STMicroelectronics LED815 Evaluation Boards EVALHVLED815W15 EVALHVLED815W15 Hoja De Datos
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EVALHVLED815W15
Device description
HVLED815PF
22/36
Doc ID 023409 Rev 4
Equation 6
This formula shows that the average output current I
OUT
does not depend anymore on the
input voltage V
IN
or the output voltage V
OUT
, neither on transformer inductance values. The
external parameters defining the output current are the transformer ratio n and the sense
resistor R
resistor R
SENSE
.
The previous formula (Equation
) 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.
sensed through a voltage partition of the auxiliary winding, just before the auxiliary rectifier
diode.
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 V
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 V
REF
(2.51 V typ - see
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.
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.
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.
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.
stabilizes the overall voltage control loop, is connected between this pin and ground.
As a result, the output voltage V
OUT
at zero-load (i.e. no led on the led driver output) can be
selected trough the R
FB
resistor in according to the following formula:
Equation 7
Where N
AUX
and N
SEC
are the auxiliary and secondary turn's number respectively.
The R
DMG
resistor value can be defined depending on the application parameters (see
"
).