LTC3413IFE Datasheet, PDF (9/16 Pages) Linear Technology – Monolithic Synchronous Regulator

LTC3413
APPLICATIONS INFORMATION
and load step transients as well as the amount of bulk
capacitance that is necessary to ensure that the control
loop is stable. Loop stability can be checked by viewing
the load transient response as described in a later section.
The output ripple, ΔV_OUT, is determined by:
ΔV_OUT
≤
ΔI_L
⎛
⎝⎜^{ESR +}
1^⎞
8fC_{OUT ⎠⎟}
The output ripple is highest at maximum input voltage
since ΔI_L increases with input voltage. Multiple capaci-
tors placed in parallel may be needed to meet the ESR
and RMS current handling requirements. Dry tantalum,
special polymer, aluminum electrolytic and ceramic capaci-
tors are all available in surface mount packages. Special
polymer capacitors offer very low ESR but have lower
capacitance density than other types. Tantalum capacitors
have the highest capacitance density but it is important
to only use types that have been surge tested for use in
switching power supplies.
Aluminum electrolytic capacitors have significantly higher
ESR, but can be used in cost-sensitive applications pro-
vided that consideration is given to ripple current ratings
and long term reliability. Ceramic capacitors have excel-
lent low ESR characteristics but can have a high voltage
coefficient and audible piezoelectric effects. The high Q
of ceramic capacitors with trace inductance can also lead
to significant ringing.
Using Ceramic Input and Output Capacitors
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. However, care must
be taken when these capacitors are used at the input and
output. When a ceramic capacitor is used at the input and
the power is supplied by a wall adapter through long wires,
a load step at the output can induce ringing at the input,
V_IN. At best, this ringing can couple to the output and be
mistaken as loop instability. At worst, a sudden inrush
of current through the long wires can potentially cause a
voltage spike at V_IN large enough to damage the part.
When choosing the input and output ceramic capacitors,
choose the X5R or X7R dielectric formulations. These
dielectrics have the best temperature and voltage charac-
teristics of all the ceramics for a given value and size.
Output Voltage Programming
In most applications, V_OUT is connected directly to V_FB.
The output voltage will be equal to one-half of the volt-
age on the V_REF pin for this case.
V_OUT
=
V_REF
2
If a different output voltage relationship is desired, an
external resistor divider from V_OUT to V_FB can be used.
The output voltage will then be set according to the fol-
lowing equation:
V_OUT
=
V_REF
2
⎛⎝⎜₁₊
R2⎞
R1⎠⎟
V_OUT
R2
V_FB
LTC3413
R1
SGND
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Figure 2. Setting the Output Voltage
Soft-Start
The RUN/SS pin provides a means to shut down the
LTC3413 as well as a timer for soft-start. Pulling the
RUN/SS pin below 0.5V places the LTC3413 in a low
quiescent current shutdown state (I_Q < 1μA).
The LTC3413 contains an internal soft-start clamp that
gradually raises the clamp on I_TH after the RUN/SS pin is
pulled above 2V. The full current range becomes available
on I_TH after 1024 switching cycles. If a longer soft-start
period is desired, the clamp on I_TH can be set externally
with a resistor and capacitor on the RUN/SS pin as shown
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