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RT9605A Datasheet PDF : 12 Pages
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RT9605A
Preliminary
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.
Igd1
=
Cg1
dV
dt
=
Cgd1
12V
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
VIN + 12V
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 ×10−12 ×12
14 ×10−9
= 1.428
(A)
(5)
Igs2
=
2200 ×10-12 ×12
30 ×10−9
=
0.88
(A)
(6)
from equation. (3) and (4)
Igd1
=
380 ×10−12 ×12
14 ×10−9
=
0.326
(A)
(7)
Igd2
=
500 ×10-12 ×12
30 ×10−9
=
0.4
(A)
(8)
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 Consideration
Figure 2 shows the schematic circuit of a two-phase
synchronous buck converter to implement the either phase
of RT9605A. The converter operates at VIN 12V.
L1
VIN
12V
1.2uH
C1
1000uF
VCORE
C3
1500uF
D1
Q1
L2
2uH
Q2
C2
1uF CB BOOTX PVCCX
1uF
UGATEX PWMX
PHB83N03LT PHASEX
PHB95N03LT LGATEX GND
R1
12V
C4 10
1uF
PWM
Figure 2. Sync. Buck Converter Circuit
When layout the PC board, it should be very careful. The
power-circuit section is the most critical one. If not
configured properly, it will generate a large amount of EMI.
The junction of Q1, Q2, L2 should be very close.
Next, the trace from UGATE, and LGATE to the gates of
MOSFET should also be short to decrease the noise of
the driver output signals. The bypass capacitor C4 should
be connected to GND directly. Furthermore, the bootstrap
capacitors (CB) should always be placed as close to the
pins of the IC as possible. The trace from PHASE to the
common node of the two MOSFETs should be kept wide
since it usually carries large current.
Select the Bootstrap Capacitor
Figure 3 shows part of the bootstrap circuit of RT9605A.
The VCB (the voltage difference between BOOT and PHASE
pins 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.
1N4148
VIN
BOOT
UGATE
PHASE
CB
+
VCB
-
LGATE
GND
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10
Figure 3. Part of Bootstrap Circuit of RT9605A
All brandname or trademark belong to their owner respectively
DS9605A-05 August 2007
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