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ISL97636AIRZ Datasheet(PDF) 11 Page - Intersil Corporation

Part # ISL97636AIRZ
Description  6-Channel LED Driver
Download  18 Pages
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Manufacturer  INTERSIL [Intersil Corporation]
Direct Link  http://www.intersil.com/cda/home
Logo INTERSIL - Intersil Corporation

ISL97636AIRZ Datasheet(HTML) 11 Page - Intersil Corporation

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11
FN6566.0
May 9, 2008
For example, if the maximum required LED current (ILED(max))
is 20mA, rearranging Equation 1 yields Equation 3:
PWM CURRENT CONTROL
The average LED current of each channel can be controlled by
an external PWMI signal as shown in Equation 3:
The PWM dimming frequency can be, for example, 20kHz,
but there are a minimum on and off time requirements such
that the dimming will be in the range of 10% to 99.5%. If the
dimming frequency is below 5kHz, the dimming range can
be 1% to 99.5%.
The PWM dimming off time cannot be longer than 28ms or
else the driver will enter shutdown.
5V Low Dropout Regulator
A 5.2V 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. For applications with an input voltage
≤ 5.5V, the
VIN and VDC pins can be connected together. The VDC pin
can be used as a coarse reference with few mA sourcing
capability.
Inrush Control and Soft-start
The ISL97636A 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 ISL97636A
includes a soft-start feature where this current limit starts at a
low value (375mA). This is stepped up to the final 3A current
limit in seven further steps of 375mA. These steps will
happen over a 1ms total time, such that after 1ms the final
limit will be reached. 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 COUT when VIN is applied and it is
determined by the ramp rate of VIN and the values of COUT
and L.
Fault Protection and Monitoring
The ISL97636A 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 ISL97636A 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
detected above 8V (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 ISL97636A monitors the current in each
channel such that any string which reaches at least 75% of
the intended output current is considered “good”. Should the
current subsequently fall below 50% of the target the
channel will be considered an “open circuit”. Furthermore,
should the boost output of the ISL97636A reach 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.
R
SET
733 0.02
36.6k
Ω
==
(EQ. 2)
I
LED ave
()
I
LED
PWMI
×
=
(EQ. 3)
ISL97636A


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