RT9611A RICHTEK [Richtek Technology Corporation], RT9611A Datasheet - Page 10

• Manufacturer Part Number: RT9611A
• Description: Synchronous Rectified Buck MOSFET Drivers
• Manufacturer: RICHTEK [Richtek Technology Corporation]

RT9611A/B
In Figure 1, the current I
gate up to 12V. The operation consists of charging C
C
gate to source of the high side and the low side power
MOSFETs, respectively. In general data sheets, the C
and C
capacitors. C
drain of the high side and the low side power MOSFETs,
respectively and referred to the data sheets as “C
reverse transfer capacitance. For example, t
the rising time of the high side and the low side power
MOSFETs respectively, the required current I
are shown as below :
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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10
I
I
gs1
gs2
V
Figure1. Equivalent Circuit and Associated Waveforms
gd2
IN
, C
=
=
gs2
C
C
©
gs1
C
gs1
gs1
2012 Richtek Technology Corporation. All rights reserved.
V
V
gd1
are referred as “C
g1
g2
d
and C
1
dV
dV
gd1
I
V
dt
dt
gd1
I
g1
g1
PHASE
g2
gs2
and C
D
g
1
=
1
. C
=
I
gs1
C
+12V
C
g
gs1
gs1
2
gd2
gs1
12V
g1
t
s
t
C
r1
and C
I
r2
1
g2
and I
x 12
are the capacitors from gate to
gs1
x 12
I
I
gd2
gs2
g2
gs2
iss
are required to move the
” which are the input
C
C
are the capacitors from
gs2
gd2
s
d
2
2
L
D
r1
gs1
2
and t
t
t
and I
rss
V
GND
r2
” the
OUT
gd1
are
(1)
(2)
gs2
gs1
,
,
turned on. From Figure 1, the body diode “D
turned on before high side MOSFETs turned on.
Before the low side MOSFET is turned on, the C
been charged to V
and g
It is helpful to calculate these currents in a typical case.
Assume a synchronous rectified buck converter, input
voltage V
MOSFET is PHB83N03LT whose C
C
PHB95N03LT whose C
30ns, from the equation (1) and (2) we can obtain :
from equation. (3) and (4)
the total current required from the gate driving source can
be calculated as following equations :
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 RT9611A/
B. The V
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
I
I
I
I
I
I
I
gd1
gd2
gs1
gs2
gd1
gd2
g1
g2
rss
=
=
= 380pF, and t
is a registered trademark of Richtek Technology Corporation.
=
=
=
=
=
=
2
I
I
gs1
gs2
1660 x 10
C
is charged up to 12V, the required current is :
C
380 x 10
2200 x 10
500 x 10
gd1
gd2
CB
14 x 10
+
IN
14 x 10
+
30 x 10
I
(the voltage difference between BOOT and
gd1
I
dV
gd2
dt
dV
= 12V, V
dt
30 x 10
-12
-12
=
=
=
-12
=
-12
-9
C
-9
(
C
-9
IN
1.428 0.326
(
x 12
x 12+12
gd1
0.88 0.4
gd2
x 12
. Thus, as C
x 12
r
-9
(
= 14ns. The low side MOSFET is
g1
iss
12
t
=
r1
Vi 12
+
=
+
= 2200pF, C
=
= V
0.326 (A)
t
+
r2
1.428
0.88
)
g2
)
DS9611A/B-03 June 2012
=
=
0.4 (A)
= 12V. The high side
gd2
)
1.28 (A)
=
(A)
(A)
1.754 (A)
reverses its polarity
rss
= 500pF and t
iss
2
= 1660pF,
” had been
gd2
have
(10)
(9)
(3)
(4)
(5)
(6)
(7)
(8)
r
=