ISL6745LEVAL2Z Intersil, ISL6745LEVAL2Z Datasheet - Page 2

• Manufacturer Part Number: ISL6745LEVAL2Z
• Description: EVAL BOARD 2 FOR ISL6745
• Manufacturer: Intersil

Specifications of ISL6745LEVAL2Z

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

Operation Mechanism and Design Constraints
A simplified offline SEPIC converter for LED lighting
applications is shown in Figure 3, where the voltage across
C
placed in series with the switch S
placed in series with the LED string to sense the load
current. The control signals include the load current and
switch current. The output voltage is sensed for the purpose
of output overvoltage protection (OVP). Since the input
capacitor C
rectified input voltage, and the voltage applied to the SEPIC
converter is approximately a rectified sinusoidal. To achieve
high power factor, the envelope of the converter input
current must track the rectified input voltage waveform. This
is accomplished when the converter is operated at a
constant switching frequency, a constant duty ratio, and
while operating in the discontinuous conduction mode
(DCM). Assuming the impedance of the power source is
negligible, the voltage across C
where subscript I denotes input AC line, and V
amplitude of the input AC voltage with frequency ω
If the switching frequency of the converter is much higher
than the utility frequency, and C
voltage across C
feedback voltage, V
the amplified difference between the LED current, Io, and the
reference voltage, Ir. Varying the reference voltage causes a
proportional change in the LED brightness. The current
signal from the current sense resistor, Rcs, which is
connected in series with the boost switch S1, is compared
with the overcurrent threshold for overcurrent protection. A
sawtooth carrier signal is compared with the feedback signal
Vm and generates the PWM signal to control the turn on/off
of the switch. Typical waveforms of the inductor current,
switch and diode currents are shown in Figure 2.
v
C1
1
is the rectified AC voltage. A current sense resistor is
t ( )
=
V
l
1
sin
is very small, the voltage waveform is the
(
ω
l
2
t
)
will be equal to the voltage across C
m
, is created by the error amplifier. V
2
1
2
is given in Equation 1,
1
is properly sized, the
. A second resistor is
l
is the
Application Note 1411
l
.
(EQ. 1)
1
. A
m
is
G A T E
AC
D1~D4
I
I
L 1
L 2
I
I
Q
FIGURE 3. SEPIC CONVERTER FOR LED LIGHTING
D
D1
FIGURE 2. TYPICAL OPERATION WAVEFORMS
NAME
PWM
L
AC
C
R
C
1
S
C
~
/L
O
1
S
1
2
D4
2
TABLE 1. SEPIC CONVERTER
C1
L1
CONTROLLER
PW M
85V ~ 265VAC, 50Hz ~ 60Hz
Bridge Rectifier
Input Capacitor
SEPIC Inductors
High Voltage MOSFET
Output Capacitor
Intermediate Capacitor
Current Sense Resistor
ISL6745A PWM Controller
PW M
Rcs
O CP
Cs
S1
C2
V
ERR
DESCRIPTION
L2
C
FB
D5
-
+
-
+
C0
Rs
I
ADJ
OVP
June 9, 2009
AN1411.3
vo