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ISL97677
Switching Frequency
The boost switching frequency can be adjusted by a
resistor as shown in Equation 4:
prevents apparent faults during these transient events
from allowing any of the LED strings to fault out. See
Table 1 for more details.
( 5 × 10
)
f SW = -----------------------
10
R OSC
(EQ. 4)
Short Circuit Protection (SCP)
The short circuit detection circuit monitors the voltage on
each channel and disables faulty channels which are
where f SW is the desirable boost switching frequency
and R OSC is the setting resistor.
5V and 2.3V Low Dropout Regulators
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 for a coarse
regulator or reference but do not pull more than few mA
from it.
Similarly, a 2.3V LDO regulator is present at the
VLOGIC pin to develop the necessary low voltage supply
for the chip’s internal logic control circuitry. A 1μF
bypass capacitor or more is needed for regulation. The
VLOGIC pin can be used as a coarse regulator or
reference but do not pull more than few mA from it.
Soft-Start
The ISL97677 uses a digital soft-start where the boost
current limit is stepped up in 8 steps. The initial current
limit level is set to one ninth of the full current limit, with
subsequent steps increasing this by a ninth every 2ms.
In the event that no LEDs have been conducting during
the interval since the last step (for example, if the LEDs
are running at low duty cycle at low PWM frequency)
then the step will be delayed until the LEDs are
conducting. If the LEDs are disabled and re-enabled
again then soft start will be restarted when the LEDs are
enabled.
Fault Protection and Monitoring
The ISL97677 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
detected above the programmed short circuit threshold.
There are three selectable levels of short circuit threshold
(3V, 4V, and 5V) that can be programmed through the
Configuration Register 0x0F. When an LED becomes
shorted, the action taken is described in Table 1. The
default short circuit threshold is 4V. The detection of this
failure mode can be disabled by SMBus interface via
Register 0x0F.
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 ISL97677 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 ISL97677
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.
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 light
will be emitted. 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 channel look as if they have
LED shorts. See Table 1 for details for 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 shown in Equation 5:
other channels unless a similar fault is occurring. All LED
faults are reported via the SMBus interface to Register
OVP = 1.21V × ( R UPPER + R LOWER ) ? R LOWER
(EQ. 5)
0x02 (Fault/Status register).
Due to the lag in boost response to any load change at its
output, certain transient events (such as significant step
changes in LED duty cycle) can transiently look like LED
fault modes. The ISL97677 uses feedback from the LEDs
to determine when it is in a stable operating region and
13
These resistors should be large to minimize the power
loss. For example, a 1Mk Ω R UPPER and 30k Ω R LOWER
sets OVP to 41.2V. Large OVP resistors also allow C OUT
discharges slowly during the PWM Off time. Parallel
capacitors should be placed across the OVP resistors
such that R UPPER /R LOWER = C LOWER /C UPPER . Using a
C UPPER value of at least 30pF is recommended. These
FN6996.1
November 13, 2009
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