Datasheet

I
RMS(BUCK-BOOST)
=
1 - D
I
OUT
D(1 -D)
I
RMS(BUCK)
= I
OUT
D(1 -D)
LM5118, LM5118-Q1
SNVS566G –APRIL 2008–REVISED FEBRUARY 2013 www.ti.com
D4
A Schottky type recirculating diode is required for all LM5118 applications. The near ideal reverse recovery
characteristics and low forward voltage drop are particularly important diode characteristics for high input voltage
and low output voltage applications. The reverse recovery characteristic determines how long the current surge
lasts each cycle when the buck switch is turned on. The reverse recovery characteristics of Schottky diodes
minimize the peak instantaneous power in the buck switch during the turn-on transition. The reverse breakdown
rating of the diode should be selected for the maximum VIN plus some safety margin.
The forward voltage drop has a significant impact on the conversion efficiency, especially for applications with a
low output voltage. “Rated” current for diodes vary widely from various manufacturers. For the LM5118 this
current is user selectable through the current sense resistor value. Assuming a worst case 0.6V drop across the
diode, the maximum diode power dissipation can be high. The diode should have a voltage rating of VIN and a
current rating of IOUT. A conservative design would at least double the advertised diode rating since
specifications between manufacturers vary. For the reference design a 100V, 10A Schottky in a D2PAK package
was selected.
C1 - C5 = INPUT CAPACITORS
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.
(19)
This value is a maximum at 50% duty cycle which corresponds to VIN = 24 volts.
(20)
Checking both modes of operation we find:
I
RMS(BUCK)
= 1.5 Amps
I
RMS(BUCK-BOOST)
= 4.7 Amps
Therefore C1-C5 should be sized to handle 4.7A of ripple current. Quality ceramic capacitors with a low ESR
should be selected. To allow for capacitor tolerances, four 2.2 µF, 100V 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)
(21)
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.
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