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
prevents VCC oscillations during power sequencing and the start-up current is less than
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)]