MP2678 Rev. 1.0

www.MonolithicPower.com

10

1/6/2011

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© 2011 MPS. All Rights Reserved.

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 most MPS linear chargers has a typical I-V

curve and provides overcharge, input over

voltage, over temperature protection. The

function of the MP2678 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, MP2678 provides full protection for

these chargers whose protection function is not

so complete especially those without input

surge voltage sustain. MP2678 guarantees the

safety of the charge system with its perfect 4

protection functions: OVP, BOVP, OCP and

OTP.

An internal N-MOSFET is used for regulating

the output voltage to be constant at 5V with

input voltage up to the over voltage protection

threshold.

The MP2678 is a simple device that requires

few external components, in addition to the

linear charger circuit as shown in the Typical

Application Circuit. The selection of MP2678’s

external components is shown as follow.

the over shoot during transients. The AC

adapter is inserted live (hot insertion) and

sudden step down of the current may cause the

input voltage overshoot. During an input OVP,

the N-MOSFET is turned off in less than 1µs

and can lead to significant over shoot. Higher

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 MP2678 and the input of the

charging circuit typically share one decoupling

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 MP2678 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 4.35V 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.

pulled low. The maximum sink current must not

beyond 5mA when the worse case happens.

That means the input voltage is 30V. So the

The recommended value is between 6kΩ to

200kΩ. While a LED is used to indicate the

status, in order to drive the LED, a smaller

resistor should be selected such as 6.04kΩ.