LTC3406A LINER [Linear Technology], LTC3406A Datasheet - Page 9

• Manufacturer Part Number: LTC3406A
• Description: 1.5MHz, 600mA Synchronous Step-Down Regulator in ThinSOT
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
The selection of C
series resistance (ESR).
Typically, once the ESR requirement for C
met, the RMS current rating generally far exceeds the
I
determined by:
where f = operating frequency, C
and ∆I
voltage, the output ripple is highest at maximum input
voltage since ∆I
Aluminum electrolytic and dry tantalum capacitors are both
available in surface mount confi gurations. 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 specifi c 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
LTC3406A’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
RIPPLE(P-P)
V
OUT
L
= ripple current in the inductor. For a fi xed output
requirement. The output ripple ∆V
I
L
L
ESR +
increases with input voltage.
OUT
is driven by the required effective
8fC
1
OUT
OUT
= output capacitance
OUT
has been
OUT
is
induce ringing at the input, V
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
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:
The external resistive divider is connected to the output,
allowing remote voltage sensing as shown in Figure 1.
Effi ciency Considerations
The effi ciency 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 effi ciency and which change would produce
the most improvement. Effi ciency can be expressed as:
where L1, L2, etc. are the individual losses as a percent-
age of input power.
Effi ciency = 100% – (L1 + L2 + L3 + ...)
V
OUT
Figure 1. Setting the LTC3406A Output Voltage
= 0.6V 1+
LTC3406A
R2
R1
GND
V
FB
IN
. At best, this ringing can
0.6V ≤ V
LTC3406A
OUT
R2
R1
≤ 5.5V
IN
3406A F01
, large enough
3406afa
9
(2)