Preliminary
Figure 7 shows the DC-DC converter's gain vs. frequency.
The compensation gain uses external impedance networks
Z
C
and Z
F
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.
80
80
Loop Gain
60
40
40
20
Gain (dB)
0
-20
-
-40
40
0
RT8101/A
T
S
V
g1
V
g2
V
IN
- V
OUT
V
L
- V
OUT
T
ON
T
OFF
I
L
ΔI
L
I
L
= I
OUT
Compensation
Gain
I
S1
Modulator
Gain
I
S2
-
-60
60
1H
0z
10
db(vo)
v
10z
0H
v b c m 2
100
l )
d(op) vb o
d(
1k
10k
Feuny
r qe c
Frequency (Hz)
10H
.Kz
1Kz
0H
100k
1 0H
0Kz
10H
.Mz
1M
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 (ΔI
L
) 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.
i
S1
L
+ V
L
-
i
S2
S1
V
IN
S2
+
V
OR
-
+
Figure 8. The waveforms of synchronous step-down
converter
According to Figure 8 the ripple current of inductor can be
calculated as follows :
V
Δ
I
L
;
Δ
t
=
D ; D
=
OUT
V
IN
Δ
t
fs
V
OUT
L
=
(V
IN
−
V
OUT
)
×
V
IN
×
fs
× Δ
I
L
V
IN
−
V
OUT
=
L
Where :
V
IN
= Maximum input voltage
V
OUT
= Output Voltage
Δt
= S1 turn on time
(1)
I
L
i
C
r
C
R
L
C
OUT
I
OUT
+
V
OUT
-
ΔI
L
= Inductor current ripple
f
S
= Switching frequency
D = Duty Cycle
r
C
= Equivalent series resistor of output capacitor
+
V
OC
-
DS8101/A-01 March 2007
www.richtek.com
11
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
