ISL1904FAZ-T7A Intersil, ISL1904FAZ-T7A Datasheet - Page 16

• Manufacturer Part Number: ISL1904FAZ-T7A
• Description: LED Lighting Drivers Dimmable Buck LED Driver IC
• Manufacturer: Intersil

Specifications of ISL1904FAZ-T7A

• Rohs: yes
• Input Voltage: 9 V to 20 V
• Operating Frequency: 1 MHz
• Maximum Supply Current: 7.8 mA
• Maximum Operating Temperature: + 125 C
• Mounting Style: SMD/SMT
• Package / Case: QSOP-16

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.
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.
AC
FIGURE 10. AC DETECTION
FILTER
EMI
16
ISL1904
GND
AC
16
15
14
13
12
11
10
9
ISL1904
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.
AC
AC
FIGURE 12. ALTERNATE AC DETECTION
FILTER
FIGURE 11. AC DETECTION
EMI
FILTER
EMI
ISL1904
ISL1904
GND
GND
AC
AC
16
15
14
13
12
11
10
9
16
15
14
13
12
11
10
9
September 20, 2012
FN8286.1