TPS54339 Datasheet, PDF (12/21 Pages) Texas Instruments – 4.5 V to 23 V Input, 3-A Synchronous Step-Down SWIFT Converter

TPS54339
SLVSBT2 – JANUARY 2013
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 TPS54339. 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 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 R1 (kΩ) R2 (kΩ)
(V)
Min
C4 (pF)⁽¹⁾
Typ
Max
L1 (µH)
Min
Typ
Max
1
6.81 22.1
5
150
220
1.5
2.2
4.7
1.05
8.25 22.1
5
150
220
1.5
2.2
4.7
1.2
12.7 22.1
5
100
1.5
2.2
4.7
1.5
21.5 22.1
5
68
1.5
2.2
4.7
1.8
30.1 22.1
5
22
2.2
3.3
4.7
2.5
49.9 22.1
5
22
2.2
3.3
4.7
3.3
73.2 22.1
5
22
2.2
3.3
4.7
5
124 22.1
5
22
3.3
4.7
6.5
165 22.1
5
22
3.3
4.7
C8 + C9 + C10 (µF)
Min
Max
22
68
22
68
22
68
22
68
22
68
22
68
22
68
22
68
22
68
(1) Optional
Since the DC gain is dependent on the output voltage, the required inductor value increases as the output
voltage increases. Additional phase boost can be achieved by adding a feed forward capacitor (C4) in parallel
with R1. The amount of available phase boost is dependent on the output voltage. Higher output voltages will
allow greater phase boost.
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. Use 600 kHz for
f_SW.
Use 600 kHz for f_SW. Make sure the chosen inductor is rated for the peak current of Equation 5 and the RMS
current of Equation 6.
Il_P-P
=
V_OUT
V_IN(MAX)
´
V_IN(MAX) - V_OUT
L_O ´ ¦_SW
(4)
Il_PEAK
=I_O
+
Il_P-P
2
(5)
I_LO(RMS) =
I_O2
+
1
12
Il_P-P2
(6)
For this design example, the calculated peak current is 3.38 A and the calculated RMS current is 3.01 A. The
inductor used is a TDK CLF7045T-2R2N with a peak current rating of 5.5 A and an RMS current rating of 4.3 A.
For high current designs, TDK SPM6530T-4R7M 4.7µH is also recommended. The SPM6530 series has a higher
current rating than the CLF7045 series.
The capacitor value and ESR determines the amount of output voltage ripple. The TPS54339 is intended for use
with ceramic or other low ESR capacitors. Recommended values range from 22µF to 68µF. Use Equation 7 to
determine the required RMS current rating for the output capacitor.
I_Co(RMS) =
V_OUT x (V_IN - V_OUT )
12 x V_IN x L_O x f_SW
(7)
For this design two TDK C3216X5R0J226M 22µF output capacitors are used. The typical ESR is 2 mΩ each.
The calculated RMS current is 0.22 A and each output capacitor is rated for 4A.
12
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