ISL97672_1011 Datasheet, PDF (10/17 Pages) Intersil Corporation – 6-Channel LED Driver Current Matching ±0.7%

ISL97672
For example, if the maximum required LED current
(I_LED(max)) is 20mA, rearranging Equation 1 yields
Equation 2:
R_SET = 401.8 ⁄ 0.02 = 20.1kΩ
(EQ. 2)
PWM CURRENT CONTROL
The ISL97672 employs direct PWM dimming such that
the output PWM dimming follows directly with the input
PWM signal without modifying the input frequency. The
average LED current of each channel can be calculated as
Equation 3:
I_LED(ave) = I_LED × PWM
(EQ. 3)
Switching Frequency
The boost switching frequency can be adjusted by a
resistor as Equation 4:
f_SW
=
-⁽--⁵----^×---1---0---¹---⁰----⁾
R_FSW
(EQ. 4)
where f_SW is the desirable boost switching frequency and
R_FSW is the setting resistor.
5V Low Dropout Regulator
A 5V LDO regulator is present at the VDC pin to develop
the necessary low voltage supply which is used by the
chips internal control circuitry. Because VDC is an LDO
pin, it requires a bypass capacitor of 1µF or more for the
regulation. The VDC pin can be used as a coarse
reference with few mA sourcing capability.
Inrush Control and Soft-Start
The ISL97672 has separately built-in independent inrush
control and soft-start functions. The inrush control function
is built around the short circuit protection FET, and is only
available in applications which include this device. At
start-up, the fault protection FET is turned on slowly due to
a 30µA pull-down current output from the FAULT pin. This
discharges the fault FET's gate-source capacitance, turning
on the FET in a controlled fashion. As this happens, the
output capacitor is charged slowly through the weakly
turned on FET before it becomes fully enhanced. This
results in a low inrush current. This current can be further
reduced by adding a capacitor (in the 1nF to 5nF range)
across the gate source terminals of the FET.
Once the chip detects that the fault protection FET is
turned on hard, it is assumed that inrush is complete. At
this point, the boost regulator will begin to switch and the
current in the inductor will ramp-up. The current in the
boost power switch is monitored and the switching
terminated in any cycle where the current exceeds the
current limit. The ISL97672 includes a soft-start feature
where this current limit starts at a low value (275mA).
This is stepped up to the final 2.2A current limit in seven
further steps of 275mA. This is stepped up to the final
2.2A current limit in 7 further steps of 275mA. These
steps will happen over at least 8ms, and will be extended
at low LED PWM frequencies if the LED duty cycle is low.
This allows the output capacitor to be charged to the
required value at a low current limit and prevents high
input current for systems that have only a low to medium
output current requirement.
For systems with no master fault protection FET, the
inrush current will flow towards C_OUT when VIN is applied
and it is determined by the ramp rate of VIN and the
values of C_OUT and L.
Fault Protection and Monitoring
The ISL97672 features extensive protection functions to
cover all the perceivable failure conditions. The failure
mode of a LED can be either open circuit or as a short.
The behavior of an open circuited LED can additionally
take the form of either infinite resistance or, for some
LEDs, a zener diode, which is integrated into the device
in parallel with the now opened LED.
For basic LEDs (which do not have built-in zener diodes),
an open circuit failure of an LED will only result in the loss
of one channel of LEDs without affecting other channels.
Similarly, a short circuit condition on a channel that
results in that channel being turned off does not affect
other channels unless a similar fault is occurring.
Due to the lag in boost response to any load change at its
output, certain transient events (such as LED current
steps or significant step changes in LED duty cycle) can
transiently look like LED fault modes. The ISL97672 uses
feedback from the LEDs to determine when it is in a
stable operating region and prevents apparent faults
during these transient events from allowing any of the
LED stacks to fault out. See Table 1 for more details.
A fault condition that results in an input current that
exceeds the devices electrical limits will result in a
shutdown of all output channels.
Short Circuit Protection (SCP)
The short circuit detection circuit monitors the voltage on
each channel and disables faulty channels which are above
approximately 5V (the action taken is described in Table 1).
Open Circuit Protection (OCP)
When one of the LEDs becomes open circuit, it can
behave as either an infinite resistance or a gradually
increasing finite resistance. The ISL97672 monitors the
current in each channel such that any string which
reaches the intended output current is considered
“good”. Should the current subsequently fall below the
target, the channel will be considered an “open circuit”.
Furthermore, should the boost output of the ISL97672
reaches the OVP limit or should the lower
over-temperature threshold be reached, all channels
which are not “good” will immediately be considered as
“open circuit”. Detection of an “open circuit” channel will
result in a time-out before disabling of the affected
channel. This time-out is sped up when the device is
above the lower over-temperature threshold in an
attempt to prevent the upper over-temperature trip
point from being reached.
Some users employ some special types of LEDs that have
zener diode structure in parallel with the LED for ESD
10
FN7632.1
November 1, 2010