LTC4015 datasheet(40/76 Pages) LINER

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LTC4015

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OPERATION

In order to achieve a qLSB of at least 0.385C, QCOUNT_

PRESCALE_FACTOR must be at least qLSB • (KQC •

RSNSB) = 0.385 • (8333.33 • 0.005). Doubling (to allow

for margin) and rounding to the nearest integer gives

QCOUNT_PRESCALE_FACTOR = 32.

The initial state of charge (SOC) of the battery is known

to be approximately 25%. Given that the QCOUNT_

PRESCALE_FACTOR = 32 has now been set for ap-

proximatelytwicethecapacityofthebattery,thedesired

valid range of QCOUNT is from 16384 (to represent 0%

SOC) to 49152 (to represent 100% SOC). The value of

QCOUNT is thus initialized to (32768 • 0.25) + 16384

= 24576 to represent the approximate 25% initial SOC.

In this case, the SOC of the battery at any time can be

calculated as 100% • (QCOUNT–16384)/32768.

Upon the first termination (c_over_x_term=1 or

timer_term=1), the battery is known to be very near

100% SOC, so the value of QCOUNT can be calibrated

accordingly. For example, if the initial state of charge

wasactually23%insteadoftheexpected25%,termina-

tion will occur when QCOUNT is approximately 49806.

Upon the first termination, the value of QCOUNT is over

written to 49152 to calibrate QCOUNT to the true SOC

of the battery.

EXAMPLE: Coulomb Counter Charge Termination

The Coulomb counter alert limits can be used to imple-

ment a maximum state of charge termination algorithm.

The following example demonstrates such a procedure:

1) Set the Coulomb counter register (QCOUNT) to the

battery's initial state of charge, and set QCOUNT_

PRESCALE_FACTOR based on the battery capacity, as

indicated in the example above.

2) SetQCOUNT_HI_ALERT_LIMITtothedesiredmaximum

stateofchargeofthebattery(e.g. QCOUNT_HI_ALERT_

LIMIT = 49152)

3) Set en_qcount_hi_alert=1

4) If the battery charges enough that QCOUNT exceeds

QCOUNT_HI_ALERT_LIMIT, the LTC4015 will issue

an SMBALERT and set qcount_hi_alert=1. After

completing the alert response algorithm and verifying

that the LTC4015 is issuing qcount_hi_alert=1, set

suspend_charger=1 to stop the battery from charging.

At this time, write QCOUNT_LO_ALERT_LIMIT to a

lower level corresponding to a recharge threshold

(e.g. QCOUNT_LO_ALERT_LIMIT = 0.95 • QCOUNT_

HI_ALERT_LIMIT) and set en_qcount_lo_alert=1 and

en_qcount_hi_alert=0.

5) When the battery discharges enough that QCOUNT falls

below QCOUNT_LO_ALERT_LIMIT, the LTC4015 will

issue an SMBALERT and set qcount_lo_alert=1. After

completing the alert response algorithm and verifying

that the LTC4015 is issuing qcount_lo_alert=1, set

en_qcount_lo_alert=0, en_qcount_hi_alert=1, and

suspend_charger=0toallowthebatterytochargeagain.

Step Down Switching Charger Controller

The LTC4015’s primary power path is a fully synchronous

stepdownswitchingchargercontroller.DuetoitsallNMOS

design, a diode and capacitor are required to provide high

side boosted drive. Taking error signals from four control

loops simultaneously, the feedback paths are externally

compensated with a RC network connected to the VC pin.

The switching controller is designed to charge single or

multiplebatteries.Normalchargingproceedsataconstant-

current until the batteries reach their target voltage. The

maximum charge current is determined by the value of

the sense resistor, RSNSB, usedinserieswiththeinductor.

The charge current loop servos the voltage across RSNSB

to the value determined by ICHARGE_DAC.

When charging is enabled an internal soft-start will ramp

up the charge current from zero to ICHARGE_TARGET.

Both the battery voltage and charge current can be read

back over I2C. The LTC4015 provides constant power

charging by limiting input current drawn by the switching

controller. The input current limit will reduce charge

current to limit the voltage across the input sense resistor,

RSNSI, to IIN_LIMIT_SETTING. If the combined system

load plus battery charge current is large enough to cause

the switching controller to reach the programmed input

current limit, the input current limit loop will reduce the

charge current. Even if the charge current is programmed

to exceed the allowable input current, the input current

will not be violated; the charger will reduce its current

as needed. The input current can be read back over I2C.

LTC4015 Datasheet(PDF) 40 Page - Linear Technology

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