MP2671 – CHARGE SYSTEM PROTECTION CIRCUIT
MP2671 Rev. 0.9 www.MonolithicPower.com 8
2/23/2009 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2009 MPS. All Rights Reserved.
OPERATION
The MP2671 is designed to provide a
redundant safety protection to a Li-Ion battery
from failures of the charging system. The IC
continuously monitors the input voltage, the
battery voltage, and the charge current. When
any of the above three parameters exceeds its
limit, the IC turns off an internal N-channel
MOSFET to remove the power from the
charging system. The threshold of charge
current protection is user-programmable.
Additionally, the IC also monitors its own die
temperature and turns off the N-channel
MOSFET if the temperature exceeds 140°C.
Together with the battery charging IC and the
protection module in a battery pack, the
charging system has triple-level protection from
over-charging the Li-Ion battery and is two–fault
tolerant. The MP2671 protects up to 30V input
voltage.
Power Up
The MP2671 has a power-on reset (POR)
threshold of 2.6V with a built-in hysteresis of
110mV. When the input voltage is below the
POR threshold, the internal power FET is off.
The IC resets itself and waits for approximately
10ms after the input voltage exceeds the POR
threshold, then, if the input voltage and battery
voltage are safe, the IC begins to soft-start the
internal power FET. The 10ms delay allows any
transient at the input during a hot insertion of
the power supply to settle down before the IC
stars to operate. The soft-start slowly turns on
the power FET to reduce the inrush current as
well as the input voltage drop during the
transition.
Input Over-voltage Protection (OVP)
The input voltage is continuously monitored by
the comparator CP1 in the Block Diagram
(Figure 1). The OVP threshold is set by a
resistive divider consisting of R1 and R2 and an
accurate reference generated by the IC itself.
The protection threshold is set to 5.8V. When
the input voltage exceeds the threshold, the
CP1 outputs a logic signal to turn off the power
FET within 1µs to prevent the high input voltage
from damaging the electronics in the handheld
system. The hysteresis for the input OVP
threshold is given in the Electrical Specification.
When the input over-voltage condition is
removed, the power FET is turned on again by
running through the soft-start. Because of the
10ms second delay before the soft-start, the
output is never enabled if the input rises above
the OVP threshold quickly.
Over-current Protection (OCP)
The current in the power FET is limited to
prevent charging the battery with an excessive
current. The current is sensed using the voltage
drop across the power FET after the FET is
turned on. The reference of the OCP is
generated using a sensing FET Q2, as shown
in Figure 1. The current in the sensing FET is
forced to a value which can be programmed by
an external resistor connected to the ILIM pin.
The size of the power FET Q1 is 31,250 times
the size of the sensing FET Q2. Therefore,
when the current in the power FET is 31,250
times the current in the sensing FET, the drain
voltage of the power FET falls below that of the
sensing FET. The comparator CP2 then outputs
a signal to turn off the power FET. The OCP
threshold can be programmed using the
following formula:
ILIMILIM
LIM
RR
V
I
25000
31250
8.0
=⋅=
The OCP comparator CP2 has a built-in 170µs
delay to prevent false triggering by transient
signals. When OCP happened, Vo shuts down
for about 40ms and take 10ms delay to soft
start as the OCP condition is removed. The
OCP function also has a 4-bit binary counter
that accumulates during an OCP event. When
the total count reaches 16, the power PFET is
turned off permanently unless the input power
is recycled or the enable pin is toggled.
