LTC3559-1_15 Datasheet, PDF (20/24 Pages) Linear Technology – Linear USB Battery Charger with Dual Buck Regulators

LTC3559/LTC3559-1
APPLICATIONS INFORMATION
leakage pulled to ground through a 10k resistor on the
switch (SW1 or SW2) pin when in shutdown.
Buck Switching Regulator Dropout Operation
It is possible for a step-down switching regulator’s input
voltage to approach its programmed output voltage (e.g., a
battery voltage of 3.4V with a programmed output voltage
of 3.3V). When this happens, the PMOS switch duty cycle
increases until it is turned on continuously at 100%. In this
dropout condition, the respective output voltage equals the
regulator’s input voltage minus the voltage drops across
the internal P-channel MOSFET and the inductor.
Buck Switching Regulator Soft-Start Operation
Soft-start is accomplished by gradually increasing the
peak inductor current for each switching regulator over
a 500μs period. This allows each output to rise slowly,
helping minimize the battery in-rush current required to
charge up the regulator’s output capacitor. A soft-start
cycle occurs whenever a switcher first 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 regulators contain circuitry to limit the
slew rate of the switch node (SW1 and SW2). This circuitry
is designed to transition the switch node over a period of
a couple of nanoseconds, significantly reducing radiated
EMI and conducted supply noise while maintaining high
efficiency.
Buck Switching Regulator Low Supply Operation
An undervoltage lockout (UVLO) circuit on PV_IN shuts
down the step-down switching regulators when BAT drops
below 2.45V. This UVLO prevents the step-down switching
regulators from operating at low supply voltages where loss
of regulation or other undesirable operation may occur.
Buck Switching Regulator Inductor Selection
The buck regulators are 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 efficiency,
choose an inductor with a low DC resistance. For a 1.2V
output efficiency is reduced about 2% for every 100mΩ
series resistance at 400mA load current, and about 2%
for every 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 induc-
tor should be rated to handle the maximum peak current
specified 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 efficiency. 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.
Table 2 shows several inductors that work well with the
LTC3559/LTC3559-1. These inductors offer a good compro-
mise in current rating, DCR and physical size. Consult each
manufacturer for detailed information on their entire
selection of inductors.
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