LTC3851A
18
3851afa
applicaTions inForMaTion
The required ESR due to a load current step is:
R
ESR
≤
ΔV
where ∆I is the change in current from full load to zero load
(or minimum load) and ∆V is the allowed voltage devia-
tion (not including any droop due to finite capacitance).
The amount of capacitance needed is determined by the
maximum energy stored in the inductor. The capacitance
must be sufficient 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
∆
I 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 specifications may require multiple capaci-
tors 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 capaci-
tors 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 is
given to ripple current ratings, tempera ture and long-term
reliability. A typical application will require several to many
aluminum electrolytic capacitors in parallel. A combina-
tion of the above mentioned capaci tors will often result
in maximizing performance and minimizing overall cost.
Other capacitor types include Nichicon PL series, NEC
Neocap, Panasonic SP and Sprague 595D series. Consult
manufacturers for other specific recommendations.
Like all components, capacitors are not ideal. Each ca-
pacitor has its own benefits and limitations. Combina tions
of different capacitor types have proven to be a very cost
effective solution. Remember also to include high frequency
decoupling capacitors. They should be placed as close as
possible to the power pins of the load. Any inductance
present in the circuit board traces negates their usefulness.
Setting Output Voltage
The LTC3851A output voltage is set by an external feed-
back resistive divider carefully placed across the output,
as shown in Figure 6. The regulated output volt age is
determined by:
ΔI
L(SC)
= t
ON(MIN)
•
IN
To improve the transient response, a feed-forward ca-
pacitor, C
FF
, may be used. Great care should be taken to
route the V
FB
line away from noise sources, such as the
inductor or the SW line.
LTC3851A
V
FB
V
OUT
R
B
C
FF
R
A
3851A F06
Figure 6. Settling Output Voltage
