MP2611 – 2A, 1-CELL SWITCHING CHARGER FOR USB AND ADAPTER POWER

MP2611 Rev. 1.12

www.MonolithicPower.com

17

12/7/2012

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CC

L_MAX

15%I

=

ΔI

(6)

calculated inductance is 2.66µH. The maximum

inductor peak current must exceed 2.3A. To

optimize efficiency, chose an inductor with a DC

resistance less than 50mΩ. Choose the inductor

7447745022 from Wurth Corporation with ratings

at L=2.2µH/3.5A /36mΩ.

NTC Function

Figure 6 shows that an internal resistor divider

sets the low temperature threshold and high

temperature threshold at 74%·VREF33 and

32%·VREF33, respectively. For a given NTC

the NTC window.

The thermistor (NCP18XH103) noted above has

the following electrical characteristic:

The following equations are derived assuming

that the NTC window is between 0°C and 50°C:

74%

VREF33

V

//R

R

R

//R

R

TH_Low

NTC_Cold

T2

T1

NTC_Cold

T2

=

=

+

(7)

32%

VREF33

V

//R

R

R

//R

R

TH_High

NTC_Hot

T2

T1

NTC_Hot

T2

=

=

+

(8)

approximates the values.

Figure 6: NTC Function Block

Selecting the Input Capacitor

The input capacitors C1/C2 from the typical

application circuit absorbs the maximum ripple

current from the buck converter, which is given

by:

IN_MAX

TC

IN_MAX

TC

CC_MAX

RMS_MAX

V

)

V

(V

V

I

I

−

=

(9)

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—C4 from the typical

application circuit—is in parallel with the battery.

C4 absorbs the high-frequency switching ripple

current and smoothes the output voltage. Its

impedance must be much less than that of the

battery to ensure it 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

8C

V

V

1-

V

V

Δr

2

S

O

IN

O

O

O

O

=

Δ

=

(10)

In order to guarantee the ±0.5% battery 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

battery voltage of the CC charge and the

maximum input voltage.

For

L=1.5µH,

%

1

.

0

=

Δ

O_MAX

r

, the output capacitor

can be calculated as:

TC

IN_MAX

O

2

SO_MAX

V

1-

V

C=

=18.5μF

8f LΔr

(11)

We can then choose a 22µF ceramic capacitor.