LTC3408_15 Datasheet, PDF (9/12 Pages) Linear Technology – 1.5MHz, 600mA Synchronous Step-Down Regulator with Bypass Transistor

LTC3408
APPLICATIO S I FOR ATIO
forced continuous mode, the LTC3408 will actually pull
current from the output until the command from V_REF is
satisfied. On alternate half cyles, this current actually exits
the V_IN terminal, potentially causing a rise in V_IN and
forcing current into the battery. To prevent deterioration
of the battery, use sufficient bulk capacitance with low
ESR; at least 10µF is recommended.
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
LTC3408’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.
Ceramic capacitors of Y5V material are not recommended
because normal operating voltages cause their bulk ca-
pacitance to become much less than the nominal value.
Programming the Output Voltage With a DAC
The output voltage can be dynamically programmed to any
voltage from 0.3V to 3.5V with an external DAC driving the
REF pin. When the output is commanded low, the output
voltage descends quickly in forced continuous mode
pulling current from the output and transferring it to the
input. If the input is not connected to a low impedance
source capable of absorbing the energy, the input voltage
could rise above the absolute maximum voltage of the part
and get damaged. The faster V_OUT is commanded low, the
higher is the voltage spike at the input. For best results,
ramp the REF pin from high to low as slow as the
application will allow. Avoid abrupt changes in voltage of
>0.2V/µs. If ramp control is unavailable, an RC filter with
a time constant of 10µs can be inserted between the REF
pin and the DAC as shown in Figure 3.
10k
DAC
1000pF
LTC3408
REF
GND
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Figure 3. Filtering the REF Pin
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 LTC3408 circuits: V_IN quiescent current and I²R
losses. The V_IN quiescent current loss dominates the effi-
ciency loss at low load currents whereas the I²R loss domi-
nates the efficiency loss at medium to high load currents.
In a typical efficiency plot, the efficiency curve at low load
currents can be misleading since the actual power lost is
of little consequence as illustrated in Figure 4.
1. The V_IN quiescent current consists of two components:
the DC bias current as given in the electrical characteris-
tics 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 typically larger
than the DC bias current. In continuous mode,
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