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:

ILIM

ILIM

LIM

R

R

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.