CS51033

http://onsemi.com

7

APPLICATIONS INFORMATION

DESIGNING A POWER SUPPLY WITH THE CS51033

Specifications

1) Duty Cycle Estimates

Since the maximum duty cycle D, of the CS51033 is

limited to 80% min., it is best to estimate the duty cycle for

the various input conditions to see that the design will work

over the complete operating range.

The duty cycle for a buck regulator operating in a

continuous conduction mode is given by:

D

+

VOUT ) VD

VIN * VSAT

where:

0.6 V so the equation reduces to:

D

+

VOUT

VIN

2) Switching Frequency and On and Off Time

Calculations

COSC +

95

FSW

1

*

FSW

3

30

103

FSW

2

^ 470 pF

T

+ 1.0

FSW

+ 5.0 ms

TON(MAX) + 5.0 ms

0.53

+ 2.65 ms

TON(MIN) + 5.0 ms

0.35

+ 1.75 ms

TOFF(MAX) + 5.0 ms * 0.7 ms + 4.3 ms

3) Inductor Selection

Pick the inductor value to maintain continuous mode

operation down to 0.3 Amps.

The ripple current

LMIN +

VOUT ) VD

TOFF(MAX)

DI

+

2.1 V

4.3

ms

0.6 A

^ 15 mH

The CS51033 will operate with almost any value of

inductor. With larger inductors the ripple current is reduced

and the regulator will remain in a continuous conduction

mode for lower values of load current. A smaller inductor

will result in larger ripple current. The core must not saturate

with the maximum expected current, here given by:

IMAX +

IOUT ) DI

2.0

+ 3.0 A ) 0.6 A 2.0 + 3.3 A

4) Output Capacitor

The output capacitor limits the output ripple voltage. The

CS51033 needs a maximum of 15 mV of output ripple for

the feedback comparator to change state. If we assume that

all the inductor ripple current flows through the output

capacitor and that it is an ideal capacitor (i.e. zero ESR), the

minimum capacitance needed to limit the output ripple to

50 mV peak−to−peak is given by:

CO

+

DI

8.0

FSW

DV

+

0.6 A

8.0

(200

103 Hz)

(33

10*3 V)

^ 11.4 mF

The minimum ESR needed to limit the output voltage

ripple to 50 mV peak−to−peak is:

ESR

+ DV

DI +

50

10*3

0.6 A

+ 55 mW

The output capacitor should be chosen so that its ESR is

at least half of the calculated value and the capacitance is at

least ten times the calculated value. It is often advisable to

use several capacitors in parallel to reduce ESR.

Low impedance aluminum electrolytic, tantalum or

organic semiconductor capacitors are a good choice for an

output capacitor. Low impedance aluminum are the

cheapest but are not available in surface mount at present.

Solid tantalum chip capacitors are available from a number

of suppliers and offer the best choice for surface mount

applications. The capacitor working voltage should be

greater than the output voltage in all cases.

VOUT + 1.25 V

R1

) R2

R2

+ 1.25 V R1

R2

) 1.0

The input bias current to the comparator is 4.0

mA. The

resistor divider current should be considerably higher than

this to ensure that there is sufficient bias current. If we

choose the divider current to be at least 250 times the bias

current this gives a divider current of 1.0 mA and simplifies

the calculations.

1.5 V

1.0 mA

+ R1 ) R2 + 1.5 kW

Let R2 = 1.0 k

Rearranging the divider equation gives:

R1

+ R2

VOUT

1.25

* 1.0 + 1.0 kW 1.5 V

1.25

+ 200 W