Datasheet

2008-2013 Microchip Technology Inc. DS20002090C-page 25
MCP73871
5.0 DETAILED DESCRIPTION
5.1 Analog Circuitry
5.1.1 LOAD SHARING AND LI-ION
BATTERY MANAGEMENT INPUT
SUPPLY (V
IN
)
The V
IN
input is the input supply to the MCP73871
device. The MCP73871 device can be supplied by
either AC Adapter (V
AC
) or USB Port (V
USB
) with SEL
pin. The MCP73871 device automatically powers the
system with the Li-Ion battery when the V
IN
input is not
present.
5.1.2 FAST CHARGE CURRENT
REGULATION SET (PROG1)
For the MCP73871 device, the charge current
regulation can be scaled by placing a programming
resistor (R
PROG1
) from the PROG1 pin to V
SS
. The
program resistor and the charge current are calculated
using the following equation:
EQUATION 5-1:
The fast charge current is set for maximum charge
current from AC-DC adapter and USB port. The
preconditioning current is 10% (0.1C) of the fast charge
current.
5.1.3 BATTERY CHARGE CONTROL
OUTPUT (V
BAT
)
The battery charge control output is the drain terminal
of an internal P-channel MOSFET. The MCP73871
device provides constant current and voltage
regulation to the battery pack by controlling this
MOSFET in the linear region. The battery charge
control output should be connected to the positive
terminal of the battery pack.
5.1.4 TEMPERATURE QUALIFICATION
(THERM)
The MCP73871 device continuously monitors battery
temperature during a charge cycle by measuring the
voltage between the THERM and V
SS
pins. An internal
50 μA current source provides the bias for most
common 10 k NTC or Positive Temperature Coeffi-
cient (PTC) thermistors.The current source is con-
trolled, avoiding measurement sensitivity to
fluctuations in the supply voltage (V
DD
). The
MCP73871 device compares the voltage at the
THERM pin to factory set thresholds of 1.24V and
0.25V, typically. Once a voltage outside the thresholds
is detected during a charge cycle, the MCP73871
device immediately suspends the charge cycle.
The MCP73871 device suspends charge by turning off
the pass transistor and holding the timer value. The
charge cycle resumes when the voltage at the THERM
pin returns to the normal range.
If temperature monitoring is not required, place a
standard 10 k resistor from THERM to V
SS
.
5.2 Digital Circuitry
5.2.1 STATUS INDICATORS AND
POWER-GOOD (PG)
The charge status outputs have two different states:
Low-Impedance (L) and High-Impedance (Hi-Z). The
charge status outputs can be used to illuminate LEDs.
Optionally, the charge status outputs can be used as an
interface to a host microcontroller. Table 5-1
summarizes the state of the status outputs during a
charge cycle.
TABLE 5-1: STATUS OUTPUTS
I
REG
1000V
R
PROG1
--------------------=
Where:
R
PROG
= kilo-ohms (k
I
REG
= milliampere (mA)
CHARGE CYCLE STATE STAT1 STAT2 PG
Shutdown (V
DD
= V
BAT
) Hi-Z Hi-Z Hi-Z
Shutdown (V
DD
= IN) Hi-Z Hi-Z L
Shutdown (CE = L) Hi-Z Hi-Z L
Preconditioning L Hi-Z L
Constant Current L Hi-Z L
Constant Voltage L Hi-Z L
Charge Complete - Standby Hi-Z L L
Temperature Fault L L L
Timer Fault L L L
Low Battery Output L Hi-Z Hi-Z
No Battery Present Hi-Z Hi-Z L
No Input Power Present Hi-Z Hi-Z Hi-Z