NCP1403_05 datasheet(10/18 Pages) ONSEMI

No de pièce	NCP1403

Description	15 V/50 mA PFM Step?뭊p DC?묭C Converter

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NCP1403

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10

APPLICATIONS CIRCUIT INFORMATION

External Component Selection

Inductor

The NCP1403 is designed to work well with a range of

inductance values, the actual inductance value depends on

the specific application, output current, efficiency, and

output ripple voltage. For step up conversion, the device

works well with inductance ranging from 22

mH to 47 mH.

Inductor with small DCR, usually less than 1.0

W, should be

used to minimize loss. It is necessary to choose an inductor

with saturation current greater than the peak switching

current in the application.

If 22

mH inductance is used, lower profile surface mount

inductor can be selected for the same current rating.

Moreover, it permits the converter to switch at higher

frequency up to 300 kHz since the inductor current will ramp

up faster and hit the current limit at a shorter time for smaller

inductance value. However, current output are slightly

lower because the off−time is limited by the minimum

off−time. If 47

mH inductance is selected, higher efficiency

and output current capability are achieved, but the converter

will switch at a lower frequency and the inductor size will be

slightly larger for the same current rating.

For lower inductance value, the inductor current

ramp−down time will be shorter than the minimum off−time.

Consequently,

the

converter

can

only

operate

in

discontinuous conduction mode and lower output current

can be generated. For higher inductance value, if the

inductance is sufficiently large, the maximum on−time will

expire before the current limit is reached. As a result, the

available output power and output current are reduced.

Besides, instability may occur when operation enters CCM.

To ensure the current limit is reached before the maximum

on−time expires, L can be selected according to the

inequality below:

L

v

(VIN

* VS)

ILIM

@ ton(MAX)

where VS = 0.75 V which is the MOSFET saturation voltage,

and ILIM is the current limit which can be referred to in

Figure 11, and ton(MAX) = 6.0

ms.

If the above condition is satisfied, IPK = ILIM; where IPK

is the peak inductor current. Then, step−up converter with

inductor satisfy the following condition will operate in

DCM only,

ILIM

@ L

(VOUT ) VD * VIN)

v toff(MIN)

If the IPK = ILIM, step−up converter with inductor satisfy

the following condition will operate in CCM at maximum

output current,

ILIM

@ L

(VOUT ) VD * VIN)

u toff(MIN)

where VD is the Schottky diode forward voltage drop,

toff(MIN) = 1.3

ms.

For step−up converter operates in DCM only, the

maximum output current can be calculated from the

equation below:

IOUT(MAX) +

(ILIM)2 L

2(VOUT ) VD * VIN)

ILIM L

VIN*VS )

toff(MIN)

For step−up converter operates in CCM, the maximum

output current can be calculated from the equation below:

IOUT(MAX) + ILIM *

(VOUT ) VD * VIN)toff(MIN)

2L

@

(VIN

* VS)

(VOUT ) VD * VS)

Diode

The diode is the main source of loss in DC−DC converters.

The most importance parameters which affect their

efficiency are the forward voltage drop, VF, and the reverse

recovery time, trr. The forward voltage drop creates a loss

just by having a voltage across the device while a current

flowing through it. The reverse recovery time generates a

loss when the diode is reverse biased, and the current appears

to actually flow backwards through the diode due to the

minority carriers being swept from the P−N junction. A

Schottky diode with the following characteristics is

recommended:

1. Small forward voltage, VF < 0.3 V

2. Small reverse leakage current

3. Fast reverse recovery time / switching speed

4. Rated current larger than peak inductor current,

Irated > IPK

5. Reverse voltage larger than output voltage,

Vreverse > VOUT

Input Capacitor

The input capacitor can stabilize the input voltage and

minimize peak current ripple from the source. The value of

the capacitor depends on the impedance of the input source

used. Small ESR (Equivalent Series Resistance) Tantalum

or ceramic capacitor with value of 10

mF should be suitable.

Output Capacitor

The output capacitor is used for sustaining the output

voltage when no current is delivering from the input, and

smoothing the ripple voltage. Low ESR Tantalum capacitor

should be used to reduce output ripple voltage since the

output ripple voltage is dominated by the ESR value of the

Tantalum capacitor. In general, a 22

mF to 47 mF low ESR

(0.2

W to 0.4 W) Tantalum capacitor should be appropriate.

The output ripple voltage can be approximately given by the

following equation:

Vripple [ (IPK * IOUT) @ ESR

Feedback Resistors

Choose the RFB2 value from the range 10 k

W to 200 kW

for positive output voltage. The value of RFB1 can then be

calculated from the equation below:

RFB1 + RFB2

VOUT

0.8

* 1

1% tolerance resistors should be used for both RFB1 and

RFB2 for better VOUT accuracy.

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