SC810

18

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. Current limited adapter operation of the SC810

ensures charge cycle integrity if the device load pulls the

adapter voltage down to the battery voltage plus charger

dropout voltage at the CC current, or even deeper into

dropout if necessary to further reduce the charge current

to power the device load.

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.

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 SC810 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. Capacitance must be evalu-

ated at the expected bias voltage (4.2V for the BAT pin

capacitor, the expected V

VIN

supply regulation voltage for

the VIN pin capacitor), rather than the zero-volt capaci-

tance 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)

Device

Load

2.2

μF

2.2

μF

VIN

STATB

IPRGM

GND

ENB

BAT

SC810

Opt.

Figure 7 — Battery Isolation and Power Path Bypass, Powering the Load Directly From the Charging Adapter