PA5711 Datasheet, PDF (6/8 Pages) Protek Devices – High Voltage LED Driver

ANALOG PRODUCTS DIVISION
2.2.5 Linear Dimming
The PA5711 comes with a linear dimming function which is
one of the four different ways the LED can be controlled.
While VCS is the maximum threshold against which the
current sense resistor voltage drop is compared against, the
LD pin can be used to reduce this threshold. An external
resistive divider setup will allow the user to adjust the LED
light intensity.
The RCS voltage drop is compared against the lower of
these two voltages. Hence decreasing the voltage provided
at LD below 250mV would reduce the intensity of the LED
light. This allows the user to choose from available resistors
rather than calculating the resistor value in accordance with
the internal threshold.
Since a voltage lower than the internal threshold can enable
linear dimming it is advisable to connect LD to VDD when
linear dimming is not required.
A soft start function can be implemented on the PA5711 by
using RC network to slowly ramp the voltage applied to the
LD pin from GND potential to over 300mV, at which the VCS
overrides the Linear Dimming Function.
VDD
R₁
LD
R2 ^C
Figure 4. Soft-Start RC arrangement
The LD pin can also be used as a feed back to the chip,
especially in an isolated driver application, a feedback from
the isolated portion can be applied to the LD pin to achieve
greater control over the calculate current drive.
2.2.6 PWM dimming
Pulse Width Modulation dimming or PWM dimming can be
implemented on the PA5711 by using a low frequency
Square wave signal to drive the PWM pin. The square wave
frequency can be in the range of a few hundred hertz. In this
setup the PWM wave will be directly driving the output of the
Gate driver.
PWM signals can be generated using a Microcontroller or
Pulse generators. The duty cycle of the input PWM is
directly proportional to the intensity of LED light. If the PWM
PA5711
High Voltage LED Driver
input is driven low the LED’s are turned off.
2.2.7 Light sense
The PA5711 has a Light sense input to accept signal from
an external light sensor. The LED’s are turned on/off
dependent on the signal from the Light sensors. This pin
must be connected to ground if not used.
2.2.8 Motion Detection
The PA5711 has a motion detection input. An external
motion detector can drive this pin to turn the LED’s on/off.
This pin needs to be connected to VDD when not used.
2.2.9 Gate Driver
The PA5711 can drive external MOSFETs with a gate
capacitance of up to 600pF.
2.2.10 Oscillator
The PA5711 operates at a constant frequency. The external
resistor Rosc determines the oscillator frequency. The
inductor size, total chip power dissipation and filter
capacitors are the basis on which the switching frequency is
decided.
The internal oscillator has a 20% frequency accuracy. In a
non-isolated mode the driver switching frequency is
between 30 kHz and 200kHz. This large operating range
gives the designer a reasonable compromise between
switching frequency and inductor size.
Figure 5. Frequency variation across ROSC
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Rev.0. 03/12
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