ISL6745LEVAL2Z Intersil, ISL6745LEVAL2Z Datasheet - Page 4

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

Specifications of ISL6745LEVAL2Z

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

If we define the ripple factor Krpi = Irp/Il, where Il is the peak
of the input current, then the constraint for DCM operation of
the converter is as expressed in Equation 15.
In the above analysis, it is assumed that the voltage across
the input capacitor C
and follows the rectified input voltage closely. Furthermore,
the voltage across C
the assumption is not correct, the current waveform will be
distorted.
The input capacitor can be estimated by using Equation 16.
The AC source delivers power as a sine wave. Since the
input is rectified, power is delivered to the load at twice the
input line frequency. Therefore, the ripple frequency on the
output voltage is also twice of the line frequency. To reduce
the ripple, the output capacitor needs to be selected
according to Equation 17.
The duty ratio is controlled by the difference between the
reference signal I
Waveforms
L
L
C
C
1
2
1
O
FIGURE 4. INPUT VOLTAGE AND CURRENT (VAC = 80V,
>
<
>
=
⎛
⎜
⎝
t
---------------- -
ON
---------------------------------
2V
4K
----------------------------- -
K
I
V
---------------------
rp
rpv
1
rp-p
V
rpi
V
+
1
V
1
V
V
2
l
V
1
I
=
t
l
O
2
s
O
t
-----------------
dp
I
L
ω
O
K
ON
2 4I
2
rp
1
= 30mA
------------ -
t
s
V
ADJ
I
VINPUT
V
1
O
1
O
2
=
1
–
and the voltage across the current
is identical to the voltage across C
is independent of the circuit operation
2K
--------------------------- -
t
----------------------------- -
t
2K
VINPUT
ON
ON
rp
rp
(
V
V
V
V
1
O
1
2 η
O
) 2 η
4
I
I
O
O
⎞
⎟
⎠
–
1
Application Note 1411
(EQ. 15)
(EQ. 16)
(EQ. 17)
1
. If
sense resistor R
the reference signal I
dimmed. A Type I Error Amplifier is used as shown in
Figure 3, with the crossover frequency determined by the
feedback capacitor C
Equation 18.
The bandwidth should be less than the line frequency, so
that the duty ratio of the converter does not disturb the
sinusoidal power delivery from the source. Otherwise,
harmonics appear in the input line current and degrade
power factor. However, the bandwidth should be wide
enough to allow adequate control of the load current. As a
trade-off, the cut-off frequency can be around half of the line
frequency.
Overvoltage protection is needed to protect the LED and
converter from damage. Due to the nonlinear relationship
between the diode current and voltage, a small increase in
output voltage can cause a large increase in current. The
overvoltage protection is implemented by comparing the
output voltage with a constant reference voltage, and the
comparator output is used to control a small FET in parallel
with the soft-start capacitor. Once the output voltage reaches
the threshold, the FET is turned on, the soft-start capacitor
discharged, and the duty cycle is reduced to zero. The PWM
output is shutdown until the output voltage decays below the
reset threshold. A new soft-start cycle begins at this time.
This behavior repeats until the cause of the overvoltage is
removed.
ω
C
FIGURE 5. INPUT VOLTAGE AND CURRENT, VAC = 120V,
=
------------- -
RC
1
fo
I
O
= 300mA
S
. By changing the current sense resistor, or
VINPUT
ADJ
fb
VINPUT
and the resistor R, given in
, the brightness of the LED can be
June 9, 2009
(EQ. 18)
AN1411.3