Datasheet

SLUU111 – June 2002
6
1-MHz, 3.3-V, High-Efficiency Synchronous Buck Converter With TPS43000 PWM Controller
2.2 Inductance Value
The inductance value can be calculated as shown in equation (2).
L
(min)
+
V
OUT
f I
RIPPLE
ǒ
1 *
V
OUT
V
IN(max)
Ǔ
I
RIPPLE
is the ripple current flowing through the inductor, which affects the output voltage
ripple and core losses. According to the specification, the converter enters PFM mode at
200 mA, so the desired ripple current it 0.4 A. Based on this and the 1-MHz operating
frequency, the inductance value is calculated at 5.0 µH.
2.3 Input and Output Capacitors
The output capacitance and required ESR can be calculated by equations (3) and (4).
C
OUTPUT (min)
+
I
RIPPLE
8 f V
RIPPLE
ESR
OUT
+
V
RIPPLE
I
RIPPLE
With 1% output voltage ripple, the capacitance required is at least 1.5 µF and its ESR should
be less than 81.7 mΩ. A Panasonic 4-V, 120-µF capacitor is chosen with an ESR of 18 mΩ.
The required input capacitance is calculated in equation (5). The calculated value is
approximately 10 µF. A 10-µF ceramic capacitor is used in order to handle the ripple current.
C
IN (min)
+ I
OUT (max)
D
max)
T
S
V
IN (ripple)
2.4 Compensation Design
The TPS43000 uses voltage-mode control. R1, R2, and R3 along with C1, C2 and C3, form
a Type III compensator network. The L-C frequency of the power stage, f
C
is approximately
6.5 kHz and the ESR zero is around 73.7 kHz, as shown in Figure 2. The overall crossover
frequency, f
0db
, is chosen at 50 kHz for reasonable transient response and stability. The two
zeros, f
Z1
and f
Z2
from the compensator are set at 0.5 f
C
and f
C
separately. The two poles
f
P1
and f
P2
are set at ESR zero and 0.5 f. The frequency of poles and zeros are defined by
the following equations:
f
Z1
+
1
2p R2 C1
f
Z2
[
1
2p R1 C3
assuming R1 ơ R3
f
P1
+
1
2p R3 C3
f
P2
[
1
2p R2 C2
assuming C1 ơ C2
(2)
(3)
(4)
(5)