Datasheet

- 1
R9
R8
1.23V
V
OUT
=
t
SS
=
10 PA
C16 x 1.23V
I
RMS(BUCK-BOOST)
=
1 - D
I
OUT
D(1 -D)
I
RMS(BUCK)
= I
OUT
D(1 -D)
LM5118, LM5118-Q1
SNVS566H –APRIL 2008–REVISED JANUARY 2014 www.ti.com
C1 - C5 = Input Capacitor
A typical 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 buck switch on-time. When the buck switch turns on, the current into the buck switch steps from zero
to the lower peak of the inductor current waveform, then ramps up to the peak value, and then drops to the zero
at turn-off. The RMS current rating of the input capacitors depends on which mode of operation is most critical.
(23)
This value is a maximum at 50% duty cycle which corresponds to VIN = 24 V.
(24)
Checking both modes of operation we find:
• I
RMS(BUCK)
= 1.5 A
• I
RMS(BUCK-BOOST)
= 4.7 A
Therefore C1-C5 should be sized to handle 4.7 A of ripple current. Quality ceramic capacitors with a low ESR
should be selected. To allow for capacitor tolerances, five 2.2 µF, 100 V ceramic capacitors will be used. If step
input voltage transients are expected near the maximum rating of the LM5118, a careful evaluation of the ringing
and possible spikes at the device VIN pin should be completed. An additional damping network or input voltage
clamp may be required in these cases.
C20
The capacitor at the VCC pin provides noise filtering and stability for the VCC regulator. The recommended value
of C20 should be no smaller than 0.1 µF, and should be a good quality, low ESR, ceramic capacitor. A value of 1
µF was selected for this design. C20 should be 10 x C8.
If operating without VCCX, then
f
OSC
x (Q
C
Buck + Boost) + I
LOAD(INTERNAL)
(25)
must be less than the VCC current limit.
C8
The bootstrap capacitor between the HB and HS pins supplies the gate current to charge the buck switch gate at
turn-on. The recommended value of C8 is 0.1 µF to 0.47 µF, and should be a good quality, low ESR, ceramic
capacitor. A value of 0.1 µF was chosen for this design.
C16 = C
SS
The capacitor at the SS pin determines the soft-start time, i.e. the time for the reference voltage and the output
voltage, to reach the final regulated value. The time is determined from:
(26)
and assumes a current limit>Iload + ICout
For this application, a C16 value of 0.1 µF was chosen which corresponds to a soft-start time of about 12 ms.
R8, R9
R8 and R9 set the output voltage level, the ratio of these resistors is calculated from:
(27)
For a 12 V output, the R8/R9 ratio calculates to 8.76. The resistors should be chosen from standard value
resistors and a good starting point is to select resistors within power ratings appropriate for the output voltage.
Values of 309 Ω for R9 and 2.67 kΩ for R8 were selected.
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