FAN2534/FAN2535

PRODUCT SPECIFICATION

6

REV. 1.0.2 8/26/03

Control Functions

Enable Pin

Connecting 2.0V or greater to the Enable pin will enable the

output, while 0.4V or less will disable it while reducing the

quiescent current consumption to less than 1µA. If this

shutdown function is not needed, the pin can simply be

to float will cause erratic operation.

Error Flag (FAN2535-XX Only)

Fault conditions such as input voltage dropout

current), will set an error flag: The ERR pin which is an

95% or the specified output voltage. When the voltage at

the ERR pin is HIGH. A logic pullup resistor of 100K Ohm

is recommended at this output. The pin can be left discon-

nected if unused.

Thermal Protection

The FAN2534/FAN2535 is designed to supply high peak

output currents for brief periods, however sustained exces-

sive output load at high input - output voltage difference

will cause the device temperature to increase and exceed

maximum ratings due to power dissipation. During output

overload conditions, when the die temperature exceeds the

shutdown limit temperature of 155°C, an onboard thermal

protection will disable the output until the temperature drops

approximately 15°C below the limit, at which point the

output is re-enabled. During a thermal shutdown situation

the user may assert the power-down function at the Enable

pin, reducing power consumption to a minimum.

Thermal Characteristics

The FAN2534/FAN2535 is designed to supply 150mA at

the specified output voltage with an operating die (junction)

temperature of up to 125°C. Once the power dissipation and

thermal resistance is known, the maximum junction tempera-

ture of the device can be calculated. While the power dissipa-

tion is calculated from known electrical parameters, the

actual thermal resistance depends on the thermal characteris-

tics of the SOT23-5 surface-mount package and the

surrounding PC Board copper to which it is mounted.

The power dissipation is equal to the product of the input-to-

output voltage differential and the output current plus the

ground current multiplied by the input voltage, or:

provided in the Electrical Characteristics section.

The relationship describing the thermal behavior of the

package is:

ing temperature.

board layout and can be empirically obtained. While the

(junction-to-case) of the SOT23-5 package is specified at

130°C /W, the

least 235°C /W. This can be improved upon by providing a

heat sink of surrounding copper ground on the PWB.

Depending on the size of the copper area, the resulting

can range from approximately 180°C /W for one square inch

to nearly 130°C /W for 4 square inches. The addition of

backside copper with through-holes, stiffeners, and other

enhancements can also aid in reducing thermal resistance.

The heat contributed by the dissipation of other devices

located nearby must be included in the design consider-

ations. Once the limiting parameters in these two relation-

ships have been determined, the design can be modified to

ensure that the device remains within specified operating

conditions. If overload conditions are not considered, it is

possible for the device to enter a thermal cycling loop, in

which the circuit enters a shutdown condition, cools, re-

enables, and then again overheats and shuts down repeatedly

due to an unmanaged fault condition.

Adjustable Version

The FAN2534 adjustable version includes an input pin ADJ

which allows the user to select an output voltage ranging

equal to the onboard bandgap reference voltage of

1.00V(typ). The equation is:

Since the bandgap reference voltage is trimmed, 1% initial

accuracy can be achieved. The total value of the resistor

chain should not exceed 250K Ohm total to keep the error

amplifier biased during no-load conditions. Programming

supply, and temperature variations. Note that the low-leak-

age FET input to the CMOS Error Amplifier induces no bias

current error to the calculation.

P

D

V

IN

V

OUT

–

()I

OUT

V

+

=

P

D max

()

T

J max

–

θ

JA

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







=

V

OUT

1.00V

1

R

upper

R

lower

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

+

×

=