SLVS632E – JANUARY 2006 – REVISED SEPTEMBER 2013
VIN 10
-
24 V
VIN
C1
4.7
m
F
EN
U1
TPS5430DDA
7
VIN
1
BOOT
5
ENA
2
8
PH
NC
3
4
NC
VSNS
6
GND PwPd
9
C2
0.01
m
F
L1
15
m
H
3.3 V
VOUT
D1
MRBS340
C3
100
m
F
C7
0.1
m
F
C4
150 pF
R3
549
W
R2
5.9 kW
R1
10 kW
C6
1500 pF
Figure 14. Ceramic Output Filter Capacitors Circuit
Output Filter Component Selection
Using
the minimum inductor value is 12
μH.
A value of 15
μH
is chosen for this design.
When using ceramic output filer capacitors, the recommended LC resonant frequency should be no more than
7 kHz. Since the output inductor is already selected at 15
μH,
this limits the minimum output capacitor value to:
1
C
O
(MIN)
³
2
(2
p
x 7000) x L
O
(13)
The minimum capacitor value is calculated to be 34μF. For this circuit a larger value of capacitor yields better
transient response. A single 100-μF output capacitor is used for C3. It is important to note that the actual
capacitance of ceramic capacitors decreases with applied voltage. In this example, the output voltage is set to
3.3 V, minimizing this effect.
External Compensation Network
When using ceramic output capacitors, additional circuitry is required to stabilize the closed loop system. For this
circuit, the external components are R3, C4, C6, and C7. To determine the value of these components, first
calculate the LC resonant frequency of the output filter:
1
F
LC
=
2
p
Ö
L
O
x C
O
(EFF)
(14)
For this example the effective resonant frequency is calculated as 4109 Hz
The network composed of R1, R2, R3, C5, C6, and C7 has two poles and two zeros that are used to tailor the
overall response of the feedback network to accommodate the use of the ceramic output capacitors. The pole
and zero locations are given by the following equations:
V
O
Fp1 = 500000 x
F
LC
(15)
Fz1 = 0.7 x F
LC
(16)
Copyright © 2006–2013, Texas Instruments Incorporated
17
Product Folder Links:
TI [ TEXAS INSTRUMENTS ]
