NCP1521B

http://onsemi.com

11

APPLICATION INFORMATION

Output Voltage Selection

The output voltage is programmed through an external

For low power consumption and noise immunity, the

resistor from FB to GND (R2) should be in the

the current through the divider will be 3.0

mA.

desired R1 and the R1 value:

VOUT + VFB

(1 ) R1

R2

)

(eq. 2)

Input Capacitor Selection

In PWM operating mode, the input current is pulsating

with large switching noise. Using an input bypass capacitor

can reduce the peak current transients drawn from the

input supply source, thereby reducing switching noise

significantly. The capacitance needed for the input bypass

capacitor depends on the source impedance of the input

supply.

The maximum RMS current occurs at 50% duty cycle

For NCP1521B, a low profile, low ESR ceramic

capacitor of 4.7

mF should be used for most of the cases. For

effective bypass results, the input capacitor should be

Table 1. List of Input Capacitor

Murata

GRM188R60J475KE

GRM21BR71C475KA

Taiyo Yuden

JMK212BY475MG

TDK

C2012X5ROJ475KB

C1632X5ROJ475KT

Output L−C Filter Design Considerations

The NCP1521B operates at 1.5 MHz frequency and uses

current mode architecture. The correct selection of the

output filter ensures good stability and fast transient

response.

Due to the nature of the buck converter, the output L−C

filter must be selected to work with internal compensation.

For NCP1521B, the internal compensation is internally

fixed and it is optimized for an output filter of L = 2.2

mH

The corner frequency is given by:

fc +

1

2p L

COUT

(eq. 3)

+

1

2p 2.2 mH

10 mF

+ 34 kHz

The device is intended to operate with inductance values

between 1.0

mH and maximum of 4.7 mH.

If the corner frequency is moved, it is recommended to

check the loop stability depending on the output ripple

voltage accepted and output current required. For lower

frequency, the stability will be increased; a larger output

capacitor value could be chosen without critical effect on

the system. On the other hand, a smaller capacitor value

increases the corner frequency and it should be critical for

the system stability. Take care to check the loop stability.

The phase margin is usually higher than 45

°.

Table 2. L−C Filter Example

Inductance (L)

1.0

22

2.2

10

4.7

4.7

Inductor Selection

The inductor parameters directly related to device

performances are saturation current and DC resistance and

inductance value. The inductor ripple current (

decreases with higher inductance:

DIL +

VOUT

L

fSW

1−

VOUT

VIN

(eq. 4)

L inductor value

The saturation current of the inductor should be rated

higher than the maximum load current plus half the ripple

current:

IL(MAX) + IO(MAX) )

DIL

2

(eq. 5)

The inductor’s resistance will factor into the overall

efficiency of the converter. For best performances, the DC

resistance should be less than 0.3

W for good efficiency.