LTC3418 Datasheet, PDF (10/20 Pages) Linear Technology – 8A, 4MHz, Monolithic Synchronous Step-Down Regulator

LTC3418
APPLICATIO S I FOR ATIO
Different core materials and shapes will change the size/
current and price/current relationship of an inductor.
Toroid or shielded pot cores in ferrite or permalloy mate-
rials are small and don’t radiate much energy, but gener-
ally cost more than powdered iron core inductors with
similar characteristics. The choice of which style inductor
to use mainly depends on the price vs size requirements
and any radiated field/EMI requirements. New designs for
surface mount inductors are available from Coiltronics,
Coilcraft, Toko and Sumida.
C_IN and C_OUT Selection
The input capacitance, C_IN, is needed to filter the trapezoi-
dal wave current at the source of the top MOSFET. To
prevent large voltage transients from occurring, a low ESR
input capacitor sized for the maximum RMS current
should be used. The maximum RMS current is given by:
I_RMS
=
I_OUT(MAX)
V_OUT
V_IN
V_{IN – 1}
V_OUT
This formula has a maximum at V_IN = 2V_OUT, where I_RMS
= I_OUT/2. This simple worst-case condition is commonly
used for design because even significant deviations do not
offer much relief. Note that ripple current ratings from
capacitor manufacturers are often based on only 2000
hours of life which makes it advisable to further derate the
capacitor, or choose a capacitor rated at a higher tempera-
ture than required. Several capacitors may also be paral-
leled to meet size or height requirements in the design.
The selection of C_OUT is determined by the effective series
resistance (ESR) that is required to minimize voltage
ripple 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, ∆V_OUT, is determined by:
∆V_OUT
≤
∆I_L ^{⎛⎝⎜}ESR
+
1
8fC_OUT
⎞
⎠⎟
The output ripple is highest at maximum input voltage
since ∆I_L increases with input voltage. Multiple capacitors
placed in parallel may be needed to meet the ESR and RMS
10
current handling requirements. Dry tantalum, special poly-
mer, aluminum electrolytic and ceramic capacitors 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 provided that consideration is given to ripple
current ratings and long term reliability. Ceramic capaci-
tors have excellent 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
The output voltage is set by an external resistive divider
according to the following equation:
V_OUT = 0.8^{⎛⎝⎜}1+ ^{RR21⎞⎠⎟}
The resistive divider allows pin V_FB to sense a fraction of
the output voltage as shown in Figure 1.
3418f