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LTC3550

3550fa

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

VUSBIN > 3.95V and

VUSBIN > BAT + 50mV

VUSBIN < 3.95V or

VUSBIN < BAT + 50mV

VDCIN > 4.15V and

VDCIN > BAT + 50mV

Charger Powered from

Wall Adapter Source;

USBIN Current < 25µA

⎯P⎯W⎯R: LOW

Charger Powered

from Wall Adapter

Source

⎯P⎯W⎯R: LOW

VDCIN < 4.15V or

VDCIN < BAT + 50mV

Charger Powered from

USB 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)hastwostates: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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