LTC3577-4_15 Datasheet, PDF (32/52 Pages) Linear Technology – Highly Integrated Portable Product PMIC

LTC3577-3/LTC3577-4
OPERATION
Note that the power loss curves remain fairly constant for
both graphs yet changing the slew rate has a larger effect
on the 1.2V output efficiency. This is mainly because for
a given output current the 2.5V output is delivering more
than 2x the power than the 1.2V output. Efficiency will
always decrease and show more variation to slew rate as
the programmed output voltage is decreased.
Low Supply Operation
An undervoltage lockout circuit on V_OUT (V_OUT UVLO)
shuts down the step-down switching regulators when V_OUT
drops below about 2.7V. It is recommended that the step-
down switching regulator input supplies (V_IN12, V_IN3) be
connected to the power path output (V_OUT) directly. This
UVLO prevents the step-down switching regulators from
operating at low supply voltages where loss of regula-
tion or other undesirable operation may occur. If driving
the step-down switching regulator input supplies from
a voltage other than the V_OUT pin, the regulators should
not be operated outside the specified operating range as
operation is not guaranteed beyond this range.
Inductor Selection
Many different sizes and shapes of inductors are available
from numerous manufacturers. Choosing the right inductor
from such a large selection of devices can be overwhelming,
but following a few basic guidelines will make the selection
process much simpler. The step-down switching regula-
tors are designed to work with inductors in the range of
2.2μH to 10μH. For most applications a 4.7μH inductor is
suggested for step-down switching regulators providing
up to 500mA of output current while a 3.3μH inductor is
suggested for step-down switching regulators providing
up to 800mA. Larger value inductors reduce ripple current,
which improves output ripple voltage. Lower value induc-
tors result in higher ripple current and improved transient
response time, but will reduce the available output current.
To maximize efficiency, choose an inductor with a low DC
resistance. For a 1.2V output, efficiency is reduced about
2% for 100mΩ series resistance at 400mA load current,
and about 2% for 300mΩ series resistance at 100mA load
current. Choose an inductor with a DC current rating at
least 1.5 times larger than the maximum load current to
ensure that the inductor does not saturate during normal
operation. If output short circuit is a possible condition,
the inductor should be rated to handle the maximum peak
current specified for the step-down converters. Different
core materials and shapes will change the size/current
and price/current relationship of an inductor. Toroid or
shielded pot cores in ferrite or Permalloy materials are
small and don’t radiate much energy, but generally cost
more than powdered iron core inductors with similar
electrical characteristics. Inductors that are very thin or
have a very small volume typically have much higher core
and DCR losses, and will not give the best efficiency. The
choice of which style inductor to use often depends more
on the price versus size, performance, and any radiated
EMI requirements than on what the step-down switching
regulators requires to operate. The inductor value also has
an effect on Burst Mode operation. Lower inductor values
will cause Burst Mode switching frequency to increase.
Table 3 shows several inductors that work well with the
step-down switching regulators. These inductors offer a
good compromise in current rating, DCR and physical
size. Consult each manufacturer for detailed information
on their entire selection of inductors.
Input/Output Capacitor Selection
Low ESR (equivalent series resistance) ceramic capacitors
should be used at both step-down switching regulator
outputs as well as at each step-down switching regulator
input supply. Only X5R or X7R ceramic capacitors should
be used because they retain their capacitance over wider
voltage and temperature ranges than other ceramic types.
A 10μF output capacitor is sufficient for the step-down
switching regulator outputs. For good transient response
and stability the output capacitor for step-down switching
regulators should retain at least 4μF of capacitance over
operating temperature and bias voltage. Each switching
regulator input supply should be bypassed with a 2.2μF
capacitor. Consult with capacitor manufacturers for de-
tailed information on their selection and specifications
of ceramic capacitors. Many manufacturers now offer
very thin (<1mm tall) ceramic capacitors ideal for use in
height-restricted designs. Table 4 shows a list of several
ceramic capacitor manufacturers.
32
357734fb