HVLED805

Application information

Doc ID 18077 Rev 1

17/29

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.

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.

The output voltage can be defined according the formula:

Equation 1

Voltage feedforward block on page 20” section).

Figure 14.

Voltage control principle: internal schematic

2.5V

Rdmg

From Rsense

Aux

EA

R

To PWM Logic

S/H

Rfb

DEMAG

LOGIC

CV

C

COMP

DMG

DMG

REF

OUT

SEC

AUX

REF

FB

R

V

V

n

n

V

R

⋅

−

⋅

=