STMicroelectronics HVLED805 Evaluation Board EVALHVLED805 EVALHVLED805 Data Sheet

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: 

Where n

SEC

 and n

AUX

 are the secondary and auxiliary turn’s number respectively.

The R

DMG

 value can be defined depending on the application parameters (see “

” section).

         

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

⋅

−

⋅

=