Datasheet

Ilp-p=
V
OUT
V
IN(max)

•
V -V
IN(max) OUT

L f
O SW
•
I =I +
lpeak O
Ilp-p

2
I =
Lo(RMS)
√
−
I +
O
2
1
−
12
Ilp-p
2
I =
CO(RMS)
V
OUT
( )V-V
IN OUT
V
IN
L
O
f
SW
√12
−
•
•
•
•
−
TPS54425
www.ti.com SLVSA84C –AUGUST 2010– REVISED JULY 2011
At low frequencies, the overall loop gain is set by the output set-point resistor divider network and the internal
gain of the TPS54425. The low frequency phase is 180 degrees. At the output filter pole frequency, the gain rolls
off at a -40 dB per decade rate and the phase drops rapidly. D-CAP2™ introduces a high frequency zero that
reduces the gain roll off to -20 dB per decade and increases the phase to 90 degrees one decade above the
zero frequency. The inductor and capacitor selected for the output filter must be selected so that the double pole
of Equation 4 is located below the high frequency zero but close enough that the phase boost provided be the
high frequency zero provides adequate phase margin for a stable circuit. To meet this requirement use the
values recommended in Table 1
Table 1. Recommended Component Values
Output Voltage (V) R1 (kΩ) R2 (kΩ) C4 (pF)
(1)
L1 (µH) C8 + C9 (µF)
1 6.81 22.1 1.5 22 - 68
1.05 8.25 22.1 1.5 22 - 68
1.2 12.7 22.1 1.5 22 - 68
1.8 30.1 22.1 10 - 22 2.2 22 - 68
2.5 49.9 22.1 10 - 22 2.2 22 - 68
3.3 73.2 22.1 10 - 22 2.2 22 - 68
5 121 22.1 10 - 22 3.3 22 - 68
(1) Optional
For higher output voltages at or above 1.8 V, additional phase boost can be achieved by adding a feed forward
capacitor (C4) in parallel with R1.
Since the DC gain is dependent on the output voltage, the required inductor value will increase as the output
voltage increases. For higher output voltages at or above 1.8 V, additional phase boost can be achieved by
adding a feed forward capacitor (C4) in parallel with R1
The inductor peak-to-peak ripple current, peak current and RMS current are calculated using Equation 5,
Equation 6 and Equation 7. The inductor saturation current rating must be greater than the calculated peak
current and the RMS or heating current rating must be greater than the calculated RMS current. Use 700 kHz for
f
SW
.
Use 700 kHz for f
SW
. Make sure the chosen inductor is rated for the peak current of Equation 6 and the RMS
current of Equation 7.
(5)
(6)
(7)
For this design example, the calculated peak current is 4.47A and the calculated RMS current is 4.009 A. The
inductor used is a TDK SPM6530-1R5M100 with a peak current rating of 11.5 A and an RMS current rating of 11
A.
The capacitor value and ESR determines the amount of output voltage ripple. The TPS54425 is intended for use
with ceramic or other low ESR capacitors. Recommended values range from 22uF to 68uF. Use Equation 8 to
determine the required RMS current rating for the output capacitor.
(8)
For this design two TDK C3216X5R0J226M 22uF output capacitors are used. The typical ESR is 2 mΩ each.
The calculated RMS current is 0.271A and each output capacitor is rated for 4A.
Copyright © 2010–2011, Texas Instruments Incorporated 13