LTC3852
20
3852f
C
OUT
Selection
The selection of C
OUT
is primarily determined by the effec-
tive series resistance, ESR, to minimize voltage ripple. The
output ripple, DV
OUT
, in continuous mode is determined by:
ΔV
OUT
≅ΔI
L
ESR +
1
8fC
OUT
⎛
⎝
⎜
⎞
⎠
⎟
where f = operating frequency, C
OUT
= output capaci tance
and DI
L
= ripple current in the inductor. The output ripple
is highest at maximum input voltage since DI
L
increases
with input voltage. Typically, once the ESR requirement
for C
OUT
has been met, the RMS current rating gener-
ally far exceeds the I
RIPPLE(P-P)
requirement. With DI
L
=
0.3I
OUT(MAX)
and allowing 2/3 of the ripple to be due to
ESR, the output ripple will be less than 50mV at maximum
V
IN
and:
C
OUT
Required ESR < 2.2R
SENSE
C
OUT
>
1
8fR
SENSE
The fi rst condition relates to the ripple current into the ESR
of the output capacitance while the second term guaran tees
that the output capacitance does not signifi cantly discharge
during the operating frequency period due to ripple current.
The choice of using smaller output capaci tance increases
the ripple voltage due to the discharging term but can be
compensated for by using capacitors of very low ESR to
maintain the ripple voltage at or below 50mV. The I
TH
pin
OPTI-LOOP compensation compo nents can be optimized
to provide stable, high perfor mance transient response
regardless of the output capaci tors selected.
The selection of output capacitors for applications with
large load current transients is primarily determined by the
voltage tolerance specifi cations of the load. The resistive
component of the capacitor, ESR, multiplied by the load
current change, plus any output voltage ripple must be
within the voltage tolerance of the load.
The required ESR due to a load current step is:
R
ESR
≤
ΔV
ΔI
APPLICATIONS INFORMATION
where
D
I is the change in current from full load to zero load
(or minimum load) and
D
V is the allowed voltage devia-
tion (not including any droop due to fi nite capacitance).
The amount of capacitance needed is determined by the
maximum energy stored in the inductor. The capacitance
must be suffi cient to absorb the change in inductor
current when a high current to low current transition
occurs. The opposite load current transition is generally
determined by the control loop OPTI-LOOP components,
so make sure not to over compensate and slow down
the response. The minimum capacitance to assure the
inductors’ energy is adequately absorbed is:
C
OUT
>
L ΔI
()
2
2 ΔV
()
V
OUT
where DI is the change in load current.
Manufacturers such as Nichicon, United Chemi-Con and
Sanyo can be considered for high performance through-
hole capacitors. The OS-CON semiconductor electrolyte
capacitor available from Sanyo has the lowest (ESR)(size)
product of any aluminum electrolytic at a somewhat
higher price. An additional ceramic capacitor in parallel
with OS-CON capacitors is recommended to reduce the
inductance effects.
In surface mount applications, ESR, RMS current han dling
and load step specifi cations may require multiple capacitors
in parallel. Aluminum electrolytic, dry tantalum and
special polymer capacitors are available in surface mount
packages. Special polymer surface mount capaci tors offer
very low ESR but have much lower capacitive density per
unit volume than other capacitor types. These capacitors
offer a very cost-effective output capacitor solution and are
an ideal choice when combined with a controller having
high loop bandwidth. Tantalum capaci tors offer the highest
capacitance density and are often used as output capacitors
for switching regulators having controlled soft-start.
Several excellent surge-tested choices are the AVX TPS,
AVX TPSV or the KEMET T510 series of surface mount
tantalums, available in case heights rang ing from 1.5mm
to 4.1mm. Aluminum electrolytic capaci tors can be used
in cost-driven applications, provided that consideration
