Datasheet

'V
OUT
ESR
MAX
=
I
PEAK
I
OUT
x D
MAX
C
MIN
=
f x 'V
OUT
V
OUT
V
IN2
+ V
OUT
With D
MAX
=
C15 = C
RAMP
=
L x 10
-6
2 x R
SENSE
R13
(BUCK-BOOST)
=
2.5V
10 x I
PEAK
LM5118, LM5118-Q1
www.ti.com SNVS566G –APRIL 2008–REVISED FEBRUARY 2013
(13)
R13
(BUCK-BOOST)
= 18.7 mΩ
A R
SENSE
value of no more than 18.7 mΩ must be used to ensure the required maximum output current in the
buck-boost mode. A value of 15 mΩ was selected for component tolerances and is a standard value.
R13 = 15 mΩ
C15 = C
RAMP
With the inductor value selected, the value of C3 necessary for the emulation ramp circuit is:
(14)
With the inductance value (L1) selected as 10 µH, the calculated value for C
RAMP
is 333 pF. A standard value of
330 pF was selected.
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:
(15)
ESR requirements can be estimated from:
(16)
For this example, with a ΔVOUT (output ripple) of 50 mV,
C
MIN
= 141 µF
ESR
MAX
= 3.8 mΩ
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 VOUT plus
some margin. Since D1 only conducts during the buck switch off time in either mode, the current rating required
is:
I
DIODE
= I
OUT
x (1-D) Buck Mode (17)
I
DIODE
= I
OUT
Buck-Boost Mode (18)
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