LTC3851-1_15 Datasheet, PDF (18/28 Pages) Linear Technology – Synchronous Step-Down Switching Regulator Controller

LTC3851-1
APPLICATIONS INFORMATION
The required ESR due to a load current step is:
R_ESR
≤
ΔV
ΔI
where ΔI is the change in current from full load to zero load
(or minimum load) and ΔV is the allowed voltage deviation
(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 ΔI 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 handling
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 capacitors 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 capacitors 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 ranging from 1.5mm
to 4.1mm. Aluminum electrolytic capacitors can be used
in cost-driven applications, provided that consideration is
given to ripple current ratings, temperature and long-term
reliability. A typical application will require several to many
aluminum electrolytic capacitors in parallel. A combina-
tion of the above mentioned capacitors 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
capacitor 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 LTC3851-1 output voltage is set by an external feed-
back resistive divider carefully placed across the output,
as shown in Figure 6. The regulated output voltage is
determined by:
V_OUT
=
0.8V ^{⎛⎝⎜}1+
R_B ⎞
R_A ⎠⎟
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.
V_OUT
LTC3851-1
V_FB
R_B
C_FF
R_A
38511 F06
Figure 6. Settling Output Voltage
38511fa
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