SC813 datasheet(20/23 Pages) SEMTECH

20 / 23 page

SC811 / SC813

During charging, a short to ground applied to the active

current programming pin (IPRGM or IPUSB) is detected,

while a short to ground on the inactive programming pin

is ignored. Pin-short detection on an active current pro-

gramming pin forces the SC811/3 into reset, turning off

the output. A pin-short on either programming pin will

prevent startup regardless of the mode selected. When

the IPRGM or IPUSB pin-short condition is 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 programmed pre-charge current limit value

or the fast-charge current limit value, depending on the

voltage at the output.

Operation Without a Battery

The SC811/3 can be operated as a 4.2V LDO regulator

without the battery present, for example, factory testing.

If this use is anticipated, the output capacitance C

BAT

should be at least 2.2μF to ensure stability. To operate the

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

or grounded.

Capacitor Selection

Low cost, low ESR ceramic capacitors such as the X5R and

X7R dielectric material types are recommended. The BAT

pin capacitor range is 1μF to 22μF. The VIN pin capacitor

is typically between 0.1μF and 2.2μF, although larger

values will not degrade performance. Capacitance must

be evaluated at the expected bias voltage, rather than the

zero-volt capacitance rating.

PCB Layout Considerations

Layout for linear devices is not as critical as for a switching

regulator. However, careful attention to detail will ensure

reliable operation.

Place input and output capacitors close to the

device for optimal transient response and device

behavior.

Connect all ground connections directly to the

ground plane. If there is no ground plane,

connect to a common local ground point before

connecting to board ground near the GND pin.

•

Attaching the part to a larger copper footprint

will enable better heat transfer from the device,

especially on PCBs with internal ground and

power planes.

Design Considerations — Large Battery

A battery with a desired fast-charge current exceeding

500mA is most consistent with the USB fixed 1:5 current

ratio low-to-high power model of operation. For example,

consider an 800mAh battery, with maximum fast-charge

current of 800mA. The adapter input fast-charge should

be configured for 800mA max (R

IPRGM

= 2.80kΩ). Select

IPUSB

= 4.53kΩ to set USB high power fast-charge to

450mA, and the USB low power fast-charge set to

450/5 = 90mA. The MODE pin tri-level logical input can be

used to select between USB high power and USB low

power modes whenever a fixed 5:1 current ratio is

desired.

Design Considerations — Small Battery

A battery with a desired fast-charge current less than

500mA will not be charged in minimum charge time when

in USB low power mode of operation with a 1:5 low-to-

high power mode current ratio. A 300mAh battery can be

used as an example with maximum fast-charge current of

300mA. In this example, the adapter input and USB input

high power fast-charge currents should both be set to

300mA. The USB low power fast-charge current of, for

example, 90mA, for a low-to-high power current ratio of

1:3.3, would provide a shorter charge time than the 60mA

obtained with the fixed USB low-to-high power charging

current ratio of 1:5.

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, while selecting high power mode

(MODE pin driven high) for both low or high power charg-

ing. The external circuit is illustrated in Figure 5.

IPUSB

RIPUSB

RIPUSB_HI

USB Hi/Lo

Power Select

Figure 5. External programming of arbitrary USB high

power and low power charge currents.

•

Applications Information (continued)

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