Datasheet

F =
LC
1
2 Lp
O O
xC (EFF)Ö
C (MIN)
O
³
1
2
(2 x7000)xLp
O
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
TPS5420
www.ti.com SLVS642E –APRIL 2006–REVISED SEPTEMBER 2013
Circuit Using Ceramic Output Filter Capacitors
Figure 13 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.
Figure 13. Ceramic Output Filter Capacitors Circuit
Output Filter Component Selection
Using Equation 11, the minimum inductor value is 17.9 μH. A value of 18 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 18 μH, this limits the minimum output capacitor value to:
(13)
The minimum capacitor value is calculated to be 29 μF. For this circuit a larger value of capacitor yields better
transient response. Two 47 μF 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:
(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:
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