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RT8101APSP Datasheet(PDF) 11 Page - Richtek Technology Corporation

Part # RT8101APSP
Description  12V Synchronous Buck PWM DC-DC
PDF  15 Pages
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Manufacturer  RICHTEK [Richtek Technology Corporation]
Direct Link  http://www.richtek.com
Logo RICHTEK - Richtek Technology Corporation

RT8101APSP Datasheet(HTML) 11 Page - Richtek Technology Corporation

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11
RT8101/A
Preliminary
DS8101/A-01 March 2007
www.richtek.com
Figure 7 shows the DC-DC converter's gain vs. frequency.
The compensation gain uses external impedance networks
ZC and ZF to provide a stable, high bandwidth loop. High
crossover frequency is desirable for fast transient
response, but it often jeopardizes the system stability. In
order to cancel one of the LC filter poles, place the zero
before the LC filter resonant frequency. In the experience,
place the zero at 75% LC filter resonant frequency.
Crossover frequency should be higher than the ESR zero
but less than 1/5 of the switching frequency. The second
pole is placed at half of the switching frequency.
Figure 7. Bode Plot
Component Selection
1) Inductor Selection
The selection of output inductor is based on the
considerations of efficiency, output power and operating
frequency. Low inductance value has smaller size, but
results in low efficiency, large ripple current and high output
ripple voltage. Generally, an inductor that limits the ripple
current (
ΔIL) between 20% and 50% of the output current
is appropriate. Figure 8 shows the typical topology of
synchronous step-down converter and its related
waveforms.
S1
S2
V
IN
i
S1
i
S2
I
OUT
V
OUT
+
-
R
L
r
C
C
OUT
i
C
V
OR
+
-
V
OC
+
-
V
L
+
-
L
I
L
Frequency
10Hz
100Hz
1.0KHz
10KHz
100KHz
1.0MHz
vdb(vo) vdb(comp2) vdb(lo)
-40
0
40
80
-60
10
100
1k
10k
100k
1M
80
40
0
20
60
-20
-40
-60
Loop Gain
Compensation
Gain
Modulator
Gain
Frequency (Hz)
Figure 8. The waveforms of synchronous step-down
converter
According to Figure 8 the ripple current of inductor can be
calculated as follows :
(1)
Where :
VIN = Maximum input voltage
VOUT = Output Voltage
Δt = S1 turn on time
ΔIL = Inductor current ripple
fS = Switching frequency
D = Duty Cycle
rC = Equivalent series resistor of output capacitor
L
IN
OUT
OUT
IN
IN
OUT
L
OUT
IN
I
fs
V
V
)
V
(V
L
V
V
D
;
fs
D
t
;
t
I
L
V
V
Δ
×
×
×
−
=
=
=
Δ
Δ
Δ
=
−
VL
VIN - VOUT
- VOUT
IL
IL = IOUT
ΔIL
IS1
IS2
TS
TON TOFF
Vg1
Vg2



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