Datasheet

R
IN
+ ESR
+
G =
Z
S
2
1
Z
IN
Z
S
P
OUT
V
IN
2
Z
IN
= -
Z
S
=
f
S
=
1
2S
L
IN
x C
IN
L
IN
C
IN
4 x f
SW
x C
IN
I
OUT
'V
IN
=
=
4 x 250 kHz x 7 PF
7A
= 1V
'V
OUT
= 3A x
1
8 x 250 kHz x 320
0.4 m:
2
+
2
PF
'V
OUT
= 4.8 mV
'V
OUT
= I
PP
x
1
8 x f
SW
x C
OUT
ESR
2
+
€
2
LM25116
SNVS509D –APRIL 2007–REVISED FEBRUARY 2013 www.ti.com
OUTPUT CAPACITORS
The output capacitors smooth the inductor ripple current and provide a source of charge for transient loading
conditions. For this design example, five 100 µF ceramic capacitors where selected. Ceramic capacitors provide
very low equivalent series resistance (ESR), but can exhibit a significant reduction in capacitance with DC bias.
From the manufacturer’s data, the ESR at 250 kHz is 2 mΩ / 5 = 0.4 mΩ, with a 36% reduction in capacitance at
5V. This is verified by measuring the output ripple voltage and frequency response of the circuit. The
fundamental component of the output ripple voltage is calculated as:
(15)
With typical values for the 5V design example:
(16)
INPUT CAPACITORS
The regulator supply voltage has a large 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 switch steps to the valley of the inductor
current waveform, ramps up to the peak value, and then drops to zero at turn-off. The input capacitors should be
selected for RMS current rating and minimum ripple voltage. A good approximation for the required ripple current
rating is I
RMS
> I
OUT
/ 2.
Quality ceramic capacitors with a low ESR were selected for the input filter. To allow for capacitor tolerances and
voltage rating, four 2.2 µF ceramic capacitors were used for the typical application circuit. With ceramic
capacitors, the input ripple voltage will be triangular and peak at 50% duty cycle. Taking into account the
capacitance change with DC bias, the input ripple voltage is approximated as:
(17)
When the converter is connected to an input power source, a resonant circuit is formed by the line impedance
and the input capacitors. If step input voltage transients are expected near the maximum rating of the LM25116,
a careful evaluation of the ringing and possible overshoot at the device VIN pin should be completed. To
minimize overshoot make C
IN
> 10 x L
IN
. The characteristic source impedance and resonant frequency are:
(18)
The converter exhibits a negative input impedance which is lowest at the minimum input voltage:
(19)
The damping factor for the input filter is given by:
where
• R
IN
is the input wiring resistance and ESR is the series resistance of the input capacitors. The term Z
S
/ Z
IN
will
always be negative due to Z
IN
. (20)
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