SC820 datasheet(20/22 Pages) SEMTECH

20 / 22 page

SC820

= 100mA maximum. When parallel resistor R

IPUSB_HI

11.0kΩ is switched in, the equivalent IPUSB resistor is

7.50kΩ, and so I

FQ_USB

= 300mA maximum.

A large battery is any battery with a desired fast-charge

current exceeding 500mA. Large battery charging 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 config-

ured for 800mA maximum (R

IPRGM

= 2.80kΩ), the USB low

power fast-charge set to 100mA max (R

IPUSB

= 23.2kΩ), and

the USB high power fast-charge set to 500mA maximum

IPUSB_HI

= 5.62kΩ).

USB Low Power Mode Alternative

Where a USB mode selection signal is not available, or

where system cost or board space make USB low power

mode external current programming 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. The VUSB pin UVLR feature

ensures that the charging load will never pull the USB

Vbus supply voltage below V

UVLR

regardless of the USB

host or hub supply limit. The UVLR limit voltage guaran-

tees 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.

Capacitor Selection

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

X7R dielectric material types are recommended. The BAT

pin capacitor should be at least 1μF, but can be as large as

desired to accommodate the required input capacitors of

regulators connected directly to the battery terminal. BAT

pin total capacitance must be limited if the SC820 is to be

operated without the battery present. See the section

Operation Without a Battery. The VAD pin and VUSB pin

capacitors are typically between 0.1μF and 2.2μF, although

larger values will not degrade performance. Capacitance

must be evaluated at the expected bias voltage (4.2V for

the BAT pin capacitor, the expected V

VAD

and V

VUSB

supply

regulation voltages for the input pin capacitors), 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.

•

Applications Information (continued)

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