LA5744

No.A0717-4/5

Protection Circuit Functional Descriptions

1. Overcurrent protection function

The overcurrent protection function detects, on a cycle-by-cycle basis, the output transistor current and turns off that

output transistor current if it exceeds 3.1A.

2. Short circuit protection function

This IC prevents the current from increasing when the outputs are shorted by setting the switching frequency to 30kHz

if the VOS pin voltage falls below 0.8V.

Note 3 : If the soft start function is not used, the IC will start up with the overcurrent protection function operating. At this

time, the switching frequency will be cut in half. This means that the switching frequency will be 15kHz at

startup.

Note 4 : Since the switching frequency becomes 30kHz when the VOS pin voltage falls under 0.8V, the current capacity is

reduced. If a load is applied with the VOS pin voltage over 0.8V, the inductance value operates at 22µH. If a load

is to be applied when this voltage is under 0.8V, the inductance value must be increased.

Description of Functional Settings

1. Calculation equation to set the output voltage

This IC controls the switching output so that the VOS pin voltage becomes 1.23V (typ).

The equation to set the output voltage is as follows :

The VOS pin has the inrush current of 1µA (typ). Therefore, the error becomes larger when R1 and R2 resistance

values are large.

2. Startup Delay Function

The output voltage rises when the internal voltage reaches 0.4V (typical). Until that point, a capacitor is charged from

an internal 10

µA (typical) constant-current supply. The startup delay time can be calculated as shown below

Example : Assume a 1

µF capacitor is used.

VO = 1+

× 1.23V(typ)

R2

R1

Td =

=

= 40 ms

C

× V

i

1

µF × 0.4

10

µA

Limit current

Inductor current

SWOUT voltage