isl97635a Intersil Corporation, isl97635a Datasheet - Page 14

• Manufacturer Part Number: isl97635a
• Description: Smbus 6-channel Led Driver
• Manufacturer: Intersil Corporation

Fault Protection and Monitoring
The ISL97635A 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 interface to 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.
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 ISL97635A 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 two selectable levels of short circuit threshold
(3.1V and 8.0V) that can be programmed through the
Configuration Register 0x08 bit 0. When an LED becomes
shorted, the action taken is described in Table 1. The default
short circuit threshold is 8V. The detection of this failure
mode can be disabled via register 0x08 bit 1 if required.
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 ISL97635A 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
14
ISL97635A
channel will be considered an “open circuit”. Furthermore,
should the boost output of the ISL97635A 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.
Some users employ some special types of LEDs that have
zener diode structure in parallel with the LED for ESD
enhancement and enabling open circuit operation. When
this type of LED is open circuited, 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 channel making
all other channels look as if they have LED shorts. See
Table 1 for details regarding responses to fault conditions.
Overvoltage Protection (OVP)
The integrated OVP circuit monitors the output voltage and
keeps the voltage at a safe level. The OVP threshold is set
as Equation 14:
These resistors should be large to minimize the power loss.
For example, a 1MΩ R
to 32.2V. Large OVP resistors also allow C
slowly during the PWM off time.
Undervoltage Lockout
If the input voltage falls below the UVLO level of 2.45V, the
device will stop switching and reset. Operation will restart
when the voltage comes back into the operating range.
Input Overcurrent Protection
During normal switching operation, the current through the
internal boost power FET is monitored. If the current
exceeds the current limit, the internal switch will be turned
off. This monitoring happens on a cycle-by-cycle basis in a
self protecting way.
Additionally, the ISL97635A monitors the voltage at the LX
and OVP pins. At start-up, a fixed current is injected out of
the LX pins and into the output capacitor. The device will not
start-up unless the voltage at LX exceeds 1.2V. Furthermore,
should the voltage at LX not rise above this threshold during
any subsequent period where the power FET is not switched
on, it will immediately disable the input protection FET. The
OVP pin is also monitored such that if it rises above and
OVP
=
1.21V
×
(
R
UPPER
UPPER
+
R
LOWER
and 39kΩ R
) R
⁄
LOWER
LOWER
OUT
December 22, 2008
discharges
sets OVP
(EQ. 14)
FN6564.2