ISL1902 Datasheet, PDF (20/25 Pages) Intersil Corporation – Full-Featured, Dimmable AC Mains LED Driver with PFC

ISL1902
The red trace is the current source supplying the output. The blue
trace is the output capacitor voltage. The green trace is the LED
current. When the PWM signal is off, the 50mA current source
charges C_OUT and the output voltage increases. When the PWM
turns on, 100mA of current flows through the LEDs, with the initial
current slightly higher and the final current slightly lower as C_OUT
discharges. The peak-to-peak ripple voltage on C_OUT is ~160mV.
The decrease in the LED current during conduction is determined
by the size of the output capacitor and the LED current.
Linear Amplifier
The linear amplifier block is a fully accessible uncommitted
operational amplifier. It may be used for a variety of purposes,
such as interfacing sensors, direct sensing of LED current, or a
pre-load amplifier. Examples of using the linear amplifier as a
sensor interface are shown in Figures 20 through 23.
+
Q1
R5
1
24
2 PRELOAD 23
3
22
R3
4 VREF
21
5
20
6
19
7
LOUT ¹⁸
8 REFIN _LREF 17
9 LPOUT _LFB 16
R1
10
15
11
14
12 ISL1902 13
R4
R2
FIGURE 26. LINEAR AMPLIFIER CONFIGURED AS PRE-LOAD
The linear amplifier may be used as a pre-load control to provide
a larger dynamic dimming range as well as providing additional
holding current for applications using triac-based dimmers. As
shown in Figure 26, the pre-load can be configured as an active
load that increases linearly as the control loop reference level
(REFIN) decreases. The result is not only is the total load current
decreased as REFIN is lowered, but an increasing portion of the
load current is shunted to the pre-load. At some point, the
preload conducts all of the load current while allowing zero LED
current. This allows the converter to continue operation to
maintain circuit bias with zero LED current. Very high levels of
LED dimming resolution become achievable. Both the maximum
pre-load current and turn-on threshold are adjustable.
Again referring to FIgure 26, the voltage across R5 represents
the current flowing in the pre-load. The maximum level of this
signal is limited by the VOH of the linear amplifier and the gate
threshold voltage of the pre-load FET, Q1. A reasonable
maximum voltage for this signal is 3.0V. Therefore, the
maximum pre-load current, I_PL, is 3.0/R5.
V_O = V_R5 = LREF ⋅ _⎝⎛1 + ^RR-----³⁴--_⎠⎞ – _⎝⎛^RR-----³⁴-- ⋅ REFIN_⎠⎞
V
(EQ. 25)
where LREF and REFIN are the voltages at the LREF and REFIN
pins, respectively. For purposes of illustration, if R3 and R4 are
equal, Equation 25 simplifies to:
V_O = V_R5 = 2 ⋅ LREF – REFIN
V
(EQ. 26)
Equation 26 shows that if REFIN is greater than 2x LREF, V_O is
non-positive and the pre-load is not conducting. With proper
selection of LREF and R3/R4, the pre-load turn-on threshold and
gain characteristics can be matched to the application
requirements.
R_-----3_--
R4
=
3_{-----–-----}(_--0_----._-5_-----⋅_---%_------H_----)_----±_---------(_--0_----._-5_-----⋅_---%_-----H_{------–-----}3_---)_---²_----+-----6_-----⋅_---%_-----H_---
%H
(EQ. 27)
where%H is the selected fraction of maximum load when the pre-
load begins to conduct.
LREF
=
-^{RR---------34}-------^â‹---⁰----^.-⁵-----^â‹---^%------^H------
1
+
R_-----3_--
R4
=
VREF
⋅
--------^R-----²---------
R1 + R2
V
(EQ. 28)
As an example, assume the pre-load should begin to operate at
75% of full load, and that the maximum pre-load current, I_PL, is
50mA. Using Equation 27 to solve for the ratio of R3/R4 yields a
result of 8. Equation 28 yields a value of 0.333V for LREF.
Remembering the maximum allowed voltage across R5 is 3.0V
yields R5 = 3.0V/50mA = 60Ω at 150mW. V_O is plotted in
Figure 27.
3.0
2.5
2.0
1.5
1.0
0.5
0
0 50 100 150 200 250 300 350 400 450 500
REFIN (mV)
FIGURE 27. PRE-LOAD EXAMPLE
Alternatively, the linear amplifier may be used to control a
second LED string, either to force current sharing, or to control a
colored LED string for color correction. The second string can be
controlled from the same reference as the first LED string
allowing the string currents to track, or it can be controlled from
a separate reference that allows the two strings to work in
opposition, sharing the load current in proportion to each
reference. Figure 28 shows the tracking configuration.
20
FN7981.2
March 20, 2013