18
LTC3445
3445fa
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
LTC3445s control loop does not depend on the output
capacitors 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
CC1
. 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
CC1
, 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.
Buck Output Voltage Programming
The LTC3445 has an internal resistor divider network tied
to the FB pin. The output voltage is controlled by a DAC
(6-bit register) whose setting is controlled by the I
2
C
interface. The effective DAC bit range is from 0 to 48. Note
that any DAC settings above 48 defaults to the 48 setting.
The DAC controls the V
OUT
range of 0.85V to 1.55V in
~15mV steps. The default value for V
OUT
is 1.35V and is
reset to this value whenever V
CC1
comes up.
When the DACs value is changed, LTC3445 controls
V
OUT
s slew rate via a 2-bit RATE register. The RATE
register can be updated via the I
2
C interface. The slew rate
can be set to approximately 0.9mV/祍, 3.8mV/祍, 7.5mV/祍
or 11.3mV/祍. The default value for RATE is 10mV/祍 and
is reset to this value whenever V
CC1
comes up.
The DAC and RATE values are not lost whenever the RUN
pin is deasserted.
Once the DAC and RATE registers are programmed, a GO
bit transition is required for the buck to update. This is
accomplished by changing the GO bit (REG2[0]) from
logic low to a logic high.
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 LTC3445 buck regulator circuits: V
CC1
quiescent
current and I
2
R losses. The V
CC1
quiescent current loss
dominates the efficiency loss at very low load currents
whereas the I
2
R 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 8.
1. The V
CC1
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
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