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© 2008 Semtech Corp.

SC805A

www.semtech.com

POWER MANAGEMENT

Applications Information (Cont.)

Fault

CHRGB

VCC UVLO

High-Z

VCC OVP

Blinking

NTC Fault

Blinking

Pre-Charge Time-Out

Blinking

T

Blinking

Over-Current and Max Temperature Protection

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

tion. When the device is in charge-mode the output is cur-

rent-limited to either the pre-charge current limit value or

the fast-charge current limit value depending on the voltage

at the output. When the device is in LDO mode, the output

current is limited to the fast-charge current limit. Max die

temperature protection is included on the SC805A. This

feature allows the SC805A to operate with maximum power

dissipation by disabling the output current when the die

temperature reaches OT. The result is that the SC805A

will operate as a pulse charger in extreme power dissipa-

tion applications delivering the maximum allowable output

current while regulating the internal die temperature to a

safe level.

Layout Guidelines

The two most critical aspects of the PCB layout are the

power path and thermal layout. The power path starts

at the Adapter + input and runs to the VCC input of the

SC805A, then from the VOUT pin of the SC805A to the bat-

tery + terminal, and completes with the return trace from

the battery - terminal to the adapter - input. All of these

traces need to be designed to handle the required charging

current. The trace from VOUT of the SC805A is most critical

and should be made as wide as possible to minimize I×R

drops between the regulated voltage at the SC805A VOUT

pin and the battery terminal. Keep the Adapter+ to SC805A

VCC input trace wide to minimize voltage drops that will

add to the dropout voltage of the SC805A. The GND pin

of the SC805A should be connected in a Kelvin fashion at

the battery-terminal to eliminate voltage drops in the return

path which reduce the regulated battery voltage.

The thermal performance of the SC805A package requires

a low impedance connection from the heat slug on the

bottom of the package to an external ground plane. This

is best accomplished by using a single large via under the

device connected to a ground plane on the bottom exposed

side of the PCB. The evaluation board uses 1 square inch

of copper on the bottom of the PCB and is capable of 1A

charging current.

The input and output bulk decoupling capacitors for the

SC805A should be placed near the external terminals for

the adapter and battery. This short low impedance loop

is for the high current spikes that result from input/output

hot-plugging of the charger. To minimize these current

spikes the value of the decoupling capacitor should be mini-

mized. A typical application requires a 0.1μF input/output

capacitor. If the distance from the external terminals to

the SC805A is greater than 1 inch, 0.1μF local decoupling

capacitors at the SC805A may be required.