HVLED815PF datasheet(22/36 Pages) STMICROELECTRONICS

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Device description

HVLED815PF

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Doc ID 023409 Rev 4

Equation 6

This formula shows that the average output current IOUT does not depend anymore on the

input voltage VIN or the output voltage VOUT, neither on transformer inductance values. The

external parameters defining the output current are the transformer ratio n and the sense

resistor RSENSE.

The previous formula (Equation

6) is valid for both standard and high power factor

implementation.

4.6

Constant voltage operation

The IC is specifically designed to work in primary regulation and the output voltage is

sensed through a voltage partition of the auxiliary winding, just before the auxiliary rectifier

diode.

Figure 18 shows the internal schematic of the constant voltage mode and the external

connections.

Due to the parasitic wires resistance, the auxiliary voltage is representative of the output just

when the secondary current becomes zero. For this purpose, the signal on DMG pin is

sampled-and-held at the end of transformer's demagnetization to get an accurate image of

the output voltage and it is compared with the error amplifier internal reference voltage VREF

(2.51 V typ - see

Table 5: Electrical characteristics).

During the MOSFET's OFF-time the leakage inductance resonates with the drain

capacitance and a damped oscillation is superimposed on the reflected voltage. The S/H

logic is able to discriminate such oscillations from the real transformer's demagnetization.

When the DMG logic detects the transformer's demagnetization, the sampling process

stops, the information is frozen and compared with the error amplifier internal reference.

The internal error amplifier is a transconductance type and delivers an output current

proportional to the voltage unbalance of the two outputs: the output generates the control

voltage that is compared with the voltage across the sense resistor, thus modulating the

cycle-by-cycle peak drain current.

The COMP pin is used for the frequency compensation: usually, an RC network, which

stabilizes the overall voltage control loop, is connected between this pin and ground.

As a result, the output voltage VOUT at zero-load (i.e. no led on the led driver output) can be

selected trough the RFB resistor in according to the following formula:

Equation 7

Where NAUX and NSEC are the auxiliary and secondary turn's number respectively.

The RDMG resistor value can be defined depending on the application parameters (see

Section 4.7: Voltage feed-forward block).

HVLED815PF Datasheet(PDF) 22 Page - STMicroelectronics

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