ELECTRICAL CHARACTERISTICS (continued)

Symbol

Parameter

Test Conditions

Min.

Typ.

Max.

Unit

∆T

Output Voltage Drift

1.2

mV/

°C

SVR

Supply Voltage Rejection

Regulator

Filter Mode

71

35(*)

dB

Short Circuit Current

250

300

mA

Switch On Time

Regulator

Filter Mode

300

500(*)

ms

Thermal Shutdown Junction

Temperature

145

°C

PRINCIPLE OF OPERATION

During normal operation (input voltage upper than

normal voltage regulator built around the OP1 of

the block diagram.

The series pass element uses a PNP-NPN connec-

tion to reduce the dropout. The reference voltage

of the OP1 is derived from a REF through the OP2

and Q3, acting as an active zener diode of value

In this condition the device works in the range (1)

of the characteristic of the non linear drop control

unit (see Figure 1).

Figure 1 :

Nonliner Transfer Characteristic of

the Drop Control Unit

The output voltage is fixed to its nominal value :



1

+

R1

R2









1

+

R1

R2







The ripple rejection is quite high (70dB) and inde-

On the usual voltage regulators, when the input

vol-tage goes below the nominal value, the regula-

tion transistors (series element) saturate bringing

the system out of regulation and making it very

sensible to every variation of the input voltage. On

the contrary, a control loop on the L4915 consents

to avoid the saturation of the series element by

regulating the value of the reference voltage

(pin 2). In fact, whenever the input voltage de-

a non linear OTA, forces the reference voltage at

the static mode, when the input voltage goes below

condition the device works as a low pass filter in

the range (2) of the OTA characteristic. The ripple

follows :

SVR







(jΩ

)







= 1 +











6

gm







1

+

R1

R2

















Where:

gm =

=

2

OTA’S typical transconductance value on

linear region

R1

R2

= fixed ratio

µF

The reaction time of the supervisor loop is given by

the value of the ripple voltage is so high and its

negative peak is fast enough to determine an istan-

taneous decrease of the dropout till 1.2V, the OTA

works in a higher transconductance condition

[range (3) of the characteristic] and discharges the

capacitor rapidously.

If the ripple frequency is high enough the capacitor

won’t charge itself completely, and the output volt-

age reaches a small value allowing a better ripple

rejection ; the device’s again working as a filter (fast

transient range).

is obtained.

L4915

3/7