ISL97671IRZ-T Intersil, ISL97671IRZ-T Datasheet - Page 14

• Manufacturer Part Number: ISL97671IRZ-T
• Description: IC LED DVR PWM CTRL 6CH 20QFN
• Manufacturer: Intersil

Specifications of ISL97671IRZ-T

• Topology: PWM, Step-Up (Boost)
• Number Of Outputs: 6
• Internal Driver: Yes
• Type - Primary: Automotive, Backlight
• Type - Secondary: RGB, White LED
• Frequency: 600kHz ~ 1.2MHz
• Voltage - Supply: 4.5 V ~ 26.5 V
• Mounting Type: Surface Mount
• Package / Case: 20-VFQFN Exposed Pad
• Operating Temperature: -40°C ~ 85°C
• Current - Output / Channel: 40mA
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -

In-rush Control and Soft-start
The ISL97671 has separately built in independent in-rush
control and soft-start functions. The in-rush 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 in-rush 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 in-rush has
completed. 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 is terminated in any cycle where the current
exceeds the current limit. The ISL97671 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 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
in-rush current will flow towards C
applied and it is determined by the ramp rate of VIN and
the values of C
Fault Protection and Monitoring
The ISL97671 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. All LED
faults are reported via the SMBus/I
Register 0x02 (Fault/Status register). The controller is
able to determine which channels have failed via Register
0x09 (Output Masking register). The controller can also
choose to use Register 0x09 to disable faulty channels at
start-up, resulting in only further faulty channels being
reported by Register 0x02.
OUT
and L.
14
OUT
2
C interface to
when VIN is
ISL97671
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 ISL97671 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. The control device logic
will remain functional such that the Fault/Status Register
can be interrogated by the system. The root cause of the
failure will be loaded to the volatile Fault/Status Register
so that the host processor can interrogate the data for
failure monitoring.
Short Circuit Protection (SCP)
The short circuit detection circuit monitors the voltage on
each channel and disables faulty channels which are
detected above the programmed short circuit threshold.
There are three selectable levels of short circuit threshold
(3.6V, 4.8V and 5.85V) that can be programmed through
the Configuration Register 0x08 bits 0 and 1. When an
LED becomes shorted, the action taken is described in
Table 1. The default short circuit threshold is 5V. The
detection of this failure mode can be disabled via
Register 0x08, see Table 2B for additional information.
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 ISL97671 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 ISL97671
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 enhancement, thus enabling open circuit operation.
When this type of LED goes open circuit, the effect is as
if the LED forward voltage has increased, but no
lighting. Any affected string will not be disabled, unless
the failure results in the boost OVP limit being reached,
allowing all other LEDs in the string to remain
functional. Care should be taken in this case that the
boost OVP limit and SCP limit are set properly, so as to
make sure that multiple failures on one string do not
cause all other good channels to be faulted out. This is
due to the increased forward voltage of the faulty
June 24, 2010
FN7631.0