Datasheet

VIN10 24V-
C1
4.7 Fm
L1
18 Hm
R1
10kW
R2
5.9kW
R3
549 W
C6
1800pF
U1
TPS5420D
VIN
GND
VSNS
PH
BOOT
PwPd
EN
3.3V
VOUT
C2
0.01 Fm
D1
MRBS340
C4
47 Fm
C7
0.1 Fm
C4
150pF
VIN
7
1
2
4
5
8
3
6
9
ENA
NC
NC
C3
47 Fm
C (MIN)
O
≥
1
2
(2 ×7000) Lπ
O
×
(13)
F =
LC
1
2 Lπ
O O
×C (EFF)√
(14)
TPS5420-Q1
www.ti.com ................................................................................................................................................... SLVS752B – NOVEMBER 2007 – REVISED JUNE 2008
Figure 13. Ceramic Output Filter Capacitors Circuit
Output Filter Component Selection
Using Equation 11 , the minimum inductor value is 17.9 µ H. A value of 18 µ H 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 18 µ H, this limits the minimum output capacitor value to:
The minimum capacitor value is calculated to be 29 µ F. For this circuit a larger value of capacitor yields better
transient response. Two 47- µ F output capacitors are used for C3 and C4. 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, C5, C6, and C7. To determine the value of these components, first
calculate the LC resonant frequency of the output filter:
For this example, the effective resonant frequency is calculated as 4109 Hz.
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