MP2671 – CHARGE SYSTEM PROTECTION CIRCUIT

MP2671 Rev. 0.9

www.MonolithicPower.com

10

2/23/2009

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APPLICATION INFORMATION

For safe and effective charging, some strict

requirements have to be satisfied during

charging Li-Ion batteries such as high precise

power source for charging (4.2V±50mV), the

accuracy should be higher than 1%. For highly

used capacity, the voltage of the battery should

be charged to the value (4.2V) as possible as

could. Otherwise, the performance and the life

of the battery suffers overcharge. Additionally,

the pre-charge for depleted batteries, charging

voltage, charging current, as well as the

temperature detection and protection, are

required for linear battery chargers. The output

of any MP2602 family chargers has a typical I-V

curve and provides overcharge, input over

voltage, over temperature protection. The

function of the MP2671 is to add a redundant

protection layer such that, under any fault

condition, the charging system output does not

exceed the I-V limits that the battery required.

Additionally, MP2671 provides full protection for

these chargers whose protection function is not

so complete especially those without input

surge voltage sustain. MP2671 guarantees the

safety of the charge system with its perfect 4

protection functions: OVP, BOVP, OCP and

OTP

The MP2671 is a simple device that requires

few external components, in addition to the

MP2602 charger circuit as shown in the Typical

Application Circuit. The selection of MP2671’s

external components is shown as follow, and

has been introduced in the Over-current

Protection section.

the over shoot during transients. The AC

adapter is inserted live (hot insertion) and

sudden step down of the current in the power

FET will cause the input voltage overshoot.

During an input OVP, the FET is turned off in

less than 1µs and can lead to significant over

type of over shoot. However, the over shoot

caused by a hot insertion is not very dependent

on the decoupling capacitance value. Usually,

the input decoupling capacitor is recommended

to use a dielectric ceramic capacitor with a

value between 1µF to 4.7µF.

The output of the MP2671 and the input of the

charging circuit typically share one decoupling

capacitor. The selection of that capacitor is

mainly determined by the requirement of the

charging circuit. When using the MP2602 family

chargers, a 1µF to 4.7uF ceramic capacitor is

recommended.

battery terminal in case the MP2671 fails. The

recommended value is between 200kΩ to 1MΩ.

With 200kΩ resistance, during the failure

operation, assuming the VB pin voltage is 30V

and the battery voltage is 4.2V. The worst case

the current flowing from the VB pin to the

charger output is,

(30V - 4.2V)/200kΩ = 130µA,

Such small current can be easily absorbed by

the

bias

current

of

other

components.

case current, but at the same time increases

the error for the 4.4V battery OVP threshold.

As the typical VB pin leakage current is 20nA,

the error of the battery OVP threshold can be

resistor, the worst-case additional error is 4mV

and with a 1MΩ resistor, the worst-case

additional error is 20mV.

.