ISL1902FAZ-T7A Intersil, ISL1902FAZ-T7A Datasheet - Page 21

• Manufacturer Part Number: ISL1902FAZ-T7A
• Description: LED Lighting Drivers Dimmable AC Mains LED Driver PFC
• Manufacturer: Intersil

Specifications of ISL1902FAZ-T7A

• Rohs: yes
• Input Voltage: 4 V
• Operating Frequency: 320 Hz
• Maximum Supply Current: 14.5 mA
• Output Current: 1 A
• Maximum Operating Temperature: + 125 C
• Mounting Style: SMD/SMT
• Package / Case: QSOP-24
• Minimum Operating Temperature: - 40 C

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.
FIGURE 29. DIRECT LED CURRENT SENSING
FIGURE 28. SECOND LED STRING CONTROL
10
11
12
6
7
8
9
FB1
ISL1902
LOUT
LREF
10
11
12
1
2
3
4
5
6
7
8
9
LFB
R E FIN
LPO U T
21
ISL1902
19
18
17
16
15
14
13
R1
LO U T
LR E F
+
LFB
24
23
22
21
20
19
18
17
16
15
14
13
+
R2
R 5
Q 1
R3 = R1||R2
R4
C 1
R 3
ISL1902
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
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
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
where A
the gain of the external resistor divider on IOUT (R2/(R1 + R2)),
V
maximum output current. In most applications, R
to minimize power dissipation while providing adequate signal
level. The minimum value of the I
to achieve 0.5V on IOUT.
REF
S
=
is the maximum reference level (0.5V), and I
------------------------------------------------------------
A
IOUT
IOUT
⋅
is the IOUT buffer gain (nominally 4x), A
A
V
DIVIDER
REF
S
determines the amplitude of the CS+ signal.
⋅
I
O
Ω
O
R
S
product is 125mV, required
S
O
will be sized
is the
March 20, 2013
DIVIDER
(EQ. 29)
FN7981.2
is
O
.