SC820 datasheet(19/22 Pages) SEMTECH

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SC820

During charging, a short to ground applied to the selected-

input active current programming pin (IPRGM or IPUSB) is

detected by a different mechanism, while a short to

ground on the inactive programming pin is ignored. Pin-

short detection on an active current programming pin

forces the SC820 into reset, turning off the output. A pin-

short on either programming pin will then prevent startup

regardless of the input selected. When the IPRGM and

IPUSB pin-short conditions are removed, the charger

begins normal operation automatically without input

power cycling.

Over-Current Protection

Over-current protection is provided in all modes of opera-

tion, including CV regulation. The output current is limited

to either the pre-charge or the fast-charge current (as

programmed by IPRGM or IPUSB, determined by input

selection), depending on the voltage at the output.

Operation Without a Battery

The SC820 can be operated as a 4.2V LDO regulator

without the battery present, for example, for factory

testing. If this use is anticipated, the total output capaci-

tance, C

BAT

plus any other capacitors tied directly to BAT

pin network, should be at least 2.2μF but less than 22μF to

ensure stability in CV regulation. To operate the charger

without a battery, the ENB pin must be driven low or

grounded. The output current is limited by the pro-

grammed fast-charge current for the selected input. The

charger should not be disabled (V

ENB

> V

) without a

battery present.

Design Considerations — USB Charging

The USB specification restricts the load on the USB Vbus

power network to 100mA for low power devices and for

high power devices prior to granting permission for high

power operation. The specification restricts the Vbus load

to 500mA for high power devices after granting permis-

sion to operate as a high power device. This suggests that

a fixed 1:5 ratio of low power to high power charging

current is desirable. But this can result in suboptimal

charging when the battery capacity is too small to permit

fast charging at 500mA. For example, a 250mAh battery

will typically require a fast-charge current of 250mA or

less. A fixed 1:5 ratio for USB low and high power charging

will unnecessarily reduce charging current to 50mA, well

below the 100mA permitted.

An arbitrary ratio of USB low-to-high power charging cur-

rents can be obtained using an external n-channel FET

operated with a processor GPIO signal to engage a second

parallel IPUSB resistor. The external circuit is illustrated in

Figure 6.

IPUSB

RIPUSB

RIPUSB_HI

USB Hi/Lo

Power Select

Figure 6 — External programming of arbitrary USB

high power and low power charge currents.

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.

For purposes of design for dual-input adapter/USB charg-

ing, a small battery is one with a desired fast-charge

current less than 500mA. A 300mAh battery with

maximum fast-charge current of 300mA is an example.

The adapter input and USB input high power fast-charge

currents should both be set to 300mA maximum. The USB

input low power fast-charge current is 100mA maximum.

Refer to the circuit of Figure 4 and the data of Figures 1a

and 1b. For I

FQ_AD

= 300mA maximum, use R

IPRGM

= 7.50kΩ.

The fixed IPUSB resistor of R

IPUSB

= 23.2kΩ programs I

FQ_USB

Applications Information (continued)

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