TPS5430-Q1
SLVS751C–NOVEMBER 2007–REVISED JULY 2009................................................................................................................................................... www.ti.com
And the desired output capacitor value for the output filter to:
1
f
C
+
OUT
3357 L
V
OUT
CO
OUT
(8)
For a desired crossover of 18 kHz and a 15-µH inductor, the calculated value for the output capacitor is 220 µF.
The capacitor type should be chosen so that the ESR zero is above the loop crossover. The maximum ESR is:
1
ESR
+
MAX
2p C
f
OUT
CO
(9)
The maximum ESR of the output capacitor also determines the amount of output ripple as specified in the initial
design parameters. The output ripple voltage is the inductor ripple current times the ESR of the output filter.
Check that the maximum specified ESR as listed in the capacitor data sheet results in an acceptable output
ripple voltage:
ESR
× V
×
V
– V
(
× L
)
MAX
OUT
IN(MAX)
OUT
× F
OUT SW
V
(MAX) =
PP
N
× V
C
IN(MAX)
(10)
Where:
ΔVPP is the desired peak-to-peak output ripple.
NC is the number of parallel output capacitors.
FSW is the switching frequency.
For this design example, a single 220-µF output capacitor is chosen for C3. The calculated RMS ripple current is
143 mA and the maximum ESR required is 40 mΩ. A capacitor that meets these requirements is a Sanyo
Poscap 10TPB220M, rated at 10 V with a maximum ESR of 40 mΩ and a ripple current rating of 3 A. An
additional small 0.1-µF ceramic bypass capacitor may also used, but is not included in this design.
The minimum ESR of the output capacitor should also be considered. For good phase margin, the ESR zero
when the ESR is at a minimum should not be too far above the internal compensation poles at 24 kHz and
54 kHz.
The selected output capacitor must also be rated for a voltage greater than the desired output voltage plus
one-half the ripple voltage. Any derating amount must also be included. The maximum RMS ripple current in the
output capacitor is given by Equation 11:
ǒV
Ǔ
C
V
* V
ȡ
ȣ
OUT
IN(MAX)
OUT
N ȧ
1
I
+
ȧVIN(MAX)
COUT(RMS)
Ǹ
L
F
12
OUT
SW
(11)
Where:
NC is the number of output capacitors in parallel.
FSW is the switching frequency.
Other capacitor types can be used with the TPS5430, depending on the needs of the application.
Output Voltage Setpoint
The output voltage of the TPS5430 is set by a resistor divider (R1 and R2) from the output to the VSENSE pin.
Calculate the R2 resistor value for the output voltage of 5 V using Equation 12:
R1 1.221
R2 +
V
* 1.221
OUT
(12)
For any TPS5430 design, start with an R1 value of 10 kΩ. R2 is then 3.24 kΩ.
Boot Capacitor
The boot capacitor should be 0.01 µF.
14
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