RT9619 查看數據表（PDF） - Richtek Technology
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RT9619 Datasheet PDF : 11 Pages
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RT9619/A
low side power MOSFETs, respectively and referred to the
data sheets as "Crss" the reverse transfer capacitance. For
example, tr1 and tr2 are the rising time of the high side and
the low side power MOSFETs respectively, the required
current Igs1 and Igs2 are showed below :
,
Igs1
=
Cgs1
dVg1
dt
=
Cgs1 ×12
tr1
(1)
Igs2
=
Cgs1
dVg2
dt
=
Cgs1 ×12
tr2
(2)
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
Vi +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)
-12
Igd1 = 380 ×10
×12
-9
=
0.326
(A)
(7)
14 ×10
-12
Igd2 = 500 ×10
× (12
-9
+ 12)
=
0.4
(A)
(8)
30 ×10
DS9619/A-06 April 2011
the total current required from the gate driving source is
Ig1 = Igs1 + Igd1 = (1.428 + 0.326) = 1.754 (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 RT9619/A.
The VCB (the voltage difference between BOOT and PHASE
on RT9619/A) 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
VCC
BOOT
UGATE
PHASE
VCC
CB
+
VCB
-
LGATE
PGND
Figure 2. Part of Bootstrap Circuit of RT9619/A
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.
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