Application information

HVLED002

12/19

DocID028720 Rev 1

5

Application information

5.1

Supply voltage and undervoltage lockout

The HVLED002 device is able to operate with a very wide range of supply voltage between

8.4 V and 30 V. The UVLO circuit insures that VCC is adequate to make the HVLED002 fully

operational before enabling the output stage. Figure 16 shows that the UVLO turn-on and

turn-off thresholds are fixed internally at 8.4 V and 7.6 V respectively. The hysteresis

1 mA.

During UVLO, the output driver is in a low state and it can easily sink 1 mA, enough to

insure the MOSFET is held off.

5.2

Reference voltage

The HVLED002 contains a precision reference voltage (5 V) that generates all the internal

reference voltages such as the error amplifier's reference (connected to its non-inverting

input), current sense clamp limit (MAXCS) and oscillator's internal bias currents and

thresholds.

The reference voltage is also available on the VREF pin that, thanks to its high output

current capability (over 20 mA), is able to supply not only nearby passive circuitries but also

auxiliary microcontrollers.

The pin must be bypassed with at least a 0.1 µF ceramic capacitor placed as close as

possible to the respective VREF and GND pins.

5.3

Oscillator

The HVLED002 oscillator is programmed as shown in Figure 14. The timing capacitor CT is

charged from a reference voltage (e.g.: VREF) through the timing resistor RT, and

discharged by an internal current source.

The MOSFET is turned on (GD pin high) when the oscillator starts the charge of the CT. As

soon as the voltage of the CT reaches an upper threshold the internal discharge current is

activated until the CT voltage reaches a lower threshold. This occurrence initiates a new

oscillator cycle.

The difference between the upper and the lower thresholds (Vosc) determines the duration

of charging and discharging time. During the discharging time (also called deadtime) the

MOSFET is off and any spurious GD triggering is avoided. The deadtime also limits the

maximum obtainable duty cycle.

The oscillator can be differently connected to external circuitry to obtain different operating

schemes. Connecting the RT to VREF a very accurate fixed frequency operation is

achieved: the RT,CT combinations are plot into Figure 5 on page 8, Figure 6 and Figure 8

for a quick reference, or calculated as follows:

Equation 1

FOSC (kHz) = 1.72 / [RT (k

) x CT (µF)]