( )
Ox INx Ox
COx(RMS)
INx Ox SW
V V V
I =
12 V L
´ -
´ ´ ´ f
2 2
LOx(RMS) Ox L
1
I = I + ΔI
12
INx(MAX) Ox
Ox
L1x
INx(MAX) SW
V V
V
ΔI =
V L1x
-
´
´ f
TPS54394
SLVSBE6 –JUNE 2012
www.ti.com
At low frequencies, the overall loop gain is set by the output set-point resistor divider network and the internal
gain of the TPS545394. 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 3 is located below the high frequency zero but close enough that the phase boost provided by 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) R1x (kΩ) R2x (kΩ) Cffx (pF)
(1)
L1x (µH) C2x (µF)
1 6.81 22.1 1.5 - 2.2 20 - 68
1.05 8.25 22.1 1.5 - 2.2 20 - 68
1.2 12.7 22.1 1.5 - 2.2 20 - 68
1.5 21.5 22.1 1.5 - 2.2 20 - 68
1.8 30.1 22.1 5 - 22 2.2 - 3.3 20 - 68
2.5 49.9 22.1 5 - 22 2.2 - 3.3 20 - 68
3.3 73.2 22.1 5 - 22 2.2 - 3.3 20 - 68
5 124 22.1 5 - 22 4.7 20 - 68
6.5 165 22.1 5 - 22 4.7 20 - 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 (Cff) in parallel with R1.
The inductor peak-to-peak ripple current, peak current and RMS current are calculated using Equation 4,
Equation 5 and Equation 6. 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.
For the calculations, use 700 kHz as the switching frequency, f
SW
. Make sure the chosen inductor is rated for the
peak current of Equation 5 and the RMS current of Equation 6.
(4)
(5)
(6)
For the above design example, the calculated peak current is 3.46 A and the calculated RMS current is 3.01 A
for VO1. The inductor used is a TDK CLF7045-2R2N with a rated current of 5.5A based on the inductance
change and of 4.3A based on the temperature rise.
The capacitor value and ESR determines the amount of output voltage ripple. The TPS54394 is intended for use
with ceramic or other low ESR capacitors. The recommended value range is from 20µF to 68µF. Use Equation 7
to determine the required RMS current rating for the output capacitor(s).
(7)
For this design two TDK C3216X5R0J226M 22µF output capacitors are used. The typical ESR is 2 mΩ each.
The calculated RMS current is 0.19A and each output capacitor is rated for 4A.
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