SC813 datasheet(16/23 Pages) SEMTECH

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SC811 / SC813

Applications Information (continued)

which is the safest default mode with the lowest fast-

charge current.

Enable Input

The ENB pin is a tri-level logical input that allows selection

of the following behaviors:

charging enabled with float-charging after ter-

mination (ENB = low range)

charging enabled with float-charging disabled

and battery monitoring at termination (ENB =

mid range)

charging disabled (ENB = high range).

If the ENB input voltage is permitted to float to mid-range,

the charger is enabled but it will turn off its output follow-

ing charge termination and will enter the monitor state.

This state is explained in the next section. Mid-range can

be selected either by floating the input (sourcing or

sinking less than 5μA) or by being externally forced such

that V

ENB

falls within the midrange limits specified in the

Electrical Characteristics table.

When driven low (V

ENB

< Max V

), the charger is enabled

and will continue to float-charge the battery following

termination. If the charger is already in monitor state fol-

lowing a previous termination, it will exit the monitor state

and begin float-charging.

When ENB is driven high (V

ENB

> Min V

), the charger is

disabled and the ENB input pin enters a high impedance

state, suspending tri-level functionality. The specified

high level input current I

is required only until a high

level is recognized by the SC811/3 internal logic. The tri-

level float circuitry is then disabled and the ENB input

becomes high impedance. Once forced high, the ENB pin

will not float to mid range. To restore tri-level operation,

the ENB pin must first be pulled down to mid or low range

(at least to V

ENB

< Max V

), then, if desired, released (by

reconfiguring the GPIO as an input) to select mid-range. If

the ENB GPIO has a weak pull-down when configured as

an input, then it is unnecessary to drive ENB low to restore

tri-level operation; simply configure the GPIO as an input.

When the ENB selection changes from high-range to mid-

or low-range, a new charge cycle begins and STATB goes

low.

•

Note that if a GPIO with a weak pull-up input configura-

tion is used, its pull-up current will flow from the GPIO into

the ENB pin while it is floating to mid-range. Since the

GPIO is driving a 1V equivalent voltage source through a

resistance (looking into ENB), this current is small — pos-

sibly less than 1

μA. Nevertheless, this current is drawn

from the GPIO peripheral power supply and, therefore,

from the battery after termination. (See the next section,

Monitor State.) For this reason, it is preferable that the

GPIO chosen to operate the ENB pin should provide a true

high impedance (CMOS) configuration or a weak pull-

down when configured as an input. When pulled below

the float voltage, the ENB pin output current is sourced

from VIN, not from the battery.

Monitor State

If the ENB pin is floating, the charger output and STATB pin

will turn off and the device will enter the monitor state

when a charge cycle is complete. If the battery voltage

falls below the re-charge threshold (V

- V

ReQ

) while in the

monitor state, the charger will automatically initiate a re-

charge cycle. 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.

While in the monitor state, the ENB tri-level input pin

remains fully active, and although in midrange, is sensitive

to both high and low levels. The SC811/3 can be forced

from the monitor state (no float-charging) directly to float-

charging operation by driving ENB low. This operation will

turn on the charger output, but will not assert the STATB

output. If the ENB pin is again allowed to float to mid-

range, the charger will remain on only until the output

current becomes less than the termination current, and

charging terminates. The SC811/3 turns off its charging

output and returns to the monitor state within a millisec-

ond. This forced re -charge behavior is useful for

periodically testing the battery state-of-charge and

topping-off the battery, without float-charging and

without requiring the battery to discharge to the auto-

matic re-charge voltage. ENB should be held low for at

least 1ms to ensure a successful forced re-charge.

Forced re-charge can be requested at any time during the

charge cycle, or even with no charging source present,

with no detrimental effect on charger operation. This

allows the host processor to schedule a forced re-charge

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