LTC3406-1.2 Datasheet, PDF (8/12 Pages) Linear Technology – 1.5MHz, 600mA Synchronous Step-Down Regulator in ThinSOT

LTC3406-1.2
APPLICATIO S I FOR ATIO
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 ∆V_OUT is deter-
mined by:
∆V_OUT
≅
∆I_L
⎛
⎝⎜
ESR
+
1⎞
8fC_OUT ⎠⎟
where f = operating frequency, C_OUT = output capacitance
and ∆I_L = ripple current in the inductor. For a fixed output
voltage, the output ripple is highest at maximum input
voltage since ∆I_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 LTC3406-
1.2’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.
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 LTC3406-1.2 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 consequence as illustrated in Figure 2.
1
0.1
0.01
0.001
0.0001
0.00001
0.1
V_IN = 2.7V
V_IN = 3.6V
V_IN = 4.2V
1
10
100
LOAD CURRENT (mA)
1000
340612 F02
Figure 2. Power Loss vs Load Current
340612f
8