L4916 datasheet(4/8 Pages) STMICROELECTRONICS

4 / 8 page

Figure 2 : P.C. Board and Component Layout of Fig. 1 (1 : 1 scale).

PRINCIPLE OF OPERATION

During normal operation (input voltage upper than

VI MIN =VOUT NOM +

∆VI/O). The device works as a

normal voltage regulator built aroundthe OP1 of the

block diagram.

The series pass element use a PNP-NPN connec-

tion to reduce the dropout. The reference voltage of

the OP1 is derived from a REF through the OP2and

Q3, acting as an active zener diode of value VREF.

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

the characteristic of the non linear drop control unit

(see fig.3).

The output voltage is fixed to its nominal value:

VOUT NOM =VREF (1 +

) =

VCFT (1 +

)

= INTERNALLY FIXED RATIO = 2.4

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

pendent from CFT value.

On the usual voltage regulators, when the input vol-

tage goes below the nominal value, the regulation

transistors (series element) saturate bringing the

system out of regulation making it very sensible to

every variation of the input voltage. On the contrary,

a control loop on the L4916 consents to avoid the

saturation of the series element by regulating the

value of the reference voltage (pin 2). In fact, when-

ever the input voltage decreases below VI MIN the

supervisor loop, utilizing a non linear OTA, forces

the reference voltage at pin 2 to decrease by dis-

charging CFT. So, during the static mode, when the

input voltage goes below VMIN the drop out is kept

fixed to about 1.6V. In this condition the device

works as a low pass filter in the range (2) of the OTA

characteristic. The ripple rejection is externally ad-

justable acting on CFT as follows :

VI (jw)

SVR (jw) =

Vout (jw)

-6

(1 +

)

jwCFT

Where:

gm = 2 . 10

-5 Ω-1 =OTA’S typical transconductance

value on linear region

= fixed ratio

CFT

= value of capacitor in

µF

The reaction time of the supervisor loop is given by

the transconductanceof the OTA and by CFT. When

the value of the ripple voltage is so high and its ne-

gative peak is fast enough to determine an istanta-

neous decrease of the dropout till 1.2 V, the OTA

works in a higher transconductance condition

[range (3) of the characteristic] and discharge 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 allowinga betterripple re-

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

transient range).

With CFT =10

µF; f= 100Hz a SVRof35is obtained.

L4916

4/8

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