TPS5430-Q1
SLVS751C–NOVEMBER 2007–REVISED JULY 2009................................................................................................................................................... www.ti.com
Circuit Using Ceramic Output Filter Capacitors
Figure 14 shows an application circuit using all ceramic capacitors for the input and output filters, which
generates a 3.3-V output from a 10-V to 24-V input. The design procedure is similar to those given for the design
example, except for the selection of the output filter capacitor values and the design of the additional
compensation components required to stabilize the circuit.
U1
TPS5430DDA
C2
0.01 mF
L1
15 mH
VIN 10-24 V
7
5
2
3
6
3.3 V
VIN
VIN
1
VOUT
BOOT
PH
EN
ENA
NC
C1
4.7 mF
8
4
C3
100 mF
D1
MRBS340
NC
VSNS
PwPd
GND
9
R1
10 kW
C7
0.1 mF
C4
150 pF
R2
5.9 kW
C6
1500 pF
R3
549 W
Figure 14. Ceramic Output Filter Capacitors Circuit
Output Filter Component Selection
Using Equation 11, the minimum inductor value is 12 µH. A value of 15 µH is chosen for this design.
When using ceramic output filter 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
(MIN) ≥
O
2
(2π × 7000) × L
O
(13)
The minimum capacitor value is calculated to be 34 µF. 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
2π √ L × C (EFF)
O
O
(14)
For this example, the effective resonant frequency is calculated as 4109 Hz.
16
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