Datasheet
R
COMP
= 2Sx R
S
x A
S
x C
OUT
x R
FB2
x f
CROSS
[:@
10
f
CROSS
=
= 23 kHz
f
SW
R
FB2
- 1
R
FB1
V
OUT
0.8V
=
0.47 µF x 1.25V
10 µA
= 59 ms
t
RES
=
0.047 µF x 0.8V
t
SS
=
10 µA
= 3.8 ms
'V
IN
=
4 x 230 kHz x 2.2 PHx 7
9A
= 0.63V
'V
IN
=
4 x x C
IN
f
SW
I
OUT
[V]
LM25117
LM25117-Q1
www.ti.com
SNVS714E –APRIL 2011–REVISED MARCH 2013
(45)
(46)
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.
VIN FILTER R
VIN
, C
VIN
An R-C filter (R
VIN
, C
VIN
) on VIN is optional. The filter helps to prevent faults caused by high frequency switching
noise injection into the VIN pin. 0.47μF ceramic capacitor is used for C
VIN
in the example.
SOFT-START CAPACITOR C
SS
The capacitor at the SS pin (C
SS
) determines the soft-start time (t
SS
), which is the time for the output voltage to
reach the final regulated value. The t
SS
for a given C
SS
can be calculated from Equation 8 as follows:
(47)
For this example, a value of 0.047μF was chosen for a soft-start time of 3.8ms.
RESTART CAPACITOR C
RES
The capacitor at the RES pin (C
RES
) determines t
RES
, which is the time the LM25117 remains off before a restart
attempt is made in hiccup mode current limiting. t
RES
for a given C
RES
can be calculated from Equation 13 as
follows:
(48)
For this example, a value of 0.47μF was chosen for a restart time of 59ms.
OUTPUT VOLTAGE DIVIDER R
FB2
and R
FB1
R
FB1
and R
FB2
set the output voltage level. The ratio of these resistors is calculated as:
(49)
The ratio between R
COMP
and R
FB2
determines the mid-band gain, A
FB_MID
. A larger value for R
FB2
may require a
corresponding larger value for R
COMP
. R
FB2
should be large enough to keep the total divider power dissipation
small. 3.24kΩ was chosen for R
FB2
in this example, which results in a R
FB1
value of 1.05kΩ for 3.3V output.
LOOP COMPENSATION COMPONENTS C
COMP
, R
COMP
and C
HF
R
COMP
, C
COMP
and C
HF
configure the error amplifier gain and phase characteristics to produce a stable voltage
loop. For a quick start, follow the 4 steps listed below. For detailed information, see Application Information.
STEP1: Select f
CROSS
By selecting one tenth of the switching frequency, f
CROSS
is calculated as follows:
(50)
STEP2: Determine required R
COMP
Knowing f
CROSS
, R
COMP
is calculated as follows:
(51)
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