FAN5038

PRODUCT SPECIFICATION

10

REV. 1.0.2 7/6/00

Output Voltage Selection

The FAN5038 precision reference is trimmed to be 1.5V

nominally. When using the FAN5038, the system designer

has complete flexibility in choosing the output voltage for

each regulator from 1.5V to 3.6V. This is done by appropri-

ately selecting the feedback resistors. These could be 0.1%

resistors to realize optimum output accuracy. The following

equations determine the output voltages of the two regulators:

Switching Regulator:

Linear Regulator:

where R8 > 1.5k

Ω and (R8 + R3) ≤ 25kΩ and R10 > 1.5kΩ

and (R9 + R10)

≤ 25kΩ

Example:

For 3.3V,

Input Capacitors

The number of input capacitors required for the FAN5038 is

dependent on their ripple current rating, which assures their

rated life. The number required may be determined by

1.5V output at 4A, 5V input, and using the Sanyo capacitors

specified in Table 1 which have a 1.9A ripple current rating,

we have DC = 1.5/5 = 0.3, and

so that we need 1 input capacitor.

Linear Regulator Design Considerations

Figure 1 shows the application schematic for the FAN5038

with an NPN used for the linear regulator.

Careful consideration must be given to the base current of

the power NPN device. The base current to the power NPN

device is limited by:

• The FAN5038 op-amp output current (50mA)

• The internal power dissipation of the FAN5038 package

• The

β of the power NPN device.

The internal FAN5038 power dissipation is the most impor-

tant limitation for this application. For optimum reliability,

we require that the junction temperature not exceed 130°C;

thus we can calculate the maximum power dissipation allow-

able for this 16-lead SOIC package as follows:

the thermal resistance of the 16-lead SOIC package is

112°C/W, then the maximum power dissipation for the IC is:

determined by the current used.

op-amp output current is:

The power NPN transistor must have a minimum

β of 12.5 at

limit of the 16-SOIC package.

V

OUT

1.5

R8

R3

+

R8

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





×

=

V

OUT

1.5

R10

R9

+

R10

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





×

=

V

OUT

1.5

R10

R9

+

R10

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





×

1.5

12k

10k

+

10k

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





×

3.3V

==

=

No. Caps

I

out

DC

DC

2

–

*

I

rating

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

=

(2)

No. Caps

4∗ 0.3

0.3

2

–

1.9

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

0.96

==

P

D

T

J max

–

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

=

P

D

130

70

–

112

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

0.533W

≤

=

P

D

P

SW

P

LR

+

==

35mA

5.25V

×

()

12.6V

V

OUT

–V

BE

–

() I

OL

0.533W

≤

×

+

I

OL

0.533W

35mA

5.25V

×

()

–

12.6V

3.135V

–

0.8V

–

()

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

40mA

≤

=

β

500mA

40mA

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

≥

12.5

=