Datasheet
MCP73871
DS20002090C-page 22 2008-2013 Microchip Technology Inc.
Table 4-1 shows the chip behavior based upon the operating conditions.
TABLE 4-1: CHIP BEHAVIOR REFERENCE TABLE
4.1 UnderVoltage Lockout (UVLO)
An internal undervoltage lockout (UVLO) circuit
monitors the input voltage and keeps the charger in
shutdown mode until the input supply rises above the
UVLO threshold.
In the event a battery is present when the input power
is applied, the input supply must rise approximately
100 mV above the battery voltage before the
MCP73871 device becomes operational.
The UVLO circuit places the device in Shutdown mode
if the input supply falls to within approximately 100 mV
of the battery voltage.
The UVLO circuit is always active. At any time the input
supply is below the UVLO threshold or falls within
approximately 100 mV of the voltage at the V
BAT
pin,
the MCP73871 device is placed in Shutdown mode.
During any UVLO condition, the battery reverse
discharge current is less than 2 μA.
4.2 System Load Sharing
The system load sharing feature gives the system
output pin (OUT) priority, allowing the system to power
up with deeply depleted battery packs.
With the SEL input active Low, the MCP73871 device
is designed to provide system power and Li-Ion battery
charging from a USB input while adhering to the current
limits governed by the USB specification.
With the SEL input active High, the MCP73871 device
limits the total supply current to 1.8A (system power
and charge current combined).
FIGURE 4-2: System Load Sharing
Diagram.
4.3 Charge Qualification
For a charge cycle to begin, all UVLO conditions must
be met and a battery or output load must be present.
A charge current programming resistor must be
connected from PROG1 to V
SS
when SEL = High.
When SEL = Low, PROG2 needs to be tied High or
Low for proper operation.
V
IN
? V
BAT
V
IN
> 2V
V
IN
> UVLO
CE V
BAT
? V
OUT
State
Bias + V
REF
Thermal
Block
Synchronous
Diode
I
OUT
Charge
1
V
BAT
> V
IN
00
0
—
Shutdown OFF
OFF
2 1 Battery
powered
system
ON
3
V
IN
> V
BAT
0 0 X — Shutdown OFF
4
V
IN
> V
BAT
1
0
0
—
Shutdown
ON
OFF
OFF
5
1
Battery
powered
system
ON
6
1
0
V
BAT
< V
OUT
Standby
ON
OFF
ON
OFF
7
V
BAT
> V
OUT
IN + BAT
powered
system
ON
8
1
V
BAT
< V
OUT
IN powered,
Charge
possible
OFF ON/OFF
9
V
BAT
> V
OUT
IN + BAT
powered
system
ON OFF
0.2
Ideal
Diode,
Synchronous
Switch
Direction
Control
0.2
Current
Limit
Direction
Control
Charge
FET
System
Power
FET
V
BAT
IN OUT
Charge
Control