Datasheet

( )
( )
= ´ ´ - = ´ ´ - =
LOAD
RMS cin
I I D 1 D 6 A 0.5 (1 0.5) 3.0 A
= = = W
+ ´
RIPPLE(esr)
MAX
1
LOAD RIPPLE
2
V
100mV
ESR 14.4m
I I 6.93A
´
´
= = = m
´ ´ ´ ´
LOAD OUT
IN(min)
RIPPLE(cap) IN SW
I V
6 A 5 V
C 25 F
V V f 400mV 10 V 300kHz
( )
( )
= + ´ + = + ´ + =
1 1
OUT(max) RIPPLE CHARGE
L peak
2 2
I I I I 6 A 1.86 A 0.08 A 7.01A
( )
´ ´ m + ´ m
´
= = =
OUT OUT
CHARGE
SS
5 V 2 22 F 2 10 F
V C
I 0.08 A
t 4ms
æ ö
æ ö
-
-
ç ÷
ç ÷
-
´ ´
´ m ´
è ø è ø
= = = = W
RIPPLE
RIPPLE(tot)
RIPPLE(tot) RIPPLE(cap)
OUT SW
MAX
RIPPLE RIPPLE
I
1.86 A
V
100mV
V V
8 C f
8 59 F 300kHz
ESR 47m
I I 1.86 A
( )
( )
( )
´
´ m
= = = m
´ ´
2
2
TRAN(max)
OUT min
OUT OVER
I L
3 8.2 H
C 59 F
V V 5 250mV
( )
( )
( )
´
´
< ´ D = ´ =
- - ´
2
TRAN
TRAN TRAN TRAN
UNDER
OUT OUT
IN OUT IN OUT OUT
I L
I I I L
V T
C C
V V V V C
TPS40170
SLUS970A –MARCH 2011–REVISED NOVEMBER 2013 www.ti.com
(22)
If V
IN(min)
> 2 x V
OUT
, use overshoot to calculate minimum output capacitance. If V
IN(min)
< 2 x V
OUT
, use
undershoot to calculate minimum output capacitance.
(23)
With a minimum capacitance, the maximum allowable ESR is determined by the maximum ripple voltage and is
approximated Equation 24.
(24)
Two 1210, 22-µF, 16-V X7R ceramic capacitors plus two 0805 10-µF, 16-V X7R ceramic capacitors are selected
to provide more than 59 µF of minimum capacitance (including tolerance and DC bias derating) and less than
47 mΩ of ESR (parallel ESR of approximately 4 mΩ).
Peak Current Rating of Inductor
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 in Equation 25.
(25)
(26)
An IHLP5050FDER8R2M01 8.2 µH is selected. This 10-A, 16-mΩ inductor exceeds the minimum inductor ratings
in a 13 mm × 13 mm package.
Input Capacitor Selection (C1, C6)
The input voltage ripple is divided between capacitance and ESR. For this design VRIPPLE(cap) = 400mV and
VRIPPLE(ESR) = 100mV. The minimum capacitance and maximum ESR are estimated by:
(27)
(28)
The RMS current in the input capacitors is estimated in Equation 29.
(29)
To achieve these values, four 1210, 2.2-µF, 100 V, X7R ceramic capacitors plus a 120-µF electrolytic capacitor
are combined at the input. This provides a smaller size and overall cost than 10 ceramic input capacitors or an
electrolytic capacitor with the ESR required.
Table 2. Inductor Summary
PARAMETER VALUE UNIT
L Inductance 8.2 µH
I
L(rms)
RMS current (thermal rating) 6.02 A
I
L(peak)
Peak current (saturation rating) 7.01 A
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