Datasheet

( )
( )
LOAD RMS
RMS cin
1 1
I I D 1 D 25 A 1 8.3 A
8 8
æ ö
= ´ ´ - = ´ ´ - =
ç ÷
è ø
( )
( ) ( )
IN _ RIPPLE(ESR)
CIN max
1 1
OUT P P
2 2
V
50mV
ESR 1.8m
I I 25A 5.8A
-
= = = W
+ +
OUT OUT
IN(min)
IN _ RIPPLE(CAP) IN(min) SW
I V
25 1.0V
C 41.7 F
V V f 150mV 8 V 500kHz
´
´
= = = m
´ ´ ´ ´
( )
( ) ( )
1 1
OUT(max) RIPPLE
L peak max
2 2
I I I 30 A 5.8A 32.9 A= + ´ = + ´ =
( )
( ) ( )
1 1
OUT(max) RIPPLE CHARGE
L peak
2 2
I I I I 25 A 5.8A 0.25 A 28.2 A= + ´ + = + ´ + =
OUT OUT
CHARGE
SS
V C
1.0 V 500 F
I 0.25 A
t 2ms
´
´ m
= = =
TPS56221
www.ti.com SLUSAH5B –MARCH 2011–REVISED SEPTEMBER 2012
Inductor Peak Current Rating
With output capacitance, it is possible to calculate the charge current during start-up and determine the minimum
saturation current rating for the inductor. The start-up charging current is approximated by Equation 8.
(8)
The peak current in the inductor I
L(peak)
is approximated by Equation 9.
(9)
With the short circuit current trip point I
OUT(max)
set at 32 A, the maximum allowable peak current I
L(peak max)
is
(10)
The selection of output capacitor meets the maximum allowable peak current requirement.
Table 2. Inductor Requirements Summary
PARAMETER VALUE UNITS
L Inductance 320 nH
I
L(rms)
RMS current (thermal rating) 25.1 A
I
L(peak max)
Peak current (saturation rating) 32.9 A
The PA0513.321NLT, 320-nH, 0.32-mΩ, 41-A inductor is selected.
Input Capacitor Selection
The input voltage ripple is divided between capacitance and ESR. For this design V
IN_RIPPLE(CAP)
= 150 mV and
V
IN_RIPPLE(ESR)
= 50 mV. The minimum capacitance and maximum ESR are estimated in Equation 11.
(11)
(12)
The RMS current in the input capacitors is estimated by Equation 13.
(13)
Four 1210, 22-µF, 25-V, X5R ceramic capacitors with about 2.5-mΩ of ESR and a 2.5-A RMS current rating are
selected. Higher voltage capacitors are selected to minimize capacitance loss at the DC bias voltage to ensure
the capacitors will have sufficient capacitance at the working voltage while a 1.0-µF capacitor in smaller case
size is used to reduce high frequency noise from the MOSFET switching.
Boot-Strap Capacitor (C14)
The bootstrap capacitor maintains power to the high-side driver during the high-side switch ON time. Per the
requirements of the integrated MOSFET, C
BOOT
is 100 nF with a minimum 10-V rating.
Boot-Strap Resistor (R2)
The bootstrap resistor slows the rising edge of the SW voltage to reduce ringing and improve EMI. Per the
datasheet recommendation a 5.10-Ω resistor is selected.
RC Snubber (R9 and C18)
To effectively limit the switch node ringing, a 1.0-Ω resistor and a 2200-pF capacitor are selected.
Copyright © 2011–2012, Texas Instruments Incorporated 17
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