PT6909
Power Factor Correction LED Driver
DESCRIPTION
PT6909 is a fixed frequency PWM controller IC
designed to control high brightness LED driver using a
single-stage PFC boost-buck topology. It can achieve
a well power factor and a higher dc-bus voltage. This
topology allows reducing the filter capacitors and using
non-electrolytic capacitors to improve reliability. The
PT6909 uses open-loop peak current control
technique eliminates a need for loop compensation,
limits the input inrush current, and is inherently
protected from input under-voltage condition.
PT6909 provides a low frequency PWM dimming input
that can accept an external control signal with a duty
cycle of 0-100% and a frequency of between 100Hz
and a few KHz. The PWM dimming capability enables
PT6909 phase control solutions that can work with
standard TRIAC.
FEATURES
•
•
•
•
•
•
•
•
Simple power factor correction Integrated
Low start-up current
8V to 12V input range
Constant-current driver
Programmable fixed frequency operation
Input and output current sensing
Input current limit
Boost circuit over-voltage protection(LED open
protection)
•
Input voltage limitation
•
Low Input current harmonic distortion
•
PWM Low-frequency dimming and phase
Dimming compatible
APPLICATIONS
•
•
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LED replacement tube
AC/DC LED driver
LED Lamp with phase dimmable
Street lamps
Traffic signals
BLOCK DIAGRAM
Tel: 886-66296288
‧
Fax: 886-29174598
‧
http://www.princeton.com.tw
‧
2F, 233-1, Baociao Road, Sindian, Taipei 23145, Taiwan
PT6909
TYPICAL APPLICATION
Typical 110V/AC 8W LED Tube With PFC Application
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PT6909
ORDER INFORMATION
Valid Part Number
PT6909-S
Package Type
8 Pins, SOP, 150MIL
Top Code
PT6909-S
PIN CONFIGURATION
PIN DESCRIPTION
Pin Name
CS1
GND
GD
VIN
PWMD
OVP
CS2
RF
Description
Input or Output Current Sense
Ground
Drives the Gate of External MOSFET
Input Voltage 8V-12V DC
Low Frequency PWM Dimming pin, also Enable pin
Over Voltage Detection
Input or Output Current Sense
Oscillator Frequency Setting
Pin No.
1
2
3
4
5
6
7
8
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April 2010
PT6909
FUNCTION DESCRIPTION
POWER TOPOLOGY
PT6909 is optimized to drive non-isolated topology, cascading an input PFC boost stage and an output buck converter
power stage. This power converter topology offers numerous advantages useful for driving high-brightness LED. These
advantages include power factor correction, low harmonic distortion of the input AC line current. The power converter
topology also permits reducing the size of a filter capacitor needed, enabling use of non-electrolytic capacitors. The latter
advantage greatly improves reliability of the overall solution.
PT6909 is a peak current-mode controller that is specifically designed to drive a constant current buck power converter.
PT6909 controls two identical current sense comparators for detecting input and output current. One of the comparators
regulates the output LED current, while the other is used for sensing the input inductor current. The comparator sensing
the input inductor current is mainly responsible for the converter start-up. The control scheme can achieve a low inrush
current, also avoid a high input current while the AC input is under-voltage.
OVER VOLTAGE DETECTION
PT6909 includes output over voltage detection function. The OVP pin is used for sensing the output voltage of boost
stage. The PT6909 will latch the GD output at shutdown state when the output voltage reaches OVP threshold voltage.
In the typical application circuit, the system will go into hiccup mode if the LED load is open. When the LED load is
disconnected, the output voltage rises as the output capacitor C1 starts charging. When the PT6909 detects an over
voltage condition and turns off the converter. The PT6909’s power supply VIN drops under UVLO threshold voltage and
goes into low current start-up status. The PT6909 restarts and turns on the converter. The system starts another cycle
until the LED load is re-connected.
INPUT VOLTAGE LIMITATION
PT6909 built-in power-supply clamped circuit. In the typical application circuit, when the VIN voltage rises higher than
14V typically, it will be clamped at this level with internal 5mA current sink capability.
SETTING INPUT AND OUTPUT CURRENT
Two current sense comparators are included in the PT6909. Both comparators have their inverting inputs internally
connected to 240mV. The CS1 and CS2 inputs are non-inverting inputs of the comparators. When either the CS1 or CS2
pin voltage rises up 240mV, the GD pulse is terminated. A leading edge blanking delay of 300ns(typ) is added. The GD
voltage becomes high again upon receiving the next clock pulse of the oscillator circuit.
Referring to the Typical Application Diagram, the CS1 comparator is responsible for regulating output current. The
output LED current can be programmed using the following equation:
0.24 1
I
LED
=
− Δ
I
L
1
R
CS1
2
Where
△I
L
1
is the peak-to-peak current ripple in L1. The CS2 comparator limits the current in the input inductor L2.
Therefore, the PT6909 starts-up in the input current limiting mode. The CS2 voltage sensing must be programmed such
that no input current limiting occurs in normal steady-state operation. The L2 current sensing resistor R
CS2
can be
programmed using the following equation:
R
CS2
<
0.24
I
L2P
Where, I
L2P
is the maximum peak current in L2.
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April 2010
PT6909
PWM DIMMING AND TRAIC DIMMING
PWM dimming scheme can be implemented by external low frequency PWM signal to the PWMD pin. When the PWMD
pin is pulled high, the gate driver is enabled and the circuit operates normally. When the PWMD pin is left open or
connected to GND, the gate driver is disabled and the external MOSFET turns off. By using the duty cycle of the PWM
control method that can change the working and the closing time, and the light output can be adjusted between zero and
the internal set. In addition, the PWM dimming method is limited because the output current cannot be exceeded the
internal set.
The power topology does not require filter capacitance at the output side of rectifier bridge. Therefore, sampling the
phase-controlled voltage waveform of TRAIC connects PT6909’s PWMD pin can realize TRAIC dimming.
OSCILLATOR
Connecting an external resistor from RF pin to GND programs switching frequency:
2.6 x 10
10
Fosc
=
R
RF
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