SC802AEVB datasheet(11/18 Pages) SEMTECH

11 / 18 page

SC802/SC802A

page 1, an increasing battery temperature produces a

decreasing NTC pin voltage, designated V

EN_NTC

When V

EN_NTC

is greater than the high (cold) threshold (but

below the Charger Enable threshold) or less than the low

(hot) threshold (but above the Disable threshold), the

charge cycle is suspended, turning off the output. This

suspends but does not reset the charge timer, and indi-

cates a charging fault by blinking the CHRGB status indi-

cator. Hysteresis is provided for both high and low NTC

thresholds to avoid chatter at the NTC temperature fault

thresholds. When V

EN_NTC

returns to the Temperature-OK-

to-Charge range, the charge timer resumes, CHRGB is

asserted (pulled low), the charging output is enabled, and

the charge cycle continues. The timer will expire when

the output on-time exceeds the timer setting, regardless

of how long it has been disabled due to an NTC fault.

All EN_NTC input thresholds are proportional to the VCC

pin voltage (V

VCC

). When the recommended external NTC

circuit is used, the external EN_NTC pin voltage is also

proportional to V

VCC

, varying with the thermistor resis-

tance. This ensures that all EN_NTC thresholds are insensi-

tive to V

VCC

. The ratiometric hot and cold thresholds are

given by the parameters RT

NTC_HF

and RT

NTC_CR

. V

EN_NTC

between RT

NTC_HF×V

VCC

and RT

NTC_CR×V

VCC

indicates the

battery temperature is safe to charge, and enables charg-

ing. See the following table.

EN_NTC Pin Ratiometric Thresholds

% of V

VCC

Range

EN_NTC

Ratiometric

Thresholds

NTC_EN

= 90%

Charger Enable

NTC Cold Fault

NTC_CR

= 75%

NTC Temperature-

OK-to-Charge

NTC_HF

= 30%

NTC Hot Fault

NTC_DIS

= 1V

Charger Disable

When V

EN_NTC

< 1V approximately, the SC802/A charger is

disabled. This allows the system controller to asynchro-

nously disable or reset the device by pulling EN_NTC to

ground, using for example an n-channel FET. When dis-

abled, the charger is turned off, the charge timer is reset,

Applications Information (continued)

and CHRGB is turned off (high). The behavior of CPB

differs between the SC802 and SC802A when the device

is disabled. See the Status Indicators section.

Charger Enable is selected when the battery (along with

the thermistor) is removed, determined by the NTC pin

exceeding RT

NTC_EN

× V

VCC

, nominally 90% of the VCC pin

voltage. Charger Enable behaves identically to NTC safe-

to-charge. Charger Enable also provides a convenient

means to operate the SC802/A in applications without a

battery thermistor, without requiring a passive resistor

network to obtain 50% of V

VCC

. Connect EN_NTC directly

to VCC, or via a pullup resistor if it will be necessary to

disable the charger by pulling EN_NTC to ground.

The response of the SC802/A to an EN_NTC pin voltage

above the NTC Cold Fault threshold (but below RT

NTC_EN

)

or below the low NTC Hot Fault threshold (but above

NTC_DIS) is the same. Therefore the EN_NTC network can

be configured with the battery pack thermistor between

EN_NTC and VCC, and a fixed resistor between EN_NTC

and ground, reversing the designation of the hot and cold

thresholds. This configuration may be used to disable the

charger when the battery pack is removed.

For detailed design guidance for ratiometric NTC inter-

faces, including thermistor selection guidelines, see the

Semtech Application Note AN–PM–0801, NTC Thermistor

Network Design for Ratiometric Thresholds.

Status Indicators

There are two status indicator outputs on the SC802/A —

CHRGB (Charge) and CPB (Charger Present). These

outputs are open drain n-channel MOSFET drivers suit-

able for driving LEDs directly. The following table defines

each output state.

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