LTC3550-1_15 Datasheet, PDF (14/24 Pages) Linear Technology – Dual Input USB/AC Adapter Li-Ion Battery Charger with 600mA Buck Converter

LTC3550-1
U
OPERATIO
The power supply status output (P⎯ W⎯ R⎯ ) has two states: pull-
down and high impedance. The pull-down state indicates
that power is present at either DCIN or USBIN. If no power
is applied at either pin, the ^⎯P^⎯W^⎯R pin is high impedance,
indicating that the LTC3550-1 lacks sufficient power to
charge the battery. The pull-down state is strong enough
to drive an LED and is capable of sinking up to 10mA.
The USB power status output (USBPWR) has two states:
pull-down and high impedance. The high impedance state
indicates that the LTC3550-1 is being powered from the
USBIN input. The pull-down state indicates that the charger
is either powered from DCIN or is in a UVLO condition
(see Table 1). The pull-down state is capable of sinking
up to 1mA.
Thermal Limiting
An internal thermal feedback loop reduces the programmed
charge current if the die temperature attempts to rise
above a preset value of approximately 105°C. This feature
protects the LTC3550-1 from excessive temperature and
allows the user to push the limits of the power handling
capability of a given circuit board without risk of damag-
ing the device. The charge current can be set according
to typical (not worst-case) ambient temperature with the
assurance that the charger will automatically reduce the
current in worst case conditions. DFN package power
considerations are discussed further in the Applications
Information section.
Charge Current Soft-Start and Soft-Stop
The battery charger includes a soft-start circuit to minimize
the inrush current at the start of a charge cycle. When a
charge cycle is initiated, the charge current ramps from
zero to full-scale current over a period of 250µs. Like-
wise, internal circuitry slowly ramps the charge current
from full-scale to zero in approximately 30µs when the
charger shuts down or self terminates. This minimizes
the transient current load on the power supply during
start-up and shut-off.
Short-Circuit Protection
When the regulator output (V_OUT) is shorted to ground, the
frequency of the oscillator is reduced to about 210kHz, one
seventh the nominal frequency. This frequency foldback
ensures that the inductor current has more time to decay,
thereby preventing runaway. The oscillator’s frequency
will progressively increase to 1.5MHz when V_OUT rises
above 0V.
35501f
14