HV9805 Datasheet, PDF (15/34 Pages) Microchip Technology – Off-Line LED Driver with True DC Output Current

4.5 Headroom Voltage Regulator
Headroom voltage at the LED current regulator is
controlled by the headroom voltage regulator.
Minimize the power dissipation of the LED current
regulator and therefore minimize the DC level of the
headroom voltage (V_HDC), the dissipation being
calculated using Equation 4-2.
EQUATION 4-2:
P_DIS = I_LED  V_HDC
Where:
P_DIS = Average power dissipation within the
LED current regulator during normal
operation, which includes the dissipation
in the pass transistor M_CRX and the
dissipation in the sense resistor R_CRS
V_HDC = Desired DC level of the headroom
voltage, see Section 4.5.2.
The DC level of the boost converter output voltage, the
bus voltage, adjusts to the sum total of the DC level of
the headroom voltage and the operating voltage of the
LED load. The bus voltage adapts during regular
operation to any changes in the operating voltage of
the LED load.
4.5.1 THE REGULATION PROCESS
The headroom voltage regulator adjusts the DC level of
the headroom voltage by adjusting the on-time of the
boost converter switch.
The headroom voltage regulator includes an internal
control amplifier with external gain setting network, an
internally generated reference voltage and a feedback
voltage, provided at the HVS pin by an external resistor
divider connected at the drain of the LED current
regulator pass FET.
The regulation process can be described as follows:
1. A deviation of the headroom voltage from the
desired headroom voltage produces a current at
the output of the control amplifier in proportion to
the deviation.
2. The current produces a change in the output
voltage at the control amplifier output.
3. The change in voltage at the control amplifier
output produces a change in the boost converter
switch on-time.
4. The change in on-time produces a change in the
boost converter output current.
5. The change in the boost converter output cur-
rent produces a change in the bus voltage.
6. The change in the bus voltage then reduces the
deviation in headroom voltage.
HV9805
The response characteristic of the control amplifier is
determined by the compensation network at the output
of the control amplifier.
In order to prevent distortion of the line current, It is
preferable to drive the boundary conduction mode
boost converter with a constant on-time during the
course of a line cycle. Accordingly, variations in on-time
due to headroom voltage variation within the line cycle
should be suppressed by tailoring the frequency
response characteristic of the control amplifier.
A particularly large headroom voltage variation at twice
the line frequency is present due to the pulsating nature
of power delivery from an AC line. The control amplifier
can be effectively compensated with a capacitor in the
1 µF to 10 µF range in series with a 0.1 k to 1 k
resistor. Larger capacitance leads to less on-time
variation and less line current distortion, but slows
down the response to line voltage changes. Larger
resistance leads to larger distortion of the line current
waveform due to a proportional change between the
headroom voltage ripple and the on-time, but results in
better damping of the transient response to a line
voltage or load voltage disturbance.
The headroom voltage is programmed to the desired
level using Equation 4-3.
EQUATION 4-3:
V_{REF, HVR} = V_HDC  K_DIV
Where:
K_DIV = _R----_H----_V--^R-_B---^H--+--^V---_R^B---_H----_V---_T-
V_{REF, HVR} = Reference voltage for the headroom
voltage regulator, see DC and AC
Characteristics table
K_DIV = Attenuation of the headroom voltage
divider
R_HVT, = Top and bottom resistor of the
R_HVB headroom voltage divider, see
Typical Application Circuit
The control amplifier produces the output voltage V_HVR
according to Equation 4-4.
 2015 Microchip Technology Inc.
DS20005374A-page 15