SC810

13

Applications Information (continued)

1.8V or as high as a fully charged battery. While a con-

nected GPIO port is configured as an output, the proces-

sor writes 0 to select ENB low-range, and 1 to select

high-range. The GPIO port is configured as an input to

select mid-range.

ENB can also be permanently grounded to select low-

range or left unconnected to select mid-range if it will not

be necessary to change the level selection.

The equivalent circuit looking into the ENB pin is a vari-

able resistance, minimum 15kΩ, to an approximately 1V

source. The input will float to mid range whenever the

external driver sinks or sources less than 5μA, a common

worst-case characteristic of a high impedance or a weak

pull-up or pull-down GPIO configured as an input. The

driving GPIO must be able to sink at least 25μA or source

at least 50μA to ensure a low or high state, respectively.

(See the Electrical Characteristics table.)

With the ENB input voltage permitted to float to mid-

range, the charger is enabled but it will turn off its output

following 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 (sourc-

ing 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

IL

), 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

IH

), the charger is dis-

abled and the ENB input pin enters a high impedance state,

suspending tri-level functionality. The specified high level

input current I

IH

is required only until a high level is recog-

nized by the SC810 internal logic. The tri-level float circuitry

is then disabled and the ENB input becomes high imped-

ance. 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

IM

), 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 opera-

tion; 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 more information. For this reason, it is

preferable that the GPIO chosen to operate the ENB pin

should provide a true high impedance (CMOS) configura-

tion 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 (the charging adapter), 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

CV

- 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 SC810 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 SC810 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