MP2633 – 1.5A SINGLE CELL SWITCH MODE BATTERY CHARGER

MP2633 Rev. 1.05

www.MonolithicPower.com

28

4/19/2013

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%

35

TH

//R

R

R

//R

R

V

V

NTC_Hot

T2

T1

NTC_Hot

T2

SYS

TH

=

=

+

=

(17)

the upper bound of its operating temperature

determine the upper and lower temperature limits.

This flexibility allows the MP2633 to operate with

most of NTC resistors for different temperature

follows:

)

R

R

(

TL

TH

)

TH

TL

(

R

R

R

NTC_Hot

NTC_Cold

NTC_Cold

NTC_Hot

T1

−

×

×

−

×

×

=

(18)

NTC_Hot

NTC_Cold

NTC_Hot

NTC_Cold

T2

R

TL

TH)

-

(1

-

R

TH

)

TL

1

(

R

R

)

TH

TL

(

R

×

×

×

×

−

×

×

−

=

(19)

For example, the NCP18XH103 thermistor has

the following electrical characteristic:

Based on equation (18) and equation (19),

RT1=6.47kΩ and RT2 = 21.35kΩ are suitable for

an NTC window between 0°C and 50°C. Chose

NTC

SYS

Low Temp Threshold

High Temp Threshold

Figure 12: NTC Function Block

Setting the System Voltage in Boost Mode

In the boost mode, the system voltage can be

regulated to the value customer required

between 4.2V to 6V by the resistor divider at FB

pin as R1 and R2 in the typical application circuit.

2

R

2

R

1

R

V

2

.

1

V

SYS

+

×

=

(20)

Where 1.2V is the voltage reference of SYS. With

a typical value for R2, 10kΩ, R1 can be

determined by:

)

V

(

V

2

.

1

V

2

.

1

V

2

R

1

R

×

=

(21)

For example, for a 5V system voltage, R2 is

10kΩ, and R1 is 31.6kΩ.

Setting the Output Current Limit in Boost

Mode

In boost mode, connect a resistor from the OLIM

pin to AGND to program the output current limit.

The relationship between the output current limit

and setting resistor is as follows:

7

.

1

)

m

(

1

RS

)

A

(

I

)

mV

(

40

)

k

(

70

)

k

(

R

OLIM

OLIM

×

Ω

×

×

Ω

=

Ω

(22)

be programmed up to 1.5A.

Selecting the Inductor

Inductor selection trades off between cost, size,

and efficiency. A lower inductance value

corresponds with smaller size, but results in

higher ripple currents, higher magnetic hysteretic

losses, and higher output capacitances. However,

a higher inductance value benefits from lower

ripple current and smaller output filter capacitors,

but results in higher inductor DC resistance (DCR)

loss.

Choose an inductor that does not saturate under

the worst-case load condition.

1. Charge Mode

When MP2633 works in charge mode (as a

buck

converter),

estimate

the

required

inductance as:

S

IN

BATT

MAX

_

L

BATT

IN

f

V

V

I

V

V

L

×

×

Δ

−

=

(