NCP1417 datasheet(12/16 Pages) ONSEMI

Part #	NCP1417

Description	200 mA DC-DC Step-up Converter with Dual Low Battery Protection

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NCP1417

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12

With the feedback resistor divider, additional small

capacitor, CFB1 in parallel with RFB1 is required to ensure

stability. The value can be in between 68 nF to 220 nF, the

rule is to select the lowest capacitance to ensure stability.

Also a small capacitor, CFB2 in parallel with RFB2 may also

be needed to lower the feedback ripple hence improve

output ripple and regulation. In this example, only CFB1 is

used and the value is 150 nF.

Calculate the Low Battery Detect divider:

VLB1 = 2.3 V

Select RLB2 = 330 K

RLB1 + RLB2

VLB1

VREF

* 1

RLB1 + 330 K

2.3 V

1.19 V

* 1 + 308 K

Once the VLB1 is set, the next low battery detection point,

VLB2 will be fixed automatically.

Determine the Steady State Duty Ratio, D for typical VIN,

operation will be optimized around this point:

VOUT

VIN

+

1

* D

D

+ 1 *

VIN

VOUT

+ 1 * 2.4 V

3.3 V

+ 0.273

Determine the average inductor current, ILAVG at maximum

IOUT:

ILAVG +

IOUT

1

* D +

200 mA

1

* 0.273 +

275 mA

Determine the peak inductor ripple current, IRIPPLE–P and

calculate the inductor value:

Assume IRIPPLE–P is 25% of ILAVG, the inductance of the

power inductor can be calculated as follows:

IRIPPLE–P + 0.25

275 mA

+ 68.8 mA

L

+

VIN

tON

2IRIPPLE–P

+

2.4 V

1.4

mS

2(68.8 mA)

+ 24.4 mH

Standard value of 22

mH is selected for initial trial.

Determine the output voltage ripple, VOUT–RIPPLE and

calculate the output capacitor value:

VOUT

* RIPPLE + 40 mVP–P at IOUT + 200 mA

COUT u

IOUT

tON

VOUT–RIPPLE * IOUT

ESRCOUT

where tON + 1.4 mS and ESRCOUT + 0.15 W,

COUT u

200 mA

1.4

mS

40 mV

* 200 mA

0.15

W +

28

mF

From above calculation, you need at least 28

mF in order

to achieve the specified ripple level at conditions stated.

Practically, a one level larger capacitor will be used to

accommodate factors not take into account in the

calculation. So a capacitor value of 33

mF is selected as

initial trial.

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