11

FN7360.2

January 31, 2005

Figure 18 shows the waveforms.

FIGURE 21. ADJUSTABLE START-UP

reduce input/output ripple because the pulse skipping nature

of the method.

2. CASCADE START-UP

In this configuration, EN pin of Regulator 2 is connected to

FIGURE 22. CASCADE START-UP

3. LINEAR START-UP

In the linear start-up tracking configuration, the regulator with

FIGURE 23. LINEAR START-UP TRACKING

4. OFFSET START-UP

Compared with the cascade start-up, this configuration

FIGURE 24. OFFSET START-UP TRACKING

Component Selection

INPUT CAPACITOR

The main functions of the input capacitor(s) are to maintain

the input voltage steady and to filter out the pulse current

passing through the upper switch. The root-mean-square

value of this current is:

For long-term reliability, the input capacitor or combination of

Use X5R or X7R type ceramic capacitors, or SPCAP or

POSCAP types of Polymer capacitors for their high current

handling capability.

INDUCTOR

The NFET positive current limit is set at about 0.5A. For

optimal operation, the peak-to-peak inductor current ripple

inductance value:

The peak current the inductor sees is:

When inductor is chosen, make sure the inductor can handle

OUTPUT CAPACITOR

If there is no holding time requirement for output; output

voltage ripple and transient response are the main deciding

factors in choosing the output capacitor. Initially, choose the

STP

-

+

STN

0.1µF

200K

R

C

EL7554

EL7554

EL7554

EN

PG

STP

-

+

STN

C

R

-

+

EL7554

EL7554

-

+

EL7554

EL7554

I

IN,RMS

V

O

V

IN

V

O

–

()

×

V

IN

-----------------------------------------------

I

O

1/2

≈

×

I

(

=

L

V

IN

(

V

V

O

×

–

V

IN

∆I

L

F

S

×

×

--------------------------------------------

=

I

LPK

I

O

∆I

L

2

--------

+

=

EL7554