ISL1902 Datasheet, PDF (21/25 Pages) Intersil Corporation – Full-Featured, Dimmable AC Mains LED Driver with PFC

ISL1902
+
Q1
R5
1
24
2
23
3
22
4
21
5
20
6
19
7
LOUT 18
8 REFIN LREF 17
9 LPOUT LFB 16
10
15
11
14
12
ISL1902
13
R3
C1
FIGURE 28. SECOND LED STRING CONTROL
The linear amplifier may also be used to measure and scale the
LED current directly rather than using the differential current
sensing inputs, CS+ and CS-, that measure the switching current.
Amplifying the signal allows a smaller sensing resistor value for
improved efficiency. As shown in Figure 29, the voltage across
R4 is scaled by the linear amplifier with a gain of 1 + R2/R1.
+
R4
R1
6
19
7
LOUT 18
8
LREF 17
9
LFB 16
10
15
11 ^ISL1902 14
12 FB1
13
R3 = R1||R2
R2
Control Loop
The control loop configuration is user adjustable with selection of
the external compensation components. For applications
requiring power factor correction (PFC), a very low bandwidth
integrator is used, typically 20Hz or less. In other applications,
the control loop bandwidth can be increased as required, like any
other externally compensated voltage mode PWM controller.
The ISL1902 has two error amplifiers that share a common
non-inverting input and a common output. Each EA can sink
current, but has negligible sourcing capability. An external pull-up
resistor to VREF is required. This configuration causes the EA with
lowest output to be dominant. EA1 is the principal error amplifier
and is compensated externally for low bandwidth for PFC
applications. The downside to a low bandwidth amplifier is that it
cannot respond to input transients quickly. This is where the
second EA comes in. It can be configured for a much higher
bandwidth so that transient response is greatly improved. Under
normal operating conditions EA2 is not active. Its feedback
network is set for a higher output than EA1. When an input surge
occurs, EA1 cannot respond rapidly and the surge propagates to
the output. EA2 becomes active when its feedback voltage
exceeds the reference setpoint and acts to reduce the output
transient. The difference in setpoint is accomplished by
weighting the feedback networks to the EAs appropriately.
The voltage on IOUT is a scaled version of the CS+/CS-
differential signal, having been amplified by 4x. When averaged,
it is a scaled representation of the converter output current, I_O.
By measuring IOUT in this manner, both average and
instantaneous inductor currents are known. The instantaneous
inductor current information informs the critical conduction
mode (CrCM) oscillator when the switching current has decayed
to zero.
Figure 30 shows a typical configuration for the control loop. The
sensing resistor R_S determines the amplitude of the CS+ signal.
At maximum load this signal must be scaled to match the 0.5V
maximum reference. Since IOUT is 4x the amplitude of the CS+
signal, a simple resistor divider with filtering is required to scale
IOUT prior to connecting to the FB input.
R_S
=
-----------------------^V----^R-----^E----^F------------------------
A_IOUT ⋅ A_DIVIDER ⋅ I_O
Ω
(EQ. 29)
where A_IOUT is the IOUT buffer gain (nominally 4x), A_DIVIDER is
the gain of the external resistor divider on IOUT (R2/(R1 + R2)),
V_REF is the maximum reference level (0.5V), and I_O is the
maximum output current. In most applications, R_S will be sized
to minimize power dissipation while providing adequate signal
level. The minimum value of the I_OR_S product is 125mV, required
to achieve 0.5V on IOUT.
FIGURE 29. DIRECT LED CURRENT SENSING
21
FN7981.2
March 20, 2013