DocID028720 Rev 1

13/19

HVLED002

Application information

19

Connecting the RT to a variable voltage, dependency of the operating frequency on said

voltage is introduced. A pull-down switch can be used to reset the CT during the MOSFET's

on time, for example to operate in fixed off time. A synchronous operation is also possible

using circuitries like the one proposed as an example in Figure 19. The HVLED002

oscillator can be used to a maximum of 500 kHz.

5.4

Current sense

The peak current mode operation of the HVLED002 is made by the embedded current

sense comparator: the said element turns off the MOSFET as soon as the current sense

input voltage is greater than the internal threshold derived by the COMP pin voltage

(Figure 17).

The current sense pin (ISENSE) is normally connected to a shunt resistor, put in series with

the main switch, but different connections are also possible.

Under the normal operation the threshold voltage (VCS) is controlled by the E/A according

to the following relation:

Equation 2

VCS = 1/3 * (VCOMP - 1.4 V)

VCS is upper limited to MAXCS to reduce the shunt resistor power dissipation without the

need of current transformers or offsets circuitries. This parameter is beneficial in those

applications where both the peak current accuracy and the operating power dissipation are

critical aspects (e.g.: LED secondary side LED current regulators).

When the sensing current resistor is in series with the power switch, the current waveform

will often have a large spike at its leading edge due to parasitic capacitances and gate driver

charging currents. A very simple leading edge blanking (LEB) circuit consists on an RC filter,

but more effective active circuitries are also possible.

Figure 21. Leading edge blanking circuitries