A6267
Automotive High Current LED Controller
known as discontinuous mode operation, and results in some low
frequency ripple. The average LED current, however, remains
regulated down to zero. In discontinuous mode, when the induc-
tor current drops to zero, the voltage at the drain of the external
MOSFET rings, due to the resonant LC circuit formed by the
inductor, and the switch and diode capacitance. This ringing is
low frequency and is not harmful.
LF Single diode forward voltage reference input. Measures the
forward voltage of the first LED. This value is used as a reference
against the voltage on the LA pin to detect possible shorted LEDs
in the LED string.
Circuit Operation
Converter A constant frequency, current mode control scheme
is used to regulate the current through the LEDs. There are two
control loops within the regulator. The inner loop formed by the
amplifier, AS (see the Functional Block Diagram for AS, AC, AE,
and AL), comparator, AC, and the RS bistable, controls the induc-
tor current as measured through the switch by the switch sense
Switch Current Limit The switch current is measured by the
switch sense resistor, RSS , and the switch sense amplifier, AS
(see the Functional Block Diagram). The input limit of the sense
amplifier, VIDS, and the maximum switch current, ISMAX , define
the maximum value of the sense resistor as:
resistor, RSS
.
R
SS = VIDS / ISMAX
(1)
The outer loop including the amplifier, AL, and the integrating
error amplifier, AE, controls the average LED current by provid-
ing a setpoint reference for the inner loop.
This defines the maximum measurable value of the switch (and
inductor) current.
The maximum switch current is modulated by the on-time of the
switch. An internal slope compensation signal is subtracted from
the voltage sense signal to produce a peak sense voltage which
effectively defines the current limit. This signal is applied at a
rate of –16 mV/ꢀs starting with no contribution (t=0ꢀs) at the
beginning of each switching cycle. Figure 1 illustrates how the
peak sense voltage (typical values) changes over a period of 3 ꢀs.
The LED current is measured by the LED sense resistor, RSL
and compared to the internal reference current to produce an
,
integrated error signal at the output of AE. This error signal sets
the average amount of energy required from the inductor by the
LEDs. The average inductor energy transferred to the LEDs is
defined by the average inductor current as determined by the
inner control loop.
For example, the maximum current (typical) through the switch
at t= 1.5ꢀs (D=50%) would be 145 mV/RSS , however, if the
switch remained on for a further 1 ꢀs, the maximum current
The inner loop establishes the average inductor current by
controlling the peak switch current on a cycle-by-cycle basis.
Because the relationship between peak current and average cur-
rent is non-linear, depending on the duty cycle, the reference
level for the peak switch current is modified by a slope generator.
This compensation reduces the peak switch current measurement
by a small amount as the duty cycle increases (refer to figure 1).
The slope compensation also removes the instability inherent in a
fixed frequency current control scheme.
through the switch would be 129 mV/RSS
.
200
150
100
50
The control loops work together as follows: the switch current,
sensed by the switch current sense resistor, RSS , is compared
to the LED current error signal. As the LED current increases
the output of AE will reduce, reducing the peak switch current
and thus the current delivered to the LEDs. As the LED current
decreases the output of AE increases, increasing the peak switch
current and thus increasing the current delivered to the LEDs.
0
0
1
2
3
Under some conditions, especially when the LED current is set to
a low value, the energy required in the inductor may result in the
inductor current dropping to zero for part of each cycle. This is
Period (ꢀs)
Figure 1. Slope compensation for peak switch current control.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
8
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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