Datasheet

2S L
IN
x C
IN
1
f
S
=
Z
S
=
L
IN
C
IN
'V
IN
=
4 x 230 kHz x 15.4 PF
8A
= 0.565V
'V
IN
=
4 x x C
IN
SW
´
¶
I
OUT
'V
OUT
= 19.3 mV
1
2
¸
¹
·
¨
©
§
8 x 230 kHz x 680 PF
'
V
OUT
= 1.92A x 0.01:
2
+
1
2
¸
¹
·
¨
©
§
8 x x C
OUT
SW
´
¶
'V
OUT
= I
PP
x ESR
2
+
LM25119/25119Q
www.ti.com SNVS680G –AUGUST 2010–REVISED JANUARY 2014
OUTPUT CAPACITORS
The output capacitors smooth the inductor ripple current and provide a source of charge during transient loading
conditions. For this design example, a 680 µF electrolytic capacitor with 10 mΩ ESR was selected as the main
output capacitor. The fundamental component of the output ripple voltage is approximated as:
(21)
(22)
(23)
Two 22 µF low ERS / ESL ceramic capacitors are placed in parallel with the 680 µF electrolytic capacitor, to
further reduce the output voltage ripple and spikes.
Table 1. Performance Variation by K Factor
K < 1 1 <— K —> 3 K > 3
Cross Talk Higher Lower
Peak Inductor Current with Short Output Lower Higher
Introduces additional
Sub-harmonic
Condition
pole near cross-over
oscillation may occur
frequency
Inductor Size Smaller Larger
Power Dissipation of Rs Higher Lower
Efficiency Lower Higher
INPUT CAPACITORS
The regulator input supply voltage typically has high source impedance at the switching frequency. Good quality
input capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the switch current
during the on-time. When the buck switch turns on, the current into the buck switch steps to the valley of the
inductor current waveform, ramps up to the peak value, and then drops to the zero at turn-off. The input
capacitance should be selected for RMS current rating and minimum ripple voltage. A good approximation for the
required ripple current rating necessary is I
RMS
> I
OUT
/ 2. Seven 2.2 μF ceramic capacitors were used for each
channel. With ceramic capacitors, the input ripple voltage will be triangular. The input ripple voltage with one
channel operating is approximately:
(24)
(25)
The ripple voltage of the input capacitors will be reduced significantly with dual channel operation since each
channel operates 180 degrees out of phase from the other. Capacitors connected in parallel should be evaluated
for RMS current rating. The current will split between the input capacitors based on the relative impedance of the
capacitors at the switching frequency.
When the converter is connected to an input power source, a resonant circuit is formed by the line inductance
and the input capacitors. To minimize overshoot make C
IN
> 10 x L
IN
. The characteristic source impedance (Z
S
)
and resonant frequency (f
S
) are:
(26)
(27)
Where L
IN
is the inductance of the input wire. The converter exhibits negative input impedance which is lowest at
the minimum input voltage:
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