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LT3652EDD

Power Tracking 2A Battery Charger for Solar Power

Linear Technology 

LT3652

APPLICATIONS INFORMATION

full-charge float voltage. If a charge cycle is not successful

at EOC, the timer cycle resets and charging continues for

another full timer cycle.

When VBAT drops below 97.5% of the full-charge float

voltage, whether by battery loading or replacement of the

battery, the charger automatically reengages and starts

charging.

Preconditioning and Bad Battery Fault

A LT3652 has a precondition mode, where charge current

is limited to 15% of the programmed ICHG(MAX), as set by

RSENSE. The precondition current corresponds to 15mV

across RSENSE.

Precondition mode is engaged while the voltage on the

VFB pin is below the precondition threshold (2.3V, or

0.7 • VBAT(FLT)). Once the VFB voltage rises above the

precondition threshold, normal full-current charging can

commence. The LT3652 incorporates 70mV of threshold

hysteresis to prevent mode glitching.

When the internal timer is used for termination, bad battery

detection is engaged. There is no provision for bad battery

detection if C/10 termination is used. A bad battery fault

is triggered when the voltage on VFB remains below the

precondition threshold for greater than 1/8 of a full timer

cycle (1/8 EOC). A bad battery fault is also triggered if a

normally charging battery re-enters precondition mode

after 1/8 EOC.

When a bad battery fault is triggered, the charging cycle

is suspended, so the CHRG status pin becomes high-

impedance. The FAULT pin is pulled low to signal a fault

detection.

Cycling the charger’s power or SHDN function initiates a

new charging cycle, but a LT3652 charger does not re-

quire a reset. Once a bad battery fault is detected, a new

timer charging cycle initiates when the VFB pin exceeds

the precondition threshold voltage. During a bad battery

fault, 0.5mA is sourced from the charger, so removing

the failed battery allows the charger output voltage to rise

and initiate a charge cycle reset. As such, removing a bad

battery resets the LT3652, so a new charge cycle is started

by connecting another battery to the charger output.

Battery Temperature Monitor and Fault

The LT3652 can accommodate battery temperature moni-

toring by using an NTC (negative temperature co-efficient)

thermistor close to the battery pack. The temperature

monitoring function is enabled by connecting a 10kΩ,

B = 3380 NTC thermistor from the NTC pin to ground. If the

NTC function is not desired, leave the pin unconnected.

The NTC pin sources 50μA, and monitors the voltage

dropped across the 10kΩ thermistor. When the voltage

on this pin is above 1.36V (0°C) or below 0.29V (40°C),

the battery temperature is out of range, and the LT3652

triggers an NTC fault. The NTC fault condition remains until

the voltage on the NTC pin corresponds to a temperature

within the 0°C to 40°C range. Both hot and cold thresholds

incorporate hysteresis that correspond to 5°C.

If higher operational charging temperatures are desired,

the temperature range can be expanded by adding series

resistance to the 10k NTC resistor. Adding a 0.91k resistor

will increase the effective hot temperature to 45°C.

During an NTC fault, charging is halted and both status

pins are pulled low. If timer termination is enabled, the

timer count is suspended and held until the fault condi-

tion is relieved.

Thermal Foldback

The LT3652 contains a thermal foldback protection feature

that reduces maximum charger output current if the IC

junction temperature approaches 125°C. In most cases,

on-chip temperatures servo such that any excessive tem-

perature conditions are relieved with only slight reductions

in maximum charger current.

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