HVLED805 datasheet(18/29 Pages) STMICROELECTRONICS

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Application information

HVLED805

18/29

Doc ID 18077 Rev 1

5.5

Constant current operation

Figure 15

presents the principle used for controlling the average output current of the

flyback converter.

The output voltage of the auxiliary winding is used by the demagnetization block to generate

the control signal for the mosfet switch Q1. A resistor R in series with it absorbs a current

VC/R, where VC is the voltage developed across the capacitor C.

The flip-flop’s output is high as long as the transformer delivers current on secondary side.

This is shown in Figure 16.

The capacitor C has to be chosen so that its voltage VC can be considered as a constant.

Since it is charged and discharged by currents in the range of some ten µA (ICLED is

typically 20 µA) at the switching frequency rate, a capacitance value in the range 4.7-10 nF

is suited for switching frequencies in the ten kHz.

The average output current can be expressed as:

Equation 2

Where IS is the secondary peak current, TONSEC is the conduction time of the secondary

side and T is the switching period.

Taking into account the transformer ratio n between primary and secondary side, IS can also

be expressed is a function of the primary peak current IP:

Equation 3

As in steady state the average current IC:

Equation 4

Which can be solved for VC:

Equation 5

Where VCLED=R • ILED and is internally defined.

As VC is fed to the CC comparator, the primary peak current can be expressed as:

⎟

⎠

⎞

⎜

⎝

⎛

⋅

ONSEC

OUT

⋅

()

ONSEC

CLED

ONSEC

CLED

⋅

⎟

⎠

⎞

⎜

⎝

⎛

−

⋅

ONSEC

CLED

⋅

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