Datasheet

OUT
MAX
OUT IN(MIN) RIPPLE(BUCK-BOOST)
OUT
IN(MIN)
V
ESR
V V I
I
V 2
'
OUT MAX OUT
MIN MAX
OUT IN(MIN) OUT
I D V
C with D
f V V V
u
u '
LM5118, LM5118-Q1
www.ti.com
SNVS566H APRIL 2008REVISED JANUARY 2014
C9 - C12 = Output Capacitors
In buck-boost mode, the output capacitors C9 - C12 must supply the entire output current during the switch on-
time. For this reason, the output capacitors are chosen for operation in buck-boost mode, the demands being
much less in buck operation. Both bulk capacitance and ESR must be considered to ensure a given output ripple
voltage. Buck-boost mode capacitance can be estimated from:
(21)
ESR requirements can be estimated from:
(22)
For this example, with a ΔVOUT (output ripple) of 50 mV,
C
MIN
= 141 µF
ESR
MAX
= 4.6 m
If hold-up times are a consideration, the values of input/output capacitors must be increased appropriately. Note
that it is usually advantageous to use multiple capacitors in parallel to achieve the ESR value required. Also, it is
good practice to put a .1 µF - .47 µF ceramic capacitor directly on the output pins of the supply to reduce high
frequency noise. Ceramic capacitors have good ESR characteristics, and are a good choice for input and output
capacitors. It should be noted that the effective capacitance of ceramic capacitors decreases with dc bias. For
larger bulk values of capacitance, a low ESR electrolytic is usually used. However, electrolytic capacitors have
poor tolerance, especially over temperature, and the selected value should be selected larger than the calculated
value to allow for temperature variation. Allowing for component tolerances, the following values of COUT were
chosen for this design example:
Two 180 µF Oscon electrolytic capacitors for bulk capacitance
Two 47 µF ceramic capacitors to reduce ESR
Two 0.47 µF ceramic capacitors to reduce spikes at the output.
D1
Reverse recovery currents degrade performance and decrease efficiency. For these reasons, a Schottky diode of
appropriate ratings should be used for D1. The voltage rating of the boost diode should be equal to V
OUT
plus
some margin. D1 conducts continually in buck mode and only when the buck switch is off in Buck-Boost mode.
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 0.6 V 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 100 V, 10 A Schottky in a D2PAK
package was selected.
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