SC824

27

Applications Information (continued)

External Power Path Management

Some applications require that the battery be isolated

from the load while charging. Figure 8 illustrates a typical

charger bypass circuit. This circuit powers the load directly

from the charging source via the Schottky diode D

BYPASS

.

When the charging source is present, the p-channel

MOSFET battery isolation switch Q

ISO

source-to-gate

voltage V

SG

is equal to minus the D

BYPASS

forward-biased

voltage drop, ensuring that the switch Q

ISO

is off (open).

When the charging source is removed, the MOSFET gate is

pulled down to ground by R

ISO_PD

, closing the battery isola-

tion switch and connecting the battery to the load.

When the charging source is removed, the turn-on of Q

ISO

could be delayed due to its gate capacitance. If so, the

substrate PN diode of Q

ISO

will become forward biased,

holding the load voltage to within 0.7V of the battery

voltage until V

SG

> V

TH

, turning on Q

ISO

. This momentary

voltage drop can be mitigated by the use of an optional

Schottky diode in parallel with Q

ISO

, as shown.

With the load isolated from the battery, the charging

adapter must supply both the load current and the charg-

ing current. If the sum of these should ever exceed the

current capacity of the adapter, V

ADAPTER

will be pulled

down. Selection of either of the SC824 USB modes will

enable Under-Voltage Load Regulation. UVLR will reduce

the charge current if needed to ensure that V

ADAPTER

will

remain at or above V

UVLR

, maintaining the load supply

voltage

To better understand the trade-offs between charger

bypass and direct connection of the load to the battery,

see the Semtech Application Note AN–PM–0802, Tradeoffs

Between Direct Battery Connection vs. Bypassing the

Charger.

Device

Load

Battery

Pack

2.2μF

2.2μF

VIN

STAT0

IPRGM

MODE

GND

VSYS

EN_NTC

BAT

SC824

MODE SELECT

CPU GPIO

RTIME

STAT1

1μF

Thermistor

Opt.

Figure 8 — Battery Isolation and Power Path Bypass − Powering the Load Directly From the Charging Adapter