Technical Brief 490 Author: Tim Lok and Fred Greenfeld
ISL1902 Highly Integrated Dimming LED Driver For Commercial Lighting Introduction
circuitry to implement the design features desirable in LED lighting, driving up the solution cost.
In the world of 'Green Energy', LED lighting is quickly emerging as the popular choice to replace current lighting solutions that consume significant energy in environments such as commercial, office and outdoor lighting. The challenge with LED lighting from a offline mains power source is the AC/DC conversion needed as LEDs illuminate light by passing current at DC voltage. High Power Factor Correction (PFC) and Power Efficiency are required, which is one of the big reasons we are migrating toward LED lighting in the first place.
Intersil Solution Intersil is an industry leading manufacturer in power management and analog I/O semiconductor ICs. Since offline LED lighting requires an AC/DC conversion ballast, a controller dedicated for driving LEDs would be the optimal solution. With our design expertise in DC/DC converters and analog mixed signal products, Intersil introduces the ISL1902 Highly Integrated Offline LED driver as a solution for LED lighting that integrates many features to give high performance while keeping BOM cost low.
LEDs are a highly regarded solution for lighting because of their benefits over traditional lighting: Very little energy wasted in the form of heat, much longer life span, faster turn-on response time, wide dimming capability and even regarded as environmentally safer compared to Compact Florescent Lighting (CFL) and mercury vapor lamps that contain toxic materials.
The ISL1902 is a single-ended PWM controller for converting AC mains voltage or high voltage DC to a constant current source for driving high powered LEDs in both isolated and non-isolated inputs. The ISL1902 integrates circuitry that yields high efficiency and active PFC, is compatible with a variety of dimmers over a very wide flicker free range and includes features such as soft UVLO, OVP and OTP for maximizing the life of the design. The flexible feedback of the controller allows sensors such as an Ambient Light Sensor (ALS) or a proximity sensor for automated controlling of LED brightness, or a temperature sensor for ambient temperature compensation.
Dimming LED with Offline AC Early solutions for driving LEDs from mains voltage used standard DC/DC PWM controllers. These controllers are not ideal because they lack the features needed for LED lighting applications, caused light flickering when dimming, low PFC and poor efficiency. In addition, the solution required significant external
AC
EMI FILTER
AMBIENT LIGHT SENSOR
1 VDD
OUT 24
2 PRELOAD PWMOUT 23 3 OFFREF 4 VREF
INRUSH 22 GND 21
5 IOUT
AC 20
6 CS+
OVP 19
7 CS-
LOUT 18
8 REFIN
LREF 17
9 LPOUT 10 OC 11 FB2 12 FB1
LFB 16 RAMP 15 VERR 14 DELADJ 13
ISL1902
FIGURE 1. NON-ISOLATED FLYBACK WITH PWM DIMMING AND AMBIENT LIGHT COMPENSATION
March 5, 2012 TB490.0
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CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. Copyright Intersil Americas Inc. 2012. All Rights Reserved. 1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a trademark owned by Intersil Corporation or one of its subsidiaries. All other trademarks mentioned are the property of their respective owners.
Technical Brief 490 High PFC and Efficiency
Conclusion
The ISL1902 achieves high PFC and efficiency by operating in Critical Conduction Mode (CrCM). The converter operates in a variable frequency variable duty cycle with constant on time. By allowing the inductor current to fall to zero in CrCM, the input current waveform is forced to track the line input voltage, yielding PFC greater than 0.95. The converter also has an integrated dead time adjust that delays switching action to achieve near Zero Voltage Switching (ZVS) improving converter efficiency. Soft starting and cycle-by-cycle in-rush current limiting circuitry further reduces transient and steady state energy losses, respectively. Design optimization yields converter efficiency greater than 90%.
The ISL1902 is a full featured Offline Dimmable LED driver with active PFC and high efficiency designed to drive high powered LEDs in all types of lighting applications. The ISL1902 design solution achieves >0.95 PFC, >90% efficiency, is compatible with a large variety of dimmers and can dim to very low light levels with no flicker or cut off. The highly integrated features of the ISL1902 shrinks the driver size and BOM cost while delivering high performance.
Wide Dimming Range with Triac Operation or Analog Control The ISL1902 is highly versatile controller when it comes to dimming operation (see Figure 2). Dimming is achieved by decreasing the LED output current through DC modulation or PWM switching. In DC modulation, the output current for the LED is reduced by sensing the conduction angle, of both leading and trailing edge dimmers, to set a feedback reference voltage. DC modulation dimming is the lower cost, simpler method of dimming but it produces a non-linear dimming output because LEDs are more luminously efficient at lower drive currents. PWM switching improves on the dimming linearity by repeatedly turning the LED on/off at varying duty cycles to reduce light output. When on, PWM switching drives the full rated output current, eliminating the non-linearity. Alternatively to a triac, the ISL1902 can accept an analog input signal to the feedback reference from a Digital to Analog Converter (DAC) or Digital Controlled Potentiometer (DCP) for dimming through a serial data bus. The challenge with triac based dimmers is achieving very low brightness dimming, which may cause the LED to flicker or cut-off due to low conduction angles. The low conduction angles forces the controller to deliver a very low output current to the LED, which may be below the holding current of the triac. The ISL1902 solves this problem with active pre-loading. Active pre-loading diverts the output current to a load at very low brightness levels so the converter maintains a minimum load, preventing flickering at conduction angles as low as 5%. Minimal conduction losses occur because the load current is very small. In addition, the ISL1902 incorporates a reference cut off that will disable the controller output at a desired level (e.g. conduction angles less than 20%). This feature makes it compatible with many of the different dimmers available with various holding currents.
3
4 VREF AC 20
6
19
7
LOUT 18
8 REFIN
LREF 17
9
LPOUT
AC 20
6
19
LREF 17
16
9
LPOUT
LFB
15
10
14
11
13
12
ISL1902
Control Loop Reference from AC Mains Conduction Angle
16 15 14
19
7
LOUT 18
8
REFIN
LREF 17
9
LPOUT
LFB
16
10
15
11
14
12
FB Control Signal
13
ISL1902
13
ISL1902
Control Loop Reference from an Analog or PWM Signal with external FB using uncommited Op-Amp
Control Loop Reference from an Analog or PWM Signal
FIGURE 2. SETTING THE CONTROL LOOP REFERENCE 3 4
VREF
5
19
6
LOUT
18
8
REFIN
LREF
17
9
LPOUT
LFB
16
10
15
11 12
ALS
14
ISL1902
13
FIGURE 3. AMBIENT LIGHT SENSOR COMPENSATION
+
Q1
R5
24
1 2
PRELOAD
23 22
3
Uncommitted Op-Amp Flexibility The ISL1902 integrates a single unused op-amp that allows for a wide variety of uses that improve the design dimming features. As mentioned earlier, the op-amp can be used as the pre-load driver for improved dimming range (see Figure 4). An ambient light sensor or a proximity sensor can be used to disable the dimming driver in desired conditions to improve system efficiency (see Figure 3). A temperature sensor can be used to dim the LED when the ambient temperature is too high, allowing the ballast/lamp to cool down. In some applications, there is a need to drive a second LED string. The op-amp can be used as a current balancing driver to achieve uniform LED brightness.
AC 20
6
LOUT 18
7 REFIN
LFB
11
Analog or PWM Control
5
5
8
10
12
4 VREF
4 VREF
5
Analog or PWM Control
3
From AC Mains Detection
3
4
VREF
20 19
6 7
R1
8
REFIN
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LPOUT
LOUT
18
LREF
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LFB
16 15
10
14
11 12
R3
21
5
ISL1902
13
R4
R2
FIGURE 4. ACTIVE PRELOADING DRIVER
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TB490.0 March 5, 2012