RT9602 데이터 시트보기 (PDF) - Richtek Technology
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RT9602 Datasheet PDF : 11 Pages
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RT9602
lgs1
=
Cgs1
dVg1
dt
=
Cgs1×12
tr1
(1)
lgs2
=
Cgs2
dVg2
dt
=
Cgs2 ×12
tr2
(2)
According to the design of RT9602, 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 turned on. From Figure 1,
the body diode "D2" had been turned on before high side
MOSFETs turned on
lgd1
=
Cg1
dV
dt
= Cgd1
12V
t r1
(3)
Before the low side MOSFET is turned on, the Cgd2 have
been charged to Vi. Thus, as Cgd2 reverses its polarity and
g2 is charged up to 12V, the required current is
lgd2
=
C gd2
dV
dt
=
Cgd2
Vi+12V
tr2
(4)
It is helpful to calculate these currents in a typical case.
Assume a synchronous rectified BUCK converter, input
voltage Vi = 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
lgs1 = 1660 ×10-12 ×12 = 1.428 (A)
(5)
14 ×10-9
lgs2 = 2200 ×10-12 ×12 = 0.88 (A)
(6)
30 ×10-9
from equation. (3) and (4)
lgs1 = 380 ×10-12 ×12 = 0.326 (A)
(7)
14 ×10-9
lgs2 = 500 ×10-12 × (12+12) = 0.4(A)
(8)
30 ×10-9
the total current required from the gate driving source is
Ig1 = Igs1+Igd1 = (1.428+0.326) = 1.745(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.
Layout Consider
Figure 2. shows the schematic circuit of a two-phase
synchronous-buck converter to implement the RT9602. The
converter operates for the input rang from 5V to 12V.
L1
Vin
12V 1.2uH
C5
1500uF
D1
C1
1000uF
Cb1
C2 1uF
1uF
11
14
BOOT1 VCC
Q1
L2
PHB83N03LT
2uH
12 UGATE1 PVCC 5
13 PHASE1 PWM1 1
RT9602
Q2
PHB95N03LT 4 LGATE1
PWM2 2
C3
1000uF
9 UGATE2
C4
1uF
8 PHASE2 GND 3
C6
1500uF
Q3
L3
PHB83N03LT
7
LGATE2
PGND 6
2uH
Cb2
Q4
1uF
BOOT2
10
D2
PHB95N03LT
R1
12V
C7 10
1uF
PWM1
PWM2
VCORE
Figure 2. Two- Phase Synchronous-Buck Converter Circuit
DS9602-08 March 2007
www.richtek.com
7
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