PS081 datasheet(84/155 Pages) AMSCO

84 / 155 page

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4-27

PSØ81

Short description of the function:

Coming from total darkness, all capacitors are discharged and the output of U5 is high-Z. The input

voltage of U4 is zero. PS081 is not supplied by voltage. If light is switched on, the current from the

solar panel charges C17 and supplies the voltage detection. The voltage detection (R7,R14,Q1,U3)

is dimensioned so that U1 switches when the voltage at C17 passes 3.5 V. At that moment, the

output of U5 leaves high-Z and goes to the voltage of C17. U4 is supplied with 3.5 V and regulates

to 2.5 V for the PS081. PS081 begins to work. Because all capacitors behind VCC_R have now to

be charged to the voltage at C17, this voltage drops down as only C17 can supply the necessary cur-

rent. The solar panel is to weak for such a high current pulse. The voltage at C17 must not be lower

than 2.55 V. Otherwise U4 cannot regulate 2.5 V for the PS081. C17 is also the buffer capacitor

for low light situation. With the selected dimension under very bad light condition (20 Lux) the scale

can operate for minimum 1 minute if it is well charged before. Therefore 1000 µF is the recom-

mended value for C17 (minimum 680 µF).

Avoid to go below 680 µF. The circuit cannot start up if C17 is too low. The circuit will work with hig-

her values, too, but the start-up time from darkness will increase because the charging time for C17

increases. The circuit is evaluated in detail and it is strongly recommended to follow the recommen-

ded values for a correct operation. For further information, see application note AN022 and design

guide DG_Solar_POR.

4.6.1

Filtering / Recommendations LDO

In most circuits the voltage is regulated by a voltage regulator (LDO, low drop-out regulator). Of

course this component has a noise which basically influences the measurement quality. Therefore it

is crucial to choose suitable LDOs with a low-noise behavior, still keeping in mind that some applica-

tions need a low-current regulator as well. In this section we will give some recommendations which

LDOs to choose.

The critical factor to watch out for is the ‚output noise‘. The noise figures can normally be found in

the datasheet of the LDO and is given as a summary value over the whole frequency range, e.g.

500µV RMS or in dependency of the frequency in µV /

√Hz or as a diagram. A low output noise is

desired, the figures can easily vary by factor 10 (e.g. Linear LT1761-BYP has 20µV RMS (with by-

pass capacitor) vs. TI TPS71501 which has 575µV RMS).

NOTE : We do NOT recommend switching regulators in any case, please use linear voltage

regulators only as per the following recommendations.

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