SC820 datasheet(15/22 Pages) SEMTECH

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SC820

Applications Information (continued)

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.

While 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.

While 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 SC820 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 − possi-

bly 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 imped-

ance (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 VAD

or VUSB (the charging source), 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 SC820 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 SC820 turns off its charging

output and returns to the monitor state within a millisec-

ond. This forced re-charge behavior is useful for periodi-

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

the battery, without float-charging and without requiring

the battery to discharge to the automatic 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

at any desired interval, without regard to whether a charge

cycle is already in progress, or even whether a charging

source is present. Forced re-charge will neither assert nor

release the STATB output.

Status Output

The STATB pin is an open-drain output. It is asserted

(driven low) as charging begins after a valid charging

source is connected and the voltage on either input is

between its selection and OVP limits. STATB is also

asserted as charging begins after the ENB input returns to

either of the enable voltage ranges (mid or low voltage)

from the disable range. STATB is subsequently released

when the termination current is reached to indicate end-

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