ADP3050

–8–

REV. 0

U1

ADP3050

1

2

3

4

SWITCH

BOOST

BIAS

FB

IN

GND

SD

COMP

8

7

6

5

R1

20k

R2

21.5k

C3

0.22 F

D1

1N5817

GND

5V

C1

2

10 F

CERAMIC

+

C2

0.01 F

L1

22 H

2.5V

7.5k

4.7nF

D2

1N4148

+ C4

2

22 F

CERAMIC

Figure 23. Adjustable Output Application Circuit

APPLICATION INFORMATION

The complete process for designing a step-down switching regu-

lator using the ADP3050 is given in the following sections. Each

section includes a list of recommended devices. These lists do

not include every available device, nor every available manufac-

turer. They contain only surface-mount devices, but equivalent

through-hole devices can be substituted if needed. In choosing

components, keep in mind what is most important to the design

(efficiency, cost, size, etc.) as these things will ultimately deter-

mine which components are used. Also, make sure the design

specifications are clearly defined and that they reflect the worst-

case conditions. Key specifications include the minimum and

maximum input voltage, the output voltage and ripple, and the

minimum and maximum load current.

INDUCTOR SELECTION

The inductor value will determine the mode of operation for the

regulator: continuous mode, where the inductor current flows

continuously; or discontinuous mode, where the inductor current

reduces to zero during every switch cycle. Continuous mode is

the best choice for many applications. It provides higher output

power, lower peak currents in the switch, inductor, and diode,

and a lower inductor ripple current (which means lower output

ripple voltage). Discontinuous mode does allow the use of smaller

magnetics, but at a price: lower available load current, and higher

peak and ripple currents. Designs with a high input voltage or a

low load current often operate in discontinuous mode to mini-

mize inductor value and size. The ADP3050 is designed to work

well in both modes of operation.

Continuous Mode

The inductor current in a continuous mode system is a triangular

waveform (equal to the ripple current) centered around a dc value

(equal to the load current). The amount of ripple current is deter-

mined by the inductor value, and is usually between 20% and 40%

of the maximum load current. To reduce the inductor size, ripple

currents between 40% and 80% are often used in continuous

mode designs with a high input voltage or a low output current.

The inductor value can be calculated using the following equation:

L

VV

If

V

V

IN

OUT

RIPPLE

SW

OUT

IN

=

−

××

(MAX)

(MAX)

1

(2)

The initial choice for the amount of ripple current may seem arbi-

trary, but it will serve as a good starting point for finding a standard

off-the-shelf inductor value (i.e., 10

µH, 15 µH, 22 µH, 33 µH, and

47

µH). If a specific inductance value is to be used, simply rearrange

the above equation to find the ripple current. For an 800 mA, 12 V

to 5 V system, and a ripple current of 320 mA (40% of 800 mA) is

chosen, the inductance would be:

LH

=

− ×

×

×=

µ

12

5

032

1

200

10

5

12

45 5

3

.

.

A 47

µH inductor is the closest standard value, which gives a ripple

current of about 310 mA. The peak switch current is equal to the

load current plus one-half the ripple current (this is also the peak

current for the inductor and the catch diode):

II

1

2

I

0.8

0.155

0.95 A

SW(PK)

OUT(MAX)

RIPPLE

=+

=+

=

(3)

Pick an inductor with a dc (or saturation) current rating about

near the edge of saturation. For this example, 1.20

0.95 A =

1.14 A, so use an inductor with a dc current rating of at least 1.2 A.

The maximum switch current is internally limited to 1.5 A, and

this limit, along with the ripple current, will determine the maxi-

mum load current the system can provide.

If the load current decreases to below one-half the ripple current,

the regulator will operate in discontinuous mode.

Discontinuous Mode

For load currents less than around 0.5 A, discontinuous mode

operation can be used. This will allow the use of a smaller induc-

tor, but the ripple current will be much higher (which means a

higher output ripple voltage). If a larger output capacitor must be

used to reduce the output ripple voltage, the overall system may

actually take up more board area than if a larger inductor was

used. The operation and equations for the two modes are quite

different, but the boundary between these two modes occurs

when the ripple current is equal to twice the load current (when

minimum inductor value needed to keep the system in continu-

2

L

VV

If

V

V

DIS

IN

OUT

OUT

SW

OUT

IN

=

−

×

××

(MAX)

(MAX)

2

1

(4)

Using an inductor below this value will cause the system to operate

in discontinuous mode. For a 400 mA, 24 V to 5 V system:

L

H

−

×

×

×

×

≤µ

24

5

20 4

1

200

10

5

24

24 7

3

.

.

If the chosen inductor value is too small, the internal current

limit will trip each cycle and the regulator will have trouble

providing the necessary load current.

Inductor Core Types and Materials

Many types of inductors are currently available. Numerous core

styles along with numerous core materials often make the selec-

tion process seem even more confusing. A quick overview of the