SC824 datasheet(16/29 Pages) SEMTECH

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SC824

Applications Information (continued)

Monitor State

Upon termination (charge timer disabled) or time-out of

the top-off timer (charge timer enabled), the charger will

enter the monitor state. If the battery voltage falls below

the recharge threshold (V

- V

ReQ

) while in the monitor

state, the charger will automatically initiate a re-charge

cycle. This operation will turn on the charger output, but

will not assert the STAT1 output. A recharge cycle is

subject to normal timer operation if the timer is enabled.

The battery leakage current during monitor state is no

more than 1

μA over temperature and typically less than

0.1

μA at room temperature.

The SC824 can be forced out of the monitor state by dis-

abling and re-enabling the charger using the EN_NTC pin.

This operation will initiate a new charge cycle. This forced

re-charge behavior can be used for periodically testing

the battery state-of-charge and topping off the battery

without requiring the battery to discharge to the auto-

matic re-charge voltage. A single CPU instruction cycle is

a sufficient disable hold-time to initiate a re-charge cycle.

Following termination, the host processor can schedule a

forced re-charge at any desired interval. Forced re-charge

will assert the STAT1 output, which will remain on for 20

seconds following termination, regardless of whether the

charge timer is enabled.

VSYS pin

The voltage of the VSYS pin is regulated from the VIN input

and is present only when VIN is powered. A capacitor of at

least 1μF should be connected from VSYS to ground near

the pin. Capacitance must be rated at the expected bias

voltage of 4.6V.

The internal CC timer is selected when the RTIME pin is

connected to VSYS. VSYS provides an external voltage

reference and supply for the NTC network. The total exter-

nal load on the VSYS pin should not exceed 1mA.

EN_NTC Interface

The EN_NTC pin is the interface to a battery pack tempera-

ture sensing Negative Temperature Coefficient (NTC)

thermistor, which can be used to suspend charging if the

battery pack temperature is outside of a safe-to-charge

range. The EN_NTC interface also often serves as a charger

disable input and as a battery removal detector.

The recommended EN_NTC network is a fixed-value pull-

up resistor (designated R

NPU

) from the EN_NTC pin to the

VSYS pin, and the battery pack NTC thermistor (designated

NTC

) from the EN_NTC pin to ground. In this configura-

tion, an increasing battery temperature produces a

decreasing NTC pin voltage.

When V

EN_NTC

is greater than the high (cold) threshold (but

below the No-Battery detector threshold) or less than the

low (hot) threshold (but above the disable threshold), the

charge cycle is suspended by turning off the output. This

suspends but does not reset the charge timer, in any timer

stage, and a charging fault is indicated by asserting

(pulling low) STAT1 and releasing STAT0. Hysteresis is

included for both high and low NTC thresholds to avoid

chatter at the NTC temperature fault thresholds. When

EN_NTC

returns to the Temperature-OK-to-Charge range,

the charge timer resumes, STAT0 and STAT1 are asserted,

and the charge cycle continues. The charge 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 VSYS

pin voltage (V

VSYS

). See the Block Diagram. When the rec-

ommended external NTC circuit shown in the Block

Diagram is used, the external EN_NTC pin voltage is also

proportional to V

VSYS

, with the proportionality varying with

the thermistor resistance. This ensures that all EN_NTC

thresholds are insensitive to V

VSYS

. The ratiometric hot and

cold thresholds are given by the parameters RT

NTC_HF

and

NTC_CR

. EN_NTC pin voltage V

EN_NTC

between RT

NTC_HF×V

VSYS

and RT

NTC_CR×V

VSYS

enables charging. See Table 1.

Table 1 — EN_NTC Pin Ratiometric Thresholds

% of V

VSYS

Range

EN_NTC

Ratiometric

Thresholds

NTC_NBR

= 95%

No-Battery

NTC Cold Fault

NTC_CR

= 75%

NTC Temperature-

OK-to-Charge

NTC_HF

= 30%

NTC Hot Fault

NTC_DIS

= 10%

Charger Disabled

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