LTC3406AB Datasheet, PDF (9/16 Pages) Linear Technology – 1.5MHz, 600mA Synchronous Step-Down Regulator in ThinSOT

LTC3406AB
APPLICATIONS INFORMATION
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:
ΔV_OUT
≅
ΔI_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.
Aluminum electrolytic and dry tantalum capacitors are both
available in surface mount configurations. In the case of
tantalum, it is critical that the capacitors 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
LTC3406AB’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
In the adjustable version, 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 1.
0.6V ≤ V_OUT ≤ 5.5V
R2
V_FB
LTC3406AB
R1
GND
3406AB F03
Figure 1. Setting the LTC3406AB Output Voltage
Efficiency Considerations
The efficiency of a switching regulator is equal to the output
power divided by the input power times 100%. It is often
useful to analyze individual losses to determine what is
limiting the efficiency and which change would produce
the most improvement. Efficiency can be expressed as:
Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percent-
age of input power.
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