Datasheet

'i
L
'V
OUT
= 'i
L
12
I
Cout(RMS)
=
1
R
ESR
2
+
8f
SW
C
O
2
L
O
=
'i
L
f
SW
V
OUT
(1-D)
0.3I
OUT
f
SW
#
V
OUT
(1-D)
R
FB1
=
2SC
COMP
f
LC
1
R
COMP
=
2SC
COMP
f
ESR
1
LM2854
SNVS560C –MARCH 2008–REVISED APRIL 2013 www.ti.com
(15)
Note that the lower feedback resistor, R
FB2
, has no impact on the control loop from an AC standpoint since the
FB pin is the input to an error amplifier and effectively at AC ground. Hence, the control loop can be designed
irrespective of output voltage level. The only caveat here is the necessary derating of the output capacitance with
applied voltage. Having chosen R
FB1
as above, R
FB2
is then selected for the desired output voltage.
Table 1 and Table 2 list inductor and ranges of capacitor values that work well with the LM2854, along with the
associated compensation components to ensure stable operation. Values different than those listed may be
used, but the compensation components may need to be recalculated to avoid degradation in phase margin.
Note that the capacitance ranges specified refer to in-circuit values where the nominal capacitance value is
adequately derated for applied voltage.
FILTER INDUCTOR AND OUTPUT CAPACITOR SELECTION
In a buck regulator, selection of the filter inductor and capacitor will affect many key system parameters,
including stability, transient response and efficiency The LM2854 can accommodate relatively wide ranges of
output capacitor and filter inductor values in a typical application and still achieve excellent load current transient
performance and low output voltage ripple.
The inductance is chosen such that the peak-to-peak inductor current ripple, Δi
L
, is approximately 25 to 40% of
I
OUT
as follows
(16)
Note that the peak inductor current is the DC output current plus half the ripple current and reaches its highest
level at lowest duty cycle (or highest V
IN
). It is recommended that the inductor should have a saturation current
rating in excess of the current limit level.
When operating the LM2854 at input voltages above 5.2V, the inductor should be sized to keep the minimum
inductor current above -0.5A. For most applications this should only occur at light loads or when the inductor is
drastically undersized. To ensure the current never goes below -0.5A for any application, the peak-to-peak ripple
current (Δi
L
) in the inductor should be less than 1A. Keeping the minimum inductor current above -0.5A limits the
energy storage in the inductor and helps prevent the switch node voltage from exceeding the absolute maximum
specification when the low side FET turns off.
Table 3 lists examples of off-the-shelf powdered iron and ferrite based inductors that are suitable for use with the
LM2854. The output capacitor can be of ceramic or electrolytic chemistry. The chosen output capacitor requires
sufficient DC voltage rating and RMS ripple current handling capability.
The output capacitor RMS current and peak-to-peak output ripple are given respectively by
(17)
In general, 22 µF to 100 µF of ceramic output capacitance is sufficient for both LM2854 frequency options given
the optimal high frequency characteristics and low ESR of ceramic dielectric. It is advisable to consult the
manufacturer’s derating curves for capacitance voltage coefficient as the in-circuit capacitance may drop
significantly with applied voltage.
Tantalum or organic polymer electrolytic capacitance may be suitable with the LM2854 500 kHz option,
particularly in applications where substantial bulk capacitance per unit volume is required. However, the high
loop bandwidth achievable with the LM2854 obviates the necessity for large bulk capacitance during transient
loading conditions.
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