LTC3405A-1.5 Datasheet, PDF (9/16 Pages) Linear Technology – 1.5V, 1.8V, 1.5MHz, 300mA Synchronous Step-Down Regulators in ThinSOT

LTC3405A-1.5/LTC3405A-1.8
APPLICATIO S I FOR ATIO
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 LTC3405A
series’ 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.
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.
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 percentage
of input power.
Although all dissipative elements in the circuit produce
losses, two main sources usually account for most of the
losses in LTC3405A series parts circuits: V_IN quiescent
current and I²R losses. The V_IN quiescent current loss
dominates the efficiency loss at very low load currents
whereas the I²R loss dominates the efficiency loss at
medium to high load currents. In a typical efficiency plot,
the efficiency curve at very low load currents can be
misleading since the actual power lost is of no conse-
quence as illustrated in Figure 3.
1
V_IN = 3.6V
0.1
0.01
V_OUT = 1.8V
0.001
V_OUT = 1.5V
0.0001
0.1
1
10
100
LOAD CURRENT (mA)
1000
3405A1518 F03
Figure 3. Power Lost vs Load Current
1. The V_IN quiescent current is due to two components:
the DC bias current as given in the electrical character-
istics and the internal main switch and synchronous
switch gate charge currents. The gate charge current
results from switching the gate capacitance of the
internal power MOSFET switches. Each time the gate is
switched from high to low to high again, a packet of
charge, dQ, moves from V_IN to ground. The resulting
dQ/dt is the current out of V_IN that is typically larger than
the DC bias current. In continuous mode, I_GATECHG =
f(Q_T + Q_B) where Q_T and Q_B are the gate charges of the
internal top and bottom switches. Both the DC bias and
gate charge losses are proportional to V_IN and thus
their effects will be more pronounced at higher supply
voltages.
sn3405a1518 3405a1518fs
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