LM5117PMH/NOPB National Semiconductor, LM5117PMH/NOPB Datasheet - Page 25

• Manufacturer Part Number: LM5117PMH/NOPB
• Description: IC BUCK CONTROLLER 20-TSSOP
• Manufacturer: National Semiconductor
• Series: -r

Specifications of LM5117PMH/NOPB

• Pwm Type: Current Mode
• Number Of Outputs: 1
• Frequency - Max: 530kHz
• Duty Cycle: 95%
• Voltage - Supply: 5.5 V ~ 65 V
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 20-TSSOP (0.173", 4.40mm Width) Exposed Pad
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

example, a 470μF electrolytic capacitor with maximum
20mΩ 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 3.3μF 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-
jection into the VIN pin. 0.47μF ceramic capacitor is used for
C
RMS
VIN
> I
in the example. R
OUT
/ 2.
VIN
VIN
, C
, C
VIN
VIN
VIN
IN
) on VIN is optional. The filter helps to
is selected to be 3.9Ω
25
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 0.1μF was chosen for a soft-start
time of 8ms.
RESTART CAPACITOR C
The capacitor at the RES pin (C
is the time the LM5117 remains off before a restart attempt is
made in hiccup mode current limiting. t
can be calculated from equation (13) as follows:
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. 4.99kΩ was
chosen for R
of 357Ω for 12V 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:
FB1
COMP
and R
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-
RES
SS
FB1
can be calculated
for a given C
COMP
www.national.com
, R
RES
FB1
CROSS
COMP
, which
value
RES
is