RT9602
l
gs1
= C
gs1
dV
g1
C
gs1
×
12
=
dt
t
r1
dV
g2
C
gs2
×
12
=
dt
t
r2
from equation. (3) and (4)
(1)
-12
l
gs1
= 380
×
10
×
12 = 0.326 (A)
14
×
10
-9
(7)
l
gs2
= C
gs2
(2)
l
gs2
=
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
"D
2
"
had been turned on before high side
MOSFETs turned on
l
gd1
= C
g1
dV = C
gd1
12V
dt
t
r1
(3)
500
×
10
-12
×
(12+12)
30
×
10
-9
= 0.4(A)
(8)
the total current required from the gate driving source is
I
g1
= I
gs1
+I
gd1
= (1.428+0.326) = 1.745(A)
I
g2
= I
gs2
+I
gd2
= (0.88+0.4) = 1.28(A)
(9)
(10)
By a similar calculation, we can also get the sink current
required from the turned off MOSFET.
Layout Consider
Before the low side MOSFET is turned on, the C
gd2
have
been charged to Vi. Thus, as C
gd2
reverses its polarity and
g
2
is charged up to 12V, the required current is
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.
D1
L1
R1
14
VCC
PVCC
5
C7 10
1uF
12V
Vin
12V
l
gd2
= C
gd2
dV = C
gd2
Vi+12V
dt
t
r2
(4)
1.2uH
C1
1000uF
Q1
L2
C2
1uF
Cb1
1uF
11
BOOT1
12
PHB83N03LT
UGATE1
PHASE1
It is helpful to calculate these currents in a typical case.
Assume a synchronous rectified BUCK converter, input
voltage V
i
= 12V, V
g1
= V
g2
= 12V. The high side MOSFET
is PHB83N03LT whose C
iss
= 1660pF, C
rss
= 380pF,and t
r
= 14nS. The low side MOSFET is PHB95N03LT whose
C
iss
= 2200pF, C
rss
= 500pF, and t
r
= 30nS, from the
equation (1) and (2) we can obtain
-12
l
gs1
= 1660
×
10
×
12 = 1.428 (A)
14
×
10
-9
13
2uH
C5
1500uF
Q2
PHB95N03LT
PWM1
1
PWM1
RT9602
4
LGATE1
PWM2
9
C3
1000uF
Q3
L3
2uH
PHB83N03LT
2
PWM2
C4
1uF
UGATE2
PHASE2
LGATE2
GND
PGND
3
6
8
7
C6
1500uF
Q4
Cb2
1uF
BOOT2
10
D2
PHB95N03LT
(5)
V
CORE
l
gs2
= 2200
×
10
×
12 = 0.88 (A)
30
×
10
-9
-12
Figure 2. Two- Phase Synchronous-Buck Converter Circuit
(6)
DS9602-08 March 2007
www.richtek.com
7
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
