AN EXAMPLE OF 90W MICRO ATX

POWER SUPPLY USING L5991A, VI-

PER20 AND TSM111

Protection against accidental short circuits and fault

conditions is mandatory in PC power supplies.

These protection circuits can be realized by using

many discrete components which occupy a lot of

PCB space, design time in fine tuning the circuit and

also add to assembling costs.

ST’s single chip TSM111 IC provides complete pro-

tection circuits design easier, with fewer number of

components. TSM111 is an ideal supervisor IC for

PC power supplies.

The salient features and benefits of this device are

listed below :

a) Over voltage protectionfor 3.3V,5V and12V with-

out external component.

b) Over current protection for 3.3V, 5V and 12V.

c) Generates Power Good signal.

d) Programmable timing for Power Good signal.

e) Wide range supply operating voltage up to 36V

(44V AMR).

f) Stable internal voltage reference.

g) Two 1.6% voltage reference for Main and Auxil-

iary regulation.

h) Few external components.

i) Circuit occupies little space on PCB.

j) Easy implementation of the circuit.

FEATURE DESCRIPTIONS

a) Over voltage protectioncan be implemented with-

out any additional components. Overvoltage sense

levels for 3.3V, 5V, 12V are 4V, 6.1V, 14.2V respec-

tively. With very little tolerances, better protection is

achieved.

b) Over current protection can be implemented with

very small value sense resistors. As the current

sense threshold levels are set very low, regulation

is not affected. The current sense threshold levels

for 3.3V, 5V, 12V are 50mV, 50mV and 65mV re-

spectively.

c) The power good signal (pin9) is asserted to indi-

cate the 5V and 3.3V is above the under voltage

threshold level. PG pin goes high when the above

condition is reached. Pull up resistor R27 (3.1K) is

connected to 5V STDBY supply from this pin.

d) The timing of the power good signal can be con-

trolled by adjusting the value of the charging capaci-

tor on pin 10. With 2.2

µF/16V capacitor on pin 10,

400msec turn on delay is achieved.

e) The power supply can be operated from 14.5V to

voltage on the 12V input pin by 2V .i.e. if 14V is ap-

pearing 12V input pin the Vcc must be > 16V.

A COMPLETE PC SMPS SECONDARY SUPERVISOR

TSM111

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