DS3717 Rev 12

11/22

L4979D-E, L4979MD

Application information

21

3

Application information

3.1

Voltage regulator

The voltage regulator uses a p-channel MOS transistor as a regulating element. With this

structure, a low dropout voltage at current up to 150 mA is achieved. The output voltage is

regulated up to a transient input supply voltage of 40 V. No functional interruption due to

overvoltage pulses is generated. The high precision of the output voltage is obtained with a

pretrimmed reference voltage. A short -circuit protection to GND is provided.

Figure 3. Behavior of output current versus regulated voltage Vo

3.2

Reset

The reset circuit monitors the output voltage Vo. If the output voltage drops below Vo_th then

Res becomes low with a delay time trr. Real trr value changes as a non-linear function of

delta (Vo-th - Vo). The reset low signal is guaranteed for an output voltage Vo greater than

1 V.

When the output voltage becomes higher than Vo_th then Res goes high with a delay trd. This

delay is obtained by 512 periods of an oscillator (see fig. 5). The oscillator period is given

by:

and reset pulse delay trd is given by:

Vo

Vo_ref

Iout

Ishort Ilim

Vo

Vo_ref

Iout

Ishort Ilim

GAPGCFT00203

TOSC

Vrhth Vrlth

–

 C

tr





Icr

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

Vrhth Vrlth

–

 C

tr





Idr

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

+

=

trd

512 TOSC



=