TPS54428 Datasheet, PDF (12/22 Pages) Texas Instruments – 4.5V to 18 V Input, 4-A Synchronous Step-Down SWIFT™ Converter with Eco-Mode™

TPS54428
SLVSB42A – NOVEMBER 2011 – REVISED JANUARY 2012
www.ti.com
Output Voltage (V)
1
1.05
1.2
1.5
1.8
2.5
3.3
5
6.5
R1 (kΩ)
6.81
8.25
12.7
21.5
30.1
49.9
73.2
124
165
Table 1. Recommended Component Values
R2 (kΩ)
C4 (pF)⁽¹⁾
L1 (µH)
22.1
1.5
22.1
1.5
22.1
1.5
22.1
1.5
22.1
5 - 22
2.2
22.1
5 - 22
2.2
22.1
5 - 22
2.2
22.1
5 - 22
3.3
22.1
5 - 22
3.3
C8 + C9 (µF)
22 - 68
22 - 68
22 - 68
22 - 68
22 - 68
22 - 68
22 - 68
22 - 68
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
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 650 kHz for
f_SW.
Use 650 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.
I l_P-P
=
V_{O UT}
V_{IN(MA X)}
´
V_IN(MAX) - V_OUT
L_O ´ f_SW
(5)
I l_{PEA K}
= I_O
+
Il_P-P
2
(6)
I_{L O(RMS)} =
I_O2
+
1
12
Il_P
-P
2
(7)
For this design example, the calculated peak current is 4.51 A and the calculated RMS current is 4.01 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 TPS54428 is intended for use
with ceramic or other low ESR capacitors. Recommended values range from 22µF to 68µF. Use Equation 8 to
determine the required RMS current rating for the output capacitor.
( ) _{ICO(RMS) =}
V_Ox ´ V_IN - V_{O UT}
12 ´ V_IN ´ L_O ´ f_SW
(8)
For this design two TDK C3216X5R0J226M 22µF output capacitors are used. The typical ESR is 2 mΩ each.
The calculated RMS current is 0.286A and each output capacitor is rated for 4A.
Input Capacitor Selection
The TPS54428 requires an input decoupling capacitor and a bulk capacitor is needed depending on the
application. A ceramic capacitor over 10µF is recommended for the decoupling capacitor. An additional 0.1 µF
capacitor from pin 8 to ground is optional to provide additional frequency filtering. The capacitor voltage rating
needs to be greater than the maximum input voltage.
Bootstrap Capacitor Selection
A 0.1µF ceramic capacitor must be connected between the VBST to SW pin for proper operation. It is
recommended to use a ceramic capacitor.
12
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