SC805 datasheet(9/15 Pages) SEMTECH

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SC805

POWER MANAGEMENT

Applications Information (Cont.)

The CHRGB indicator operates the same way in both

charge and LDO mode.

The CHRGB output can be used for a VCC indicator. When

the SC805 is enabled the CHRGB output is either high

or low providing the VCC input is above UVLO. Alternately

an LED connected to VCCIL can be used for a VCC indi-

cator as shown in the typical application schematic on

the front page of the datasheet.

There are five fault modes detected by the SC805; (In-

put under voltage, input over voltage, NTC temperature,

Max die temperature or OT and pre-charge timeout). The

LED states for these fault modes are shown below.

Overcurrent and Max Temperature Protection

Overcurrent protection is inherent in all modes of opera-

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

current 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

SC805. This feature allows the SC805 to operate with

maximum power dissipation by disabling the output cur-

rent when the die temperature reaches OT. The result is

that the SC805 will operate as a pulse charger in ex-

treme power dissipation applications delivering the maxi-

mum allowable output current while regulating the inter-

nal 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

SC805, then from the Vout pin of the SC805 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 charg-

ing current. The trace from Vout of the SC805 is most

critical and should be made as wide as possible to mini-

mize IR drops between the regulated voltage at the

SC805 vout pin and the battery terminal. Keep the

Adapter+ to SC805 VCC input trace wide to minimize

voltage drops that will add to the dropout voltage of the

SC805. The GND pin of the SC805 should be connected

in a kelvin fashion at the Battery-terminal to eliminate

voltage drops in the return path which reduce the regu-

lated battery voltage.

The thermal performance of the SC805 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

SC805 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/out-

put hot-plugging of the charger. To minimize these cur-

rent spikes the value of the decoupling capacitor should

be minimized. A typical application requires a 0.1µF in-

put/output capacitor. If the distance from the external

terminals to the SC805 is greater than 1”, 0.1µ local

decoupling capacitors at the SC805 may be required.

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