LTC3558
23
3558f
APPLICATIONS INFORMATION
Buck Switching Regulator Soft-Start Operation
Soft-start is accomplished by gradually increasing the peak
inductor current for each switching regulator over a 500
μ
s
period. This allows an output to rise slowly, helping mini-
mize the battery in-rush current required to charge up the
regulator’s output capacitor. A soft-start cycle occurs when
the buck switcher fi rst turns on, or after a fault condition
has occurred (thermal shutdown or UVLO). A soft-start
cycle is not triggered by changing operating modes using
the MODE pin. This allows seamless output operation when
transitioning between operating modes.
Buck Switching Regulator
Switching Slew Rate Control
The buck switching regulator contains circuitry to limit
the slew rate of the switch node (SW1). This circuitry is
designed to transition the switch node over a period of a
couple of nanoseconds, signifi cantly reducing radiated
EMI and conducted supply noise while maintaining high
effi ciency.
Buck Switching Regulator Low Supply Operation
An undervoltage lockout (UVLO) circuit on PV
IN1
shuts
down the step-down switching regulators when BAT drops
below 2.45V. This UVLO prevents the buck switching regu-
lator from operating at low supply voltages where loss of
regulation or other undesirable operation may occur.
Buck Switching Regulator Inductor Selection
The buck switching regulator is designed to work with
inductors in the range of 2.2μH to 10μH, but for most
applications a 4.7μH inductor is suggested. Larger value
inductors reduce ripple current which improves output
ripple voltage. Lower value inductors result in higher
ripple current which improves transient response time.
To maximize effi ciency, choose an inductor with a low DC
resistance. For a 1.2V output effi ciency is reduced about 2%
for every 100m
Ω
series resistance at 400mA load current,
and about 2% for every 300m
Ω
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 specifi ed for the buck regulators.
Different core materials and shapes will change the size/cur-
rent 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 DCR losses, and
will not give the best effi ciency. The choice of which style
inductor to use often depends more on the price vs size,
performance, and any radiated EMI requirements than on
what the buck regulator 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.
