Datasheet

MCP73837/8
DS22071B-page 16 © 2007-2011 Microchip Technology Inc.
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. The UVLO circuitry has a built-in
hysteresis of 75 mV for the USB-port and 55 mV for the
AC-adapter.
In the event a battery is present when the input power
is applied, the input supply must rise 100 mV above the
battery voltage before MCP73837/8 becomes
operational.
The UVLO circuit places the device in shutdown mode
if the input supply falls to within +100 mV of the battery
voltage.
The UVLO circuit is always active. At any time the input
supply is below the UVLO threshold or within +100 mV
of the voltage at the V
BAT
pin, the MCP73837/8 is
placed in a Shutdown mode.
During any UVLO condition, the battery reverse
discharge current shall be less than 2 µA.
4.2 AUTONOMOUS POWER SOURCE
SELECTION
The MCP73837/8 devices are designed to select the
USB-port or the AC-adapter as the power source
automatically. If the AC-adapter input is not present,
the USB-port is selected. If both inputs are available,
the AC-adapter has first priority.
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 con-
nected from PROG1 to V
SS
. If the PROG1 or PROG2
pin are open or floating, the MCP73837/8 is disabled
and the battery reverse discharge current is less than
2 µA. In this manner, the PROG1 pin acts as a charge
enable and can be used as a manual shutdown.
4.4 Preconditioning
If the voltage at the V
BAT
pin is less than the
preconditioning threshold, the MCP73837/8 enters a
preconditioning mode. The preconditioning threshold is
factory set. Refer to Section 1.0 “Electrical
Characteristics” for preconditioning threshold
options.
In this mode, the MCP73837/8 supplies a percentage
of the charge current (established with the value of the
resistor connected to the PROG1 pin for AC mode,
established by PROG2 level for USB mode) to the
battery. The percentage or ratio of the current is factory
set. Refer to Section 1.0 “Electrical Characteristics”
for preconditioning current options.
When the voltage at the V
BAT
pin rises above the
preconditioning threshold, the MCP73837/8 enters the
constant current or fast charge mode.
4.5 Constant Current MODE - Fast
Charge
During the constant current mode, the programmed
(AC-adapter) or selected (USB-port) charge current is
supplied to the battery or load.
For AC-adapter, the charge current is established
using a single resistor from PROG1 to V
SS
. The
program resistor and the charge current are calculated
using the following equation:
EQUATION 4-1:
where R
PROG
is in kilo-ohms (kΩ) and I
REG
is in
milliampers (mA).
When charging from a USB-port, the host
microcontroller has the option of selecting either a one
unit load or a five unit loads charge rate based on the
PROG2 input. A logic LOW selects a one unit load
charge rate, a HIGH selects a five unit loads charge
rate, and high impedance input suspends or disables
charging.
Constant current mode is maintained until the voltage
at the V
BAT
pin reaches the regulation voltage, V
REG
.,
when constant current mode is invoked, the internal
timer is reset.
4.5.1 TIMER EXPIRED DURING
CONSTANT CURRENT - FAST
CHARGE MODE
If the internal timer expires before the recharge voltage
threshold is reached, a timer fault is indicated and the
charge cycle terminates. The MCP73837/8 remains in
this condition until the battery is removed, the input
battery is removed or the PROG1/2 pin is opened. If the
battery is removed or the PROG1/2 pin is opened, the
MCP73837/8 enters the Stand-by mode where it
Note: If the input power is switched during a
charge cycle, the power path switch-over
shall be a break-before-make connection.
As a result, the charge current can
momentarily go to zero. The charge cycle
timer shall remain continuous.
Note: USB Specification Rev. 2.0 defines the
maximum absolute current for one unit
load is 100 mA. This value is not an aver-
age over time and shall not be exceed.
I
REG
1000V
R
PROG
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