NCP1421D Просмотр технического описания (PDF) - ON Semiconductor

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NCP1421D

600 mA Sync-Rect PFM Step-Up DC-DC Converter with True-Cutoff and Ring-Killer

ON Semiconductor 

NCP1421

TYPICAL APPLICATION CIRCUIT

VIN

C1

22 mF

L

6.5 mH

Shutdown

Open Drain

Input

R2 200 k

Low Battery

Open Drain

Output

*Optional

C4 R3

10 p* 220 k

R1

350 k

NCP1421

R4

330 k

1

FB

2

LBI/EN

3

LBO

4

REF

C3

200 nF

8

OUT

7

LX

6

GND

5

BAT

C2 +

22 mF

VOUT =3.3 V

500 mA

Figure 26. Typical Application Schematic for 2 Alkaline Cells Supply

GENERAL DESIGN PROCEDURES

Switching mode converter design is considered a

complicated process. Selecting the right inductor and

capacitor values can allow the converter to provide

optimum performance. The following is a simple method

based on the basic first−order equations to estimate the

inductor and capacitor values for NCP1421 to operate in

Continuous Conduction Mode (CCM). The set component

values can be used as a starting point to fine tune the

application circuit performance. Detailed bench testing is

still necessary to get the best performance out of the circuit.

Design Parameters:

VIN = 1.8 V to 3.0 V, Typical 2.4 V

VOUT = 3.3 V

IOUT = 500 mA (600 mA max)

VLB = 2.0 V

VOUT−RIPPLE = 45 mVp−p at IOUT = 500 mA

Calculate the feedback network:

Select R2 = 200 k

ǒ Ǔ R1 + R2

VOUT

VREF

*

1

ǒ Ǔ R1 + 200 k

3.3 V

1.20 V

*

1

+ 350 k

Calculate the Low Battery Detect divider:

VLB = 2.0 V

Select R4 = 330 k

ǒ Ǔ R3 + R4

VLB

VREF

*

1

ǒ Ǔ R3 + 300 k

2.0 V

1.20 V

*

1

+ 220 k

Determine the Steady State Duty Ratio, D, for typical

VIN. The operation is optimized around this point:

VOUT

VIN

+

1

1

*

D

D

+

1

*

VIN

VOUT

+

1

*

2.4

3.3

V

V

+

0.273

Determine the average inductor current, ILAVG, at

maximum IOUT:

ILAVG

+

IOUT

1*D

+

500 mA

1 * 0.273

+

688

mA

Determine the peak inductor ripple current, IRIPPLE−P,

and calculate the inductor value:

Assume IRIPPLE−P is 20% of ILAVG. The inductance of the

power inductor can be calculated as follows:

L

+

2

VIN tON

IRIPPLE*P

+

2.4

2

V 0.75 mS

(137.6 mA)

+

6.5

mH

A standard value of 6.5 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 = 500 mA

COUT

u

IOUT

VOUT*RIPPLE *

tON

IOUT

ESRCOUT

where tON = 0.75 uS and ESRCOUT = 0.05 W,

COUT

u

45

500 mA

mV * 500

0.75

mA

mS

0.05

W

+

18.75

mF

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