LTC3560_15 Datasheet, PDF (9/16 Pages) Linear Technology – 2.25MHz, 800mA Synchronous Step-Down Regulator in ThinSOT

LTC3560
APPLICATIONS INFORMATION
more than powdered iron core inductors with similar
electrical characteristics. The choice of which style
inductor to use often depends more on the price versus
size requirements and any radiated field/EMI requirements
than on what the LTC3560 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3560 applications.
C_IN and C_OUT Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle V_OUT/V_IN. To prevent large
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum RMS
capacitor current is given by:
( ) _{CIN required IRMS ≅ IOMAX} ^{⎡⎣V}_OUT
V_IN − V_OUT
V_IN
⎤⎦^{1/ 2}
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 the capacitor
manufacturer’s ripple current ratings are often based on
2000 hours of life. This makes it advisable to further
derate the capacitor, or choose a capacitor rated at a
higher temperature than required. Always consult the
manufacturer if there is any question.
The selection of C_OUT is driven by the required effective
series resistance (ESR). Typically, once the ESR requirement
for C_OUT has been met, the RMS current rating generally
far exceeds the I_RIPPLE(P-P) requirement. The output ripple
ΔV_OUT is determined by:
V_OUT


I_L


ESR
+
1
8fC_OUT


where f = operating frequency, C_OUT = output capacitance
and ΔI_L = ripple current in the inductor. For a fixed output
voltage, the output ripple is highest at maximum input
voltage since ΔI_L increases with input voltage.
If tantalum capacitors are used, it is critical that the capaci-
tors are surge tested for use in switching power supplies.
An excellent choice is the AVX TPS series of surface mount
tantalum. These are specially constructed and tested for low
ESR so they give the lowest ESR for a given volume. Other
capacitor types include Sanyo POSCAP, Kemet T510 and
T495 series, and Sprague 593D and 595D series. Consult
the manufacturer for other specific recommendations.
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. Because the
LTC3560’s control loop does not depend on the output
capacitor’s ESR for stable operation, ceramic capacitors
can be used freely to achieve very low output ripple and
small circuit size.
However, care must be taken when ceramic capacitors
are used at the input and the 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 a resistive divider according
to the following formula:
V_OUT
=
0.6V



1+
R2 
R1
(2)
The external resistive divider is connected to the output,
allowing remote voltage sensing as shown in Figure 2.
3560fb
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