dV
dt
= Cgd1
12
tr1
(3)
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
Igd2
= Cgd2
dV
dt
= Cgd2
Vi + 12
tr2
(4)
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
Igs1
= 1660 x 10-12 x 12
14 x 10-9
= 1.428
(A)
(5)
Igs2
= 2200 x 10-12 x 12 = 0.88
30 x 10-9
(A)
(6)
from equation. (3) and (4)
Igd1
=
380 x 10-12 x 12
14 x 10-9
= 0.326
(A)
(7)
500 x 10-12 x (12+12)
Igd2 =
30 x 10-9
= 0.4 (A)
(8)
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)
By a similar calculation, we can also get the sink current
required from the turned off MOSFET.
Select the Bootstrap Capacitor
Figure 2 shows part of the bootstrap circuit of the
RT9612A/B. The VCB (the voltage difference between
BOOT and PHASE on RT9612A/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 capacitance CB has to be selected properly. It is
determined by following constraints.
is a registered trademark of Richtek Technology Corporation.
DS9612A/B-03 June 2012
Share Link: 