rt9206 Richtek Technology Corporation, rt9206 Datasheet - Page 14

• Manufacturer Part Number: rt9206
• Description: High Efficiency, Synchronous Buck With Dual Linear Controllers
• Manufacturer: Richtek Technology Corporation

RT9206
The AC impedance of output capacitor at operating
frequency is quite smaller than the load impedance, so
the ripple current ( I
through output capacitor. The output ripple voltage is
described as :
where V
For electrolytic capacitor application, typically 90~95% of
the output voltage ripple is contributed by the ESR of output
capacitor. So Equation (4) could be simplified as :
Users could connect capacitors in parallel to get calculated
ESR.
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14
Figure 2. The related waveforms of output capacitor.
V
V
OC
OR
i
i
L
c
0
0
OR
ΔV
d
dt
is caused by ESR and V
iL
ΔV
=
OUT
V
IN
ΔV
OUT
-V
t1
L
= ΔI
ΔV
OUT
L
OUT
) of the inductor current flows mainly
= ΔI
OUT
L
= ΔV
T
S
L
r +
= ΔI
C
t2
r +
C
OR
8 C L
1 V
1/2
L
d
dt
ΔI
+ ΔV
i
Co
L
r
1
OUT
=
C
L
O
t1
V
t2
OC
(1- D)T
OC
OUT
L
ic t
I
by capacitance.
d
O
Δ
ΔI
I
L
S
2
Δ
x r
L
V
c
OC
(2)
(3)
(4)
(5)
Input Capacitor Selection
The selection of input capacitor is mainly based on its
maximum ripple current capability. The buck converter
draws pulsewise current from the input capacitor during
the on time of S1 as shown in Figure 1. The RMS value of
ripple current flowing through the input capacitor is
described as :
The input capacitor must be cable of handling this ripple
current. Sometime, for higher efficiency the low ESR
capacitor is necessarily.
Power MOSFET Selection
The selection of MOSFETs is based on consideration of
maximum gate-source voltage (Vgs), drain-source voltage
(Vdss), maximum drain current (Id), drain-source on-state
resistance R
are driven by V
threshold voltage MOSFET should be selected to guarantee
that it could fully turn on at Vgs = 6.0V.
The total power dissipation of external MOSFETs consists
of conduction and switching losses. The conduction losses
of high-side and low-side MOSFETs are described by
equation (7) and (8), respectively.
Where
The total switching loss is approximated as.
Where
off time.
r
(High-side MOSFET)
(Low-side MOSFET)
is temperature dependency of R
I
f
V
t
OUT
s
r
= Switching frequency
ds(off)
and t
Psw = I
= Load current
P
L - con
P
ds(on)
is voltage from drain to source at MOSFET
f
H - con
are rise-time and fall-time, respectively.
INT
I
and thermal management. The MOSFETs
OUT
rms
= I
that is internally regulated as 6.0V. Low
= I
2
0
= I
0
2
(1- D) R
Vds(off)
O
D R
D(1- D)
2
ds(on)
ds(on)
DS9206-11 March 2007
(tr + tf) fs
ds(on)
r
r
(A)
(W)
(W)
(W)
(6)
(9)
(7)
(8)