ISL1902 Datasheet, PDF (18/25 Pages) Intersil Corporation – Full-Featured, Dimmable AC Mains LED Driver with PFC

ISL1902
EMI
FILTER
1
ISL1902 24
2
23
3
22
4
GND 21
AC 20
19
FIGURE 18. ALTERNATE AC DETECTION
The advantage to sensing the AC voltage directly, rather than the
rectified voltage, is that there is no error in detecting the AC zero
crossing. If monitored after the AC rectifier bridge, the AC signal
tracks the filter capacitor voltage, which may not discharge in
phase with the AC voltage. This can lead to incorrect detection of
the AC zero crossing. At light load, the filter capacitor may not
fully discharge before the AC voltage begins to increase again,
resulting in no detection of the AC zero crossing at all.
The AC pin and has a usable input range of 0V to 4V. The peak of
the input signal should range between 1V and 4V for
uncompromised accuracy. The AC detection circuit measures
both the duration of the AC conduction angle and half-cycle
duration. By comparing them every half-cycle, the detection
circuit creates a frequency independent reference that is
updated each AC half-cycle.
The reference generated by the AC detection circuit is available
as the LPOUT signal. Here it can be modified, or not, and
connected to REFIN for setting the control loop reference.
Examples of modification include interfacing with an external
transducer, such as an ambient light sensor (ALS) or temperature
sensor (NTC or PTC) to modify the reference based on the sensor
input.
The ISL1902 also supports analog dimming control by allowing
the control loop reference to be connected to REFIN, bypassing
LPOUT completely.
AC may be directly coupled to a 90Hz to 130Hz PWM signal to
generate a reference if dimming is desired without using an AC
dimmer, or an independent reference may be input to REFIN with
LPOUT not connected.
In the event of an AC outage, the AC mains frequency reference
is lost. The ISL1902 will force the reference to zero volts and
reset the soft-start circuit approximately 35ms after the last AC
zero crossing is detected. If AC is held above its detection
threshold for more than 35ms, the internal reference is forced to
its maximum of 0.5V.
3
4 VREF
5
6
8 REFIN
9 LPOUT
16
10
15
11
14
12 _ISL1902 13
3
4 VREF
5
6
8 REFIN
9 LPOUT
10
11
12 _ISL1902
ANALOG
OR
PWM
CONTROL
16
15
14
13
3
4 VREF
5
6
8 REFIN
9 LPOUT
16
10
15
11
14
12 _ISL1902 13
FIGURE 19. ALTERNATE CONFIGURATIONS FOR THE CONTROL LOOP
REFERENCE
3
4 VREF
5
19
6
LOUT 18
8 REFIN LREF 17
9 LPOUT LFB 16
10
15
11
14
12 _ISL1902 13
ALS
FIGURE 20. USING AN AMBIENT LIGHT SENSOR WITH AN AC LINE
DIMMER
3
4 VREF
5
19
6
LOUT 18
8 REFIN LREF 17
9 LPOUT LFB 16
10
15
11
14
12 _ISL1902 13
ANALOG
OR
PWM
CONTROL
ALS
FIGURE 21. USING AN AMBIENT LIGHT SENSOR WITH ANALOG OR
PWM INPUT
18
FN7981.2
March 20, 2013