ISL97672 Intersil, ISL97672 Datasheet - Page 11

• Manufacturer Part Number: ISL97672
• Description: 6-Channel LED Driver
• Manufacturer: Intersil

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
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
11
ISL97672
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 5:
These resistors should be large to minimize the power
loss. For example, a 1MkΩ R
OVP to 41.2V. Large OVP resistors also allow C
discharges slowly during the PWM Off time. Parallel
capacitors should also be placed across the OVP resistors
such that R
C
capacitors reduce the AC impedance of the OVP node,
which is important when using high value resistors.
Undervoltage Lockout
If the input voltage falls below the UVLO level of 2.45V,
the device will stop switching and be reset. Operation will
restart only if the V
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 ISL97672 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.
The OVP pin is also monitored such that if it rises above
and subsequently falls below 20% of the target OVP
level, the input protection FET will also be switched off.
Over-Temperature Protection (OTP)
The ISL97672 includes two over-temperature thresholds.
The lower threshold is set to +130°C. When this
threshold is reached, any channel which is outputting
current at a level significantly below the regulation target
will be treated as “open circuit” and disabled after a time-
out period. This time-out period is also reduced to 800µs
when it is above the lower threshold. The intention of the
lower threshold is to allow bad channels to be isolated
and disabled before they cause enough power dissipation
(as a result of other channels having large voltages
across them) to hit the upper temperature threshold.
The upper threshold is set to +150°C. Each time this is
reached, the boost will stop switching and the output
current sources will be switched off. Once the device has
OVP
UPPER
=
1.21V
value of at least 30pF is recommended. These
UPPER
×
(
R
UPPER
/R
LOWER
IN
is back in the normal operating
+
R
= C
LOWER
UPPER
LOWER
) R
⁄
and 30kΩ R
/C
LOWER
UPPER
www.DataSheet4U.com
. Using a
LOWER
OUT
June 24, 2010
FN7632.0
(EQ. 5)
sets