Datasheet

'V
IN
=
f
SW
Cin1
I
OUT
D(1-D)
500 kHz 24 PF
=
4A 0.41 (1 - 0.41)
= 80 mV
I
Cin(RMS)
= 4A
0.41 (1 - 0.41)
= 1.97A
I
Cin(RMS)
= I
OUT
D(1-D)
'V
OUT
= 1.25A
1
(3 m:)
2
+
8 500 kHz 60 PF
2
= 6.4 mV
'V
OUT
= 'i
L
1
R
ESR
2
+
8f
SW
C
O
1
2
www.ti.com Component Selection
7.2 Output Capacitor
The output capacitance and its equivalent series resistance (ESR) affect both the ripple voltage at the
output and the overall stability of the loop. The output capacitor provides a low impedance path for the
inductor ripple current and presents a source of charge for transient loading conditions.
In this example, one 100 µF 1210 multi-layer ceramic capacitor (MLCC) was selected. Ceramic capacitors
provide very low ESR but can exhibit a significant reduction in capacitance with applied DC bias. Using
manufacturer’s data, the ESR at 500 kHz is 3 mΩ and there is approximately 40% reduction in
capacitance at 1.2V. This is verified by measuring the output ripple voltage and frequency response of the
circuit. The fundamental component of the output ripple voltage amplitude is calculated as
(6)
and with typical values from this example
(7)
Because the load could transition quickly from no load to full load, it is sometimes common to add output
bulk capacitance in the form of aluminum electrolytic (Al-E), tantalum (Ta), solid aluminum, organic
polymer, and niobium (Nb) capacitors. This is largely unnecessary with the LM2854 as the loop crossover
frequency can be made sufficiently large to accommodate high di/dt load transients.
7.3 Input Filter
The necessary RMS current rating of the input capacitor can be estimated by the following equation
(8)
From this equation, it follows that the maximum RMS current will occur at full 4A load current with the
system operating at 50% duty cycle.
However, with the specified output voltage, the worst case occurs at minimum input voltage of 2.95V.
Hence, the relevant duty cycle is 0.41 and the maximum RMS current is given by
(9)
Ceramic capacitors feature a very large RMS current rating in a small footprint making them ideal for this
application. Choosing a 47 µF 10V ceramic capacitor to provide the necessary input capacitance and
assuming 50% capacitance voltage coefficient, the input AC ripple amplitude, neglecting ESR, is
(10)
When operating near the minimum input voltage, an electrolytic input capacitor is helpful to damp the input
for a typical bench test setup. Essentially, a resonant circuit is formed by the line impedance and input
capacitance. To this end, a second input capacitor, Cin2, is provided on the evaluation board to
accommodate such electrolytic capacitance. The 6TPE150MIC2 by Sanyo has 150 µF capacitance and an
ESR of 18 mΩ. The associated ESR is stable relative to temperature, and capacitance change is relatively
immune to bias voltage. The ripple current will split between the ceramic and electrolytic capacitors based
on the relative impedance at the switching frequency.
For improved performance, an 0603 1 µF ceramic AVIN filter capacitor is placed adjacent to the AVIN pin
and referenced to AGND. Together with a 1Ω series resistor from PVIN (optional), this small capacitor
helps to filter high frequency noise spikes on the supply rail and prevent these pulses from disturbing the
analog control circuitry of the chip.
7.4 Soft-Start Capacitor
A 10 nF soft-start capacitor has been chosen to provide a soft-start time of roughly 4 ms. This will allow
the LM2854 to start up gracefully without triggering over-current protection irrespective of operating
conditions.
5
SNVA323B–March 2008–Revised May 2013 AN-1786 LM2854 500 kHz Buck Regulator Evaluation Board
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