Datasheet

MCP73837/8
DS22071B-page 18 © 2007-2011 Microchip Technology Inc.
5.0 DETAILED DESCRIPTION
5.1 Analog Circuitry
5.1.1 BATTERY MANAGEMENT INPUT
SUPPLY (V
DD
)
The V
DD
input is the input supply to the MCP73837/8.
The MCP73837/8 can be supplied by either AC-
adapter (V
AC
) or USB-port (V
USB
) with autonomous
source selection. The MCP73837/8 automatically
enters a Power-down mode if the voltage on the V
DD
input falls to within +100 mV of the battery voltage or
below the UVLO voltage (V
STOP
). This feature prevents
draining the battery pack when both the V
AC
and V
USB
supplies are not present.
5.1.2 AC-ADAPTER CURRENT
REGULATION SET (PROG1)
For the MCP73837/8, the charge current regulation
can be scaled by placing a programming resistor
(R
PROG
) from the PROG1 input to V
SS
. The program
resistor and the charge current are calculated using
the following equation:
EQUATION 5-1:
The preconditioning current and the charge
termination current are ratiometric to the fast charge
current based on the selected device options.
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 MCP73837/8
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 MCP73837/8 continuously monitors battery
temperature during a charge cycle by measuring the
voltage between the THERM and the V
SS
pins. An
internal 50 µA current source provides the bias for the
most common 10 kΩ negative-temperature coefficient
(NTC) or positive-temperature coefficient (PTC)
thermistors. The current source is controlled, avoiding
measurement sensitivity to fluctuations in the supply
voltage (V
DD
). The MCP73837/8 compares the voltage
at the THERM pin to factory set thresholds of 1.20V
and 0.25V, typically. Once a voltage outside the
thresholds is detected during a charge cycle, the
MCP73837/8 immediately suspends the charge cycle.
The MCP73837/8 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 kΩ resistor from THERM to V
SS
.
5.1.5 SYSTEM TEST (LDO) MODE
The MCP73837/8 can be placed in a System Test
mode. In this mode, the MCP73837/8 operates as a
low dropout linear regulator (LDO). The output voltage
is regulated to the factory set voltage regulation option.
The available output current is limited to the pro-
grammed fast charge current. For stability, the V
BAT
output must be bypassed to V
SS
with a minimum
capacitance of 1 µF for output currents up to 250 mA.
A minimum capacitance of 4.7 µF is required for output
currents above 250 mA.
The system test mode is entered by driving the THERM
input greater than (V
DD
- 100 mV) with no battery
connected to the output. In this mode, the MCP73837/
8 can be used to power the system without a battery
being present.
5.2 Digital Circuitry
5.2.1 STATUS INDICATORS AND POWER
GOOD (PG
) OPTION
The charge status outputs have two different states:
Low (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. Tabl e 5-1 summarizes the state of
the status outputs during a charge cycle.
I
REG
1000V
R
PROG
-----------------=
Where:
R
PROG
=kilo-ohms (kΩ)
I
REG
= milli-ampere (mA)
Note 1: I
THERM
is disabled during shutdown,
stand-by, and system test modes.
2: A pull-down current source on the
THERM input is active only in stand-by
and system test modes.
3: During system test mode, the PROG
input sets the available output current
limit.
4: System test mode shall be exited by
releasing the THERM input or cycling
input power.