MP2612 – 2A, 24V INPUT, 600kHz 2-3CELL SWITCHING LI-ION BATTERY CHARGER

MP2612 Rev. 1.0

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19

9/7/2011

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can be realized by re-calculate equation (4),(5)

and (8).

Selecting the Inductor

A 1µH to 10µH inductor is recommended for

most applications. The inductance value can be

derived from the following equation.

OUT

IN

OUT

IN

L

OSC

V(V

V

)

L

VI

f





 

(10)

the 2/3 cell battery voltage.

Choose inductor current to be approximately

30% if the maximum charge current, 2A. The

maximum inductor peak current is:



L(MAX)

CHG

I

II

2

(11)

Under light load conditions below 100mA, larger

inductance

is

recommended

for

improved

efficiency.

For optimized efficiency, the inductor DC

resistance is recommended to be less than

200mΩ.

NTC Function

As Figure 9 shows, the low temperature

threshold and high temperature threshold are

preset internally via a resistive divider, which are

73%·VREF33 and 30%·VREF33. For a given

NTC thermistor, we can select appropriate R3

and R6 to set the NTC window.

In detail, for the thermistor (NCP18XH103) noted

in above electrical characteristic,

Assume that the NTC window is between 0ºC

and 50ºC, the following equations could be

derived:

73%

VREF33

V

R6//R

R3

R6//R

TH_Low

NTC_Cold

NTC_Cold







(12)

30%

VREF33

V

R6//R

R3

R6//R

TH_High

NTC_Hot

NTC_Hot







(13)

According to equation (12) and equation (13), we

can find that R3 = 9.63k and R6 = 505k.

To be simple in project, making R3=10k and R6

no

connect

will

approximately

meet

the

specification.

NTC

VREF33

Low Temp Threshold

High Temp Threshold

R3

R6

Figure 9— NTC function block

Selecting the Input Capacitor

The input capacitor reduces the surge current

drawn from the input and also the switching noise

from the device. The input capacitor impedance

at the switching frequency should be less than

the input source impedance to prevent high

frequency switching current passing to the input.

Ceramic capacitors with X5R or X7R dielectrics

are highly recommended because of their low

ESR and small temperature coefficients. For

most applications, a 4.7µF capacitor is sufficient.

Selecting the Output Capacitor

The output capacitor keeps output voltage ripple

small and ensures regulation loop stability. The

output capacitor impedance should be low at the

switching frequency. Ceramic capacitors with

X5R or X7R dielectrics are recommended.

PC Board Layout

The high frequency and high current paths (GND,

IN and SW) should be placed to the device with

short, direct and wide traces. The input capacitor

needs to be as close as possible to the IN and

GND pins. The external feedback resistors

should be placed next to the FB pin. Keep the

switching node SW short and away from the

feedback network.