LTC3854_15 Datasheet, PDF (15/28 Pages) Linear Technology – Small Footprint, Wide VIN Range Synchronous Step-Down DC/DC Controller

LTC3854
APPLICATIONS INFORMATION
The required ESR due to a load current step is:
ESR
≤
∆V
∆I
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 ( ∆I)²
2(∆V) V_OUT
Manufacturers such as Nichicon, United Chemi-Con
and Sanyo should 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 inductive 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 T51 0 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 alumi-
num electrolytic capacitors in parallel. A combination 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 LTC3854 output voltage is set by an external feed-
back resistive divider carefully placed across the output,
as shown in Figure 3. The regulated output voltage is
determined by:
V_OUT
=
0.8


1+
R_B
R_A


V_OUT
LTC3854
R_B
C_FF
FB
R_A
3854 F03
Figure 3. Feed-Forward Capacitor on FB Pin
To improve the frequency 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.
Fault Conditions: Current Foldback
The LTC3854 includes current foldback to help limit load
current when the output is shorted to ground. If the output
falls below 40% of its nominal output level, the maximum
3854fb
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