8

FN6654.1

December 8, 2010

completely dried out before the MOSFET is turned on again.

The input voltage source, the inductor, and the MOSFET and

output diode parasitic capacitors form a resonant circuit.

Oscillation will occur in this period. This oscillation is normal

and will not affect regulation.

At very low load, the MOSFET will skip pulses sometimes;

this is normal.

Current Limit

The MOSFET current limit is nominally 1.4A and guaranteed

on Equation 1:

where:

•

decided by Equation 2:

• D is the MOSFET turn-on ratio and is decided by

Equation 3:

Table1 gives typical values:

Component Considerations

Theoretically, the input capacitor has a ripple current of

Due to high-frequency noise in the circuit, the input current

ripple may exceed the theoretical value. A larger capacitor

will reduce the ripple further.

The inductor has peak and average current decided by

Equations 4 and 5:

The inductor should be chosen to handle this current.

Furthermore, due to fixed internal compensation, it is

recommended that maximum inductance of 10µH and 15µH

be used in the 5V and 12V or higher output voltage,

respectively.

current is the same as the inductor's peak current. A

Schottky diode is recommended and it should be able to

handle those currents.

The output voltage ripple can be calculated as Equation 6:

Where:

• The ESR is the output capacitor ESR value.

Low ESR capacitors should be used to minimize output

voltage ripple. Multilayer ceramic capacitors (X5R and X7R)

are preferred for output capacitors since they have a low

ESR and small packages. Tantalum capacitors also can be

used, but they take more board space and have higher ESR.

A minimum of 22µF output capacitor is sufficient for high

output current application. For lower output current, the

output capacitor can be smaller, like 4.7µF. The capacitor

should always have enough voltage rating. In addition to the

voltage rating, the output capacitor should also be able to

handle the RMS current, which is given by Equation 7:

Output Voltage

An external resistor divider is required to divide the output

voltage down to the nominal reference voltage. The current

drawn by the resistor network should be limited to maintain

the overall converter efficiency. The maximum value of the

resistor network is limited by the feedback input bias current

and the potential for noise being coupled into the feedback

pin. A resistor network less than 300k

Ω is recommended.

TABLE 1. MAX CONTINUOUS OUTPUT CURRENTS

(V)

(V)

L

(µH)

(kHz)

(mA)

2

5

10

750

360

2

9

10

750

190

2

12

10

750

140

3.3

5

10

750

600

3.3

9

10

750

310

3.3

12

10

750

230

5

9

10

750

470

5

12

10

750

340

9

12

10

750

630

12

15

10

750

670

1

ΔI

L

2

--------

–

⎝⎠

---------

×

=

(EQ. 1)

ΔI

L

L

---------

D

-----

×

=

(EQ. 2)

D

–

------------------------

=

(EQ. 3)

ΔI

L

2

--------

+

=

(EQ. 4)

1D

–

-------------

=

(EQ. 5)

ΔV

O

×

×

----------------------

×

+

=

(EQ. 6)

1

(

D

)

D

ΔI

L

2

2

--------------------

+

⎝

⎜

⎜

⎛

×

1

12

------

×

⎠

⎟

⎟

⎞

–

×

=

(EQ. 7)

ISL98012