SC813 datasheet(21/23 Pages) SEMTECH

21 / 23 page

SC811 / SC813

For USB low power mode charging, the external transistor

is turned off. The transistor is turned on when high power

mode is desired. The effect of the switched parallel IPUSB

resistor is to reduce the effective programming resistance

and thus raise the fast-charge current.

An open-drain GPIO can be used directly to engage the

parallel resistor R

IPUSB_HI

. Care must be taken to ensure that

the R

DS-ON

of the GPIO is considered in the selection of

IPUSB_HI

. Also important is the part-to-part and tempera-

ture variation of the GPIO R

DS-ON

, and their contribution to

the USB High Power charge current tolerance. Note also

that IPUSB will be pulled up briefly to as high as 3V during

startup to check for an IPUSB static pinshort to ground. A

small amount of current could, potentially, flow from

IPUSB into the GPIO ESD structure through R

IPUSB_HI

during

this event. While unlikely to do any harm, this effect must

also be considered.

The 300mAh battery example can be used to illustrate

how this system works. The adapter mode and USB high

power mode fast-charge currents should both be set to

300mA max. The USB input low power fast-charge current

is 100mA max. Refer to the circuit in Figure 5 and the data

of Figures 1a and 1b. For I

FQ_AD

= 300mA max, use R

IPRGM

7.50kΩ. A fixed IPUSB resistor of R

IPUSB

= 23.2kΩ programs

FQ_USB

= 100mA max for USB low power charging. When a

parallel resistor R

IPUSB_HI

= 11.0kΩ resistor is switched in, the

equivalent IPUSB resistor is 7.50kΩ, for I

FQ_USB

= 300mA

max.

USB Low Power Mode Alternative

Where a USB mode selection signal is not available, or for

a low capacity battery where system cost or board space

make USB low power mode external current program-

ming impractical, USB low power charging can be

supported indirectly. The IPUSB pin resistance can be

selected to obtain the desired USB high power charge

current. Then, with the MODE pin always configured for

USB high power mode, the UVLR feature will ensure that

the charging load on the VIN pin will never pull the USB

Vbus supply voltage below V

UVLR

regardless of the host or

hub supply limit. The UVLR limit voltage guarantees that

the voltage of the USB Vbus supply will not be loaded

below the low power voltage specification limit, as seen

by any other low power devices connected to the same

USB host or hub.

Independent Programming of Termination

Current

The USB high power mode fast-charge current is limited

to 1000mA, twice the USB high power load limit, and so

this mode may also be used for general purpose adapter

charging. The IPRGM pin resistance to ground determines

the USB high power mode pre-charge current, and the

termination threshold current for all modes. If adapter

mode will not be used in the application, R

IPRGM

can be

selected to program only the termination threshold

current independently of the fast-charge current, which is

programmed with R

IPUSB

Note that USB high power mode invokes Under-Voltage

Load Regulation, so if charging with an adapter in current

limit, the input voltage can be pulled down no lower than

UVLR

USB-only Charging of Very Large Batteries

The SC811/3 can support the charging of very large capac-

ity batteries as high as 2Ah using a USB-only charging

source. The IPRGM resistance lower limit of 2.05kΩ is

intended to limit the fast-charge current while charging in

adapter mode to less than 1A. If only USB charging modes

will be used, then the IPRGM resistor can be chosen as low

as 1kΩ. This extended programming range allows setting

the USB high power mode pre-charge current as high as

400mA (still below the USB specification limit), and the

charge termination current as high as 200mA. (Both of

these currents are determined by R

IPRGM

.) Note that with

IPRGM

< 2.05kΩ, adapter mode should not be used, as this

can result in potentially destructive fast-charge current.

The USB high power and USB low power fast-charge cur-

rents and the USB low power pre-charge current are

determined by the resistance between IPUSB and GND to

comply with USB specified current limits, and so are unaf-

fected by the IPRGM resistor. Termination detection

requires that the charger be in CV regulation. If the

IPRGM-determined termination threshold current is set

higher than the USB low power mode fast-charge current,

for example, then charge termination will occur the instant

that the battery voltage rises to V

. Thus USB low power

charging will behave as if trickle-charging until fully

charged, a perfectly safe and acceptable, although slow,

charging scenario.

Applications Information (continued)

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