SC824 datasheet(25/29 Pages) SEMTECH

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SC824

Applications Information (continued)

Operation Without a Battery

The SC824 can be operated as a 4.2V LDO regulator

without the battery present, for example, for factory

testing. If this use is required, the total output capacitance,

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 NTC pin should be pulled up to the

VSYS voltage to select No-Battery mode. This can be

accomplished automatically when a battery is absent if

the recommended NTC network is used. See the section

EN_NTC Interface for details.

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 SC824 is to be

operated without the battery present. See the section

Operation Without a Battery. The VIN pin capacitor is typi-

cally between 0.1μF and 2.2μF, although larger values will

not degrade performance. The VSYS pin capacitor must

be at least 1μF. Capacitance must be rated at the expected

bias voltage (4.2V for the BAT pin capacitor, 4.6V for the

VSYS pin capacitor, the expected V

VIN

supply regulation

voltage for the VIN pin capacitor), 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 device to a larger copper footprint

will enable better heat transfer from the device

on PCBs with internal ground and power

planes.

•

Design Considerations — Large Battery

A battery with a desired fast-charge current exceeding

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

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

consider an 800mAhr battery, with maximum fast-charge

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

be configured for 800mA max (R

IPRGM

= 2.78kΩ equivalent

is required). Select R

IPRGM

= 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. For adapter mode charging, set the MODE pin

high for USB high power mode if UVLR is desired, or low

for adapter mode if current limited adapter capability is

desired. Then switch in a parallel 7.15kΩ IPRGM resistor, as

shown in Figure 7, for an equivalent 2.77kΩ IPRGM resis-

tance. This will program the desired 800mA max adapter

mode fast charge current.

IPRGM

RIPRGM

RIPRGM_HI

Hi/Lo

Current Select

Figure 7 — External Programming of Arbitrary USB

High Power and Low Power Charge Currents

Design Considerations — Small Battery

A battery with a desired fast-charge current less than

500mA will not be charged in the minimum charge time

when in USB low power mode with a 1:5 low-to-high

power mode current ratio. A 300mAhr 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. In this case, the fixed USB low-to-high power

charging current ratio of 1:5 would provide a USB low

power mode fast charge current of 60mA.

For this example, setting the USB low power fast-charge

current to 90mA would provide a shorter charge time

without violating USB Vbus current requirements. An

arbitrary ratio of USB low-to-high power charging currents

can be obtained using an external n-channel FET oper-

ated with a processor GPIO signal to engage a second

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