ISL6745ALEVAL3Z Intersil, ISL6745ALEVAL3Z Datasheet - Page 2

• Manufacturer Part Number: ISL6745ALEVAL3Z
• Description: EVAL BOARD 3 FOR ISL6745
• Manufacturer: Intersil

Specifications of ISL6745ALEVAL3Z

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Circuit Description
Unlike the active power factor correction converters, a
voltage follower works in discontinuous conduction mode
with comparatively constant duty ratio (D) within each utility
cycle. There is no need for a PFC controller, or input
voltage/current sense circuit. A simplified block diagram is
shown in Figure 4. A typical waveform of the voltage after a
triac dimmer is shown in Figure 3, where high dv/dt is
noticed as the triac is turned on. A small capacitor C
recommended here to filter out the switching noises. C
should be very small so that the voltage across it follows
closely the rectified input voltage |V
FIGURE 2. TOP AND BOTTOM VIEW OF LED DRIVER
AC
2
AC
|. An Electrolytic
FIGURE 4. VOLTAGE FOLLOWER BLOCK DIAGRAM
Application Note 1487
D1
D2
1
is
1
D3
D4
P1
IIN
C1
CONTROLLER
capacitor is not suitable for this purpose, because of its high
ESR and dielectric loss. Instead, a high frequency film
capacitor such as
should be used
A damper resistor R
because of the high dv/dt caused by the phase-angle
dimming control of the triac. A minimum resistance should
be used to ensure as low as possible power loss, while
achieving normal dimming function. This resistor can be
shorted if the driver is not triac dimmer controlled. L
are selected so that the converter operates in DCM. The
details of the design of the two inductors and capacitor C
can be found in Application Note AN1411.
One simple and low-cost solution for LED lamp brightness
dimming is to control the LED current. It is assumed that the
brightness of the LED lamp is linearly proportional to its
current in normal conditions. Although the tune and color
may change with operating current, the current based
dimming method is acceptable for common LED luminaire
applications. The current reference signal is in proportion to
the averaged rectified voltage |V
versus the phase angle α of triac dimmer is shown in
Figure 5 for a commercial dimmer.
FIGURE 3. TYPICAL VOLTAGE AFTER THE TRIAC DIMMER
CONVERTER
POWER
POWER
PWM
Vo
.
C0
metallized mylar or polypropylene
P0
1
IO
is needed to reduce the ringing effect
R0
AC
| . The average voltage
July 28, 2009
1
and L
AN1487.0
2
2