MP2625

– SINGLE CELL SWITCHING CHARGER WITH POWER PATH CONTROL

MP2625 Rev. 1.12

www.MonolithicPower.com

25

10/29/2013

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)

V

(

V

V

V

7

R

6

R

REF

_

SYS

REF

_

SYS





(8)

For example, for a 4.2V system voltage, R7 is

10kΩ, and R6 is 26.5kΩ.

Selecting the Inductor

Inductor selection trades off among cost, size,

and

efficiency.

A

lower

inductance

value

corresponds to a smaller size, but results in

higher

ripple

currents,

higher

magnetic

hysteretic

losses,

and

higher

output

capacitances. Choosing a higher inductance

value benefits from lower ripple current and

smaller output filter capacitors, but results in

higher inductor DC resistance (DCR) loss. From

a

practical

standpoint,

the

inductor

ripple

current does not exceed 30% of the maximum

load current under worst cases conditions. For

the MP2625 operating with a typical 5V input

voltage, the maximum inductor current ripple

occurs at the corner point between trickle

required inductance as:

)

MHz

(

f

V

V

I

V

V

L

S

IN

BATT

MAX

_

L

BATT

IN









(

H)

(9)

)

2

ripple

%

1

(

I

I

)

MAX

(

LOAD

PEAK







(mA)

(10)

voltage, the TC to CC charge threshold, and the

switching frequency, respectively.

ΔI

maximum inductor ripple current, which is

usually 30% of the CC charge current.

the

calculated

inductance

is

1.3µH.

The

maximum inductor peak current exceeds 2.3A.

To optimize efficiency, chose an inductor with a

DC resistance less than 50mΩ. Choose the

inductor 7447745012 from Wurth with ratings at

L=1.2µH/4.6

A /21mΩ.

Selecting the Input Capacitor

The

input

capacitor

C1

from

the

typical

application circuit absorbs the maximum ripple

current from the PWM converter, which is given

by

MAX

_

IN

TC

MAX

_

IN

TC

MAX

_

CC

MAX

_

RMS

V

)

V

V

(

V

I

I









For

the

maximum ripple current is 1A. Select the input

capacitors so that the temperature rise due to

the ripple current does not exceed 10°C. Use

ceramic capacitors with X5R or X7R dielectrics

because

of

their

low

ESR

and

small

temperature coefficients. For most applications,

use a 22µF capacitor.

Selecting the Output Capacitor

The output capacitor C2 from the typical

application circuit is in parallel with the SYS

load. C2 absorbs the high-frequency switching

ripple current and smoothes the output voltage.

Its impedance must be much less than that of

the system load to ensure it properly absorbs

the ripple current.

Use a ceramic capacitor because it has lower

ESR and smaller size that allows us to ignore

the ESR of the output capacitor. Thus, the

output voltage ripple is given by:

%

L

f

2

C

8

V

V

1

V

V

r

2

S

IN

SYS

SYS

SYS

















(12)

In order to guarantee the ±0.5% system voltage

accuracy, the maximum output voltage ripple

must

not

exceed

0.5%

(e.g.

0.1%). The

maximum output voltage ripple occurs at the

minimum system voltage and the maximum

input voltage.

For

L=1.2µH,

be calculated as:

r

L

f

8

V

V

1

2

C

2

S

IN

MIN

_

SYS













(13)

We can then choose a 22µF ceramic capacitor.

Resistor Choose for NTC Sensor

Figure 8 shows an internal resistor divider

reference circuit to limit the low temperature

threshold and high temperature threshold at

65%·VCC and 33.5%·VCC, respectively. For a