lm25117psqx National Semiconductor Corporation, lm25117psqx Datasheet - Page 25

• Manufacturer Part Number: lm25117psqx
• Description: Wide Input Range Synchronous Buck Controller With Analog Current Monitor
• Manufacturer: National Semiconductor Corporation

OUTPUT CAPACITOR C
The output capacitors smooth the output voltage ripple
caused by inductor ripple current and provide a source of
charge during transient loading conditions. For this design
example, a 680μF electrolytic capacitor with maximum
10mΩ ESR was selected as the main output capacitor. The
fundamental component of the output ripple voltage with max-
imum ESR is approximated as:
Additional low ERS / ESL ceramic capacitors can be placed
in parallel with the main output capacitor to further reduce the
output voltage ripple and spikes. In this example, two 22μF
capacitors were added.
INPUT CAPACITOR C
The regulator input supply voltage typically has high source
impedance at the switching frequency. Good quality input ca-
pacitors are necessary to limit the ripple voltage at the VIN
pin while supplying most of the switch current during the on-
time. When the high-side NMOS device turns on, the current
into the device steps to the valley of the inductor current
waveform, ramps up to the peak value, and then drops to the
zero at turnoff. The input capacitor should be selected for
RMS current rating and minimum ripple voltage. A good ap-
proximation for the required ripple current rating necessary is
I
In this example, seven 2.2μF ceramic capacitors were used.
With ceramic capacitors, the input ripple voltage will be trian-
gular. The input ripple voltage can be approximated as:
Capacitors connected in parallel should be evaluated for RMS
current rating. The current will split between the input capac-
itors based on the relative impedance of the capacitors at the
switching frequency.
VIN FILTER R
An R-C filter (R
prevent faults caused by high frequency switching noise in-
RMS
> I
OUT
/ 2.
VIN
VIN
, C
, C
VIN
VIN
IN
) on VIN is optional. The filter helps to
O
25
jection into the VIN pin. 0.47μF ceramic capacitor is used for
C
SOFT-START CAPACITOR C
The capacitor at the SS pin (C
time (t
final regulated value. The t
from equation (8) as follows:
For this example, a value of 0.047μF was chosen for a soft-
start time of 3.8ms.
RESTART CAPACITOR C
The capacitor at the RES pin (C
is the time the LM25117 remains off before a restart attempt
is made in hiccup mode current limiting. t
C
For this example, a value of 0.47μF was chosen for a restart
time of 59ms.
OUTPUT VOLTAGE DIVIDER R
R
resistors is calculated as:
The ratio between R
gain, A
sponding larger value for R
to keep the total divider power dissipation small. 3.24kΩ was
chosen for R
of 1.05kΩ for 3.3V output.
LOOP COMPENSATION COMPONENTS C
and C
R
phase characteristics to produce a stable voltage loop. For a
quick start, follow the 4 steps listed below. For detailed infor-
mation, see Application Information.
STEP1: Select f
By selecting one tenth of the switching frequency, f
calculated as follows:
VIN
RES
FB1
COMP
in the example.
and R
can be calculated from equation (13) as follows:
SS
HF
, C
FB_MID
), which is the time for the output voltage to reach the
COMP
FB2
FB2
. A larger value for R
set the output voltage level. The ratio of these
and C
in this example, which results in a R
CROSS
COMP
HF
configure the error amplifier gain and
SS
and R
COMP
RES
for a given C
SS
. R
SS
FB2
RES
FB2
) determines the soft-start
FB2
determines the mid-band
FB2
) determines t
and R
should be large enough
may require a corre-
SS
FB1
can be calculated
RES
COMP
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for a given
, R
RES
FB1
CROSS
COMP
, which
value
is