AN1729 datasheet(8/10 Pages) STMICROELECTRONICS

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AN1729 APPLICATION NOTE

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being Vin the DC voltage on the input bulk capacitor.

Switching losses, the power dissipated on the output rectifier diode, and the power dissipated in the input stage

are not easy to be evaluated analytically but they are anyway almost independent from the burst period and

considerably lower than PDstup.

Another quantity that can be evaluated is the power absorbed by the load:

(Eq. 6)

VOmin-stb in the valley voltage at the input of the linear regulator while this is absorbing Istb and it is therefore

greater than VOmin, which corresponds to IWup consumption. We can use the previous equations in order to

have a minimum possible value of Tburst and, as a rule of thumb, we can assume that the power consumption

calculated (PDstb + PLoad) is 70% of the total power consumption in standby. Hence, considering the 1W con-

sumption target, we have:

(Eq. 7)

From equation 5 we see that increasing Tburst the power consumption in standby can be made as low as de-

sired, however there is a maximum limit to Tburst.

Considering that a minimum voltage on the input of the voltage regulator has to be guaranteed, Tburst is related

to the maximum voltage that we have at the input of the voltage regulator VOMax (Fixed by the Zener diode DZ3)

and at the value of the output capacitor C9, according to the following equation:

(Eq. 8)

VOmax in any case has to be lower then the output voltage of the converter during the normal operation; if it were

not, the normal control loop would be activated and the behaviour of the converter would be different from what

is required. Fixing a low value for VOMax means decreasing the power consumption of the voltage regulator but,

as it can be seen, it is not a big part of the total power consumption. At this point the value of Tburst and then the

values of C9 and VOMax is a trade-off between lowering the standby consumption and a not too big output ca-

pacitor.

Once fixed all the component values, the VOmin-stb can be calculated as:

(Eq. 9)

Standby calculations in the experimental board

In the board assembled for testing the standby function the Zener diode DZ3 has a Zener voltage of 7.5V.

It means that the VOMax value is about 7.5V+0.6V+0.6V=8.7V. The output Capacitor (C9) value is: 4700µF so

from Eq. 8 the maximum Tburst value has to be:

where the tolerance of the electrolytic capacitor C9 (-20%) and the maximum quiescent current Id of the linear

voltage regulator has been considered.

The maximum discharge time of the capacitor C4 can be calculated as:

L oad

OMax

Omin

stb

–

() I

stb

⋅

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

bu rst

C4 Vcc

Hy s

⋅⋅

700m W

Load

–

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

≥

Omax

Omin

burst

Wup

⋅

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

Omin stb

Om a x

bu rst

stb

⋅

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

–

Bu rs tM ax

OMax

OMin

–

() C9 0.8

⋅

()

⋅

wup

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

8.7V

5.5V

–

()

70 m A

3m A

------------------------------------ 3760

µF164ms

⋅

dis ch

C 4

–

Ma x

–

Vcc

Hys

C4 1.2

⋅

qMin

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

4.3 26.4

µF

⋅

1.6 m A

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

71m s

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