LM3621 datasheet(12/14 Pages) NSC

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Application Information (Continued)

Fault Protection

LM3621 provides redundant fault protection for a Li-Ion bat-

tery pack. If the charger’s control loop is accidentally

opened, the regulation voltage will begin to increase beyond

the level that are safe for the battery. In order to protect the

battery from a fault condition, PFD goes low to turn off the

series pass FET whenever the voltage across the battery ex-

ceeds the over voltage detection threshold (V

BAT-Det) or

whenever V

CSR exceeds 1V. A fault condition also causes

both of the LED drivers to pulse at 0.8 Hz. Enable must go

low to cause the LM3621 to transition from the Fault State to

Standby mode.

Low Current Drain of Battery

If power is inadvertently removed from the battery charger,

the voltage at the V

DD pin will collapse to the same potential

as the V

SS pin. An internal switch will disconnect the circuitry

of the LM3621 from the V

BAT+ pin so that no more than 1µA

(typical) of leakage current will be drawn from the battery.

Adjustable Charge Current

The current sense resistor (Rcs) converts the charging cur-

rent to a voltage level, V

CSR. Applying an external voltage

source (V

EXT) to the ISET pin will adjust the value of VCSR for

Fast Charge current regulation. Simultaneously the regula-

tion level for Boost mode, Hi-Z Charge mode, End-of-Charge

threshold and over current detect threshold will also be ad-

justed accordingly. The voltage at I

SET pin should be within

the range of 1V

≥ V

ISET ≥ 0.1V. The transfer functions of

CSR to VISET are:

Fast Charge Mode, V

CSR = 0.933·VISET

Hi-Z Mode, V

CSR = 0.083·VISET

Boost Mode, V

CSR = 0.48·VISET+0.65

Over Current Threshold, V

CSR = 0.36·VISET+0.79

End-Of-Charge, V

CSR = 0.07·VISET

DS100107-5

FIGURE 5. Battery Refresh

DS100107-16

FIGURE 6. Adjusting V

CSR by applying VEXT to ISET pin

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