AN2495 datasheet(17/36 Pages) STMICROELECTRONICS

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AN2495

Startup network design

Doc ID 13127 Rev 5

17/36

Startup network design

To allow the circuit to start as soon as the line voltage is applied, it is necessary to pre-

charge both C5 and C8 capacitances.

A resistor connected to the DC bus directly makes the pre-charge of C5 electrolytic

capacitance. The circuit must start at a minimum DC input voltage of 250 V. The current

required by the L6565 driver during the startup time determines the (R1+ R2+ R3+ R4) value.

Considering that the L6565 driver needs a 0.07 mA maximum startup current, we obtain:

Equation 33

Startup resistance must be lower than 3.6 M

Ω to reach the best trade-off between power

dissipation and time-to-start. As a consequence, before choosing the startup resistor, we

must determine the C5 capacitance value, which is set according to another requirement.

C5 must be able to supply the L6565 driver until the steady state behavior of the converter is

established. The time from bench verification is 20 ms maximum. Given a L6565 minimum

hysteresis-voltage (difference between startup threshold and undervoltage threshold)

of 3.7 V, the voltage across C5 must decrease less than 3.7 V during the startup period.

From the L6565 datasheet we know that maximum quiescent current after turn-on is

3.5 mA, so C5 must be chosen such that:

Equation 34

C5 = 33 µF is a good choice to guarantee a good margin.

Finally, we can set the startup resistance value to reduce time-to-start and simultaneously

optimize standby power dissipation. The L6565 has a maximum startup threshold of 14.5 V,

therefore the maximum time-to-start is approximately:

Equation 35

A good choice is to put in series four 200 k

Ω resistors (R1 = R2 = R3 = R4 = 200 kΩ), which

dissipate less than 1 W of standby power.

The pre-charge of C8 base capacitance is carries out by connecting it to the OUT pin of the

L6565 through a diode in series with a 2.2 k

Ω resistor.

()

inm in

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

250V

0.07mA

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

3.6 M

≅

ΔV

Δt

•

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

3.7V

3.5mA

20ms

•

3.7V

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

μF

≅

⇒

14.5V

•

inm in

()

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

⎝

⎛

⎠

⎞ I

–

------------------------------------------------------------------------- 2R

()

⇒

808k

≤

sec

≤

Time - to - start

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