RT9605B 데이터 시트보기 (PDF) - Richtek Technology
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RT9605B Datasheet PDF : 13 Pages
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RT9605B
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
=
Cgd1
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 ×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 ×10-12 ×12
14 ×10-9
= 1.428
(A)
(5)
Igs2 = 2200 ×10-12 ×12 = 0.88 (A)
(6)
30 ×10-9
from equation. (3) and (4)
Igd1
=
380 ×10-12 ×12
14 ×10-9
= 0.326
(A)
(7)
Igd2
=
500 ×10-12 × (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.
Select the Bootstrap Capacitor
Figure 2 shows part of the bootstrap circuit of RT9605B.
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
Figure 2. Part of Bootstrap Circuit of RT9605B
In practice, a low value capacitor CB will lead the over-
charging that could damage the IC. Therefore to minimize
the risk of overcharging and reducing the ripple on VCB,
the bootstrap capacitor should not be smaller than 0.1μF,
and the larger the better. In general design, using 1μF can
provide better performance. At least one low-ESR capacitor
should be used to provide good local de-coupling. Here,
to adopt either a ceramic or tantalum capacitor is suitable.
Power Dissipation
For not exceeding the maximum allowable power
dissipation to drive the IC beyond the maximum
recommended operating junction temperature of 125°C,
it is necessary to calculate power dissipation appro-
priately. This dissipation is a function of switching
frequency and total gate charge of the selected MOSFET.
Figure 3 shows the power dissipation test circuit. CL and
CU are the UGATE and LGATE load capacitors,
respectively. The bootstrap capacitor value is 1μF.
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DS9605B-03 March 2011
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