LTC3550_15 Datasheet, PDF (14/24 Pages) Linear Technology – Dual Input USB/AC Adapter Li-Ion Battery Charger with 600mA Buck Converter

LTC3550
U
OPERATIO
Battery Charger Power Source Selection
The LTC3550 can charge a battery from either the wall
adapter input or the USB port input. The charger automati-
cally senses the presence of voltage at each input. If both
power sources are present, the charger defaults to the wall
adapter source provided sufficient power is present at the
DCIN input. “Sufficient power” is defined as:
• Supply voltage is greater than the UVLO threshold.
• Supply voltage is greater than the battery voltage by
50mV (180mV rising, 50mV falling).
Table 1 describes the behavior of the PWR status output.
Table 1. Power Source Selection
V_DCIN > 4.15V and
V_DCIN > BAT + 50mV
V_USBIN > 3.95V and
V_USBIN > BAT + 50mV
Charger Powered from
Wall Adapter Source;
USBIN Current < 25µA
⎯P⎯W⎯R: LOW
V_DCIN < 4.15V or
Charger Powered from
V_DCIN < BAT + 50mV
USB Source;
⎯P⎯W⎯R: LOW
V_USBIN < 3.95V or
V_USBIN < BAT + 50mV
Charger Powered
from Wall Adapter
Source
⎯P⎯W⎯R: LOW
No Charging
⎯P⎯W⎯R: Hi-Z
Status Indicators
The charge status output (^⎯C^⎯H^⎯R^⎯G) has two states: pull-
down and high impedance. The pull-down state indicates
that the LTC3550 is in a charge cycle. Once the charge
cycle has terminated or the LTC3550 is disabled, the pin
state becomes high impedance. The pull-down state is
strong enough to drive an LED and is capable of sinking
up to 10mA.
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 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.
Low-Battery Charge Conditioning (Trickle Charge)
This feature ensures that deeply discharged batteries are
gradually charged before applying full charge current . If
the BAT pin voltage is below 2.9V, the LTC3550 supplies
1/10th of the full charge current to the battery until the
BAT pin rises above 2.9V. For example, if the charger is
programmed to charge at 800mA from the wall adapter
input and 500mA from the USB input, the charge current
during trickle charge mode would be 80mA and 50mA,
respectively.
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 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. Likewise,
internal circuitry 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 shutdown.
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