ISL1904 Datasheet, PDF(16/21 Page) Intersil Corporation – Dimmable AC Mains LED Driver with PFC and Primary Side Regulation

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ISL1904 Datasheet, PDF (16/21 Pages) Intersil Corporation – Dimmable AC Mains LED Driver with PFC and Primary Side Regulation

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ISL1904

Soft-Start Operation

Soft-start is not user adjustable and is fixed at ~ 350ms. Both the

duty cycle and control loop reference have soft-start. This

ensures a well behaved closed loop soft-start that results in

virtually no overshoot.

AC Detection and Reference Generation

The ISL1904 creates a 0 to 0.5V reference for the LED current

control loop by directly measuring the conduction angle of the AC

input voltage. The reference changes only with conduction angle

and is virtually unaffected by variation in either voltage

amplitude or frequency.

EMI

FILTER

EMI

FILTER

GND 13

AC 12

ISL1904

FIGURE 11. AC DETECTION

GND 13

AC 12

ISL1904 9

FIGURE 10. AC DETECTION

The ISL1904 detects the conduction angle using a divider

network across the AC line and connected to the AC pin, although

it can also be located after the AC bridge rectifier. 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 an input range of 0 to 4V. The peak of the

input signal should range between 1 and 4 volts for

uncompromised accuracy. The AC detection circuit measures

both the duration of the AC conduction angle and the half-cycle

duration. By comparing the two every half-cycle, the detection

circuit creates a frequency independent reference that is

updated each AC half-cycle.

In the event of an AC outage, the AC mains frequency reference

is lost. The ISL1904 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 it detection

threshold, the internal reference is forced to its maximum of

0.5V.

EMI

FILTER

GND 13

AC 12

ISL1904 9

FIGURE 12. ALTERNATE AC DETECTION

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.

Primary Current Sensing

The ISL1904 is configured to regulate the output current by

monitoring the primary switch current at the OC pin. The peak

primary switch current is captured, processed, and output on

IOUT as a PWM voltage signal modulated in proportion to the

output current. The IOUT PWM frequency is the same as the

converter switching frequency and its amplitude is equivalent to

4x the peak switch current during the previous ON-time. It must

be scaled and filtered before being input to the control loop at

the FB pin. The required filter time constant depends on the

compensated error amplifier bandwidth. The filter bandwidth

must be greater than the control loop bandwidth, typically an

order of magnitude greater, but it is generally not necessary to

filter the IOUT PWM signal to a low ripple DC level. The

compensated error amplifier, with its limited bandwidth,

performs that function.

FN8286.1

September 20, 2012

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