ISL97687 Datasheet, PDF (14/24 Pages) Intersil Corporation – 4-Channel LED Driver with Phase Shift Control and 10-Bit Dimming Resolution

ISL97687
by the output capacitor, kept at the correct voltage by the PFM
mode in the off-times. This allows the output to always remain
very close to the required level, so that when the LEDs are
re-enabled, the boost output is already at the correct level. This
dramatically improves LED PWM performance, providing industry
leading linearity down to sub 1% levels, and reduces the
overshoots in the boost inductor current, caused by transient
switching when the LEDs are switched on, to a minimum.
The system will continue to maintain V_OUT at the target level for
120ms after the last time the LEDs were on. If all LEDs are off for
a longer period than this, the converter will stop switching and go
into a sleep mode, allowing V_OUT to decay, in order to save power
during long backlight-off periods.
Switching Frequency
The boost switching frequency can be adjusted by the resistor on
the OSC pin, which must be connected to AGND, and follows
Equation 4:
f_SW
=
-⁽--⁵----^×---1---0---¹---⁰----⁾
R_OSC
(EQ. 4)
where f_SW is the desirable boost switching frequency and R_OSC is
the setting resistor.
5V and 2.4V Low Dropout Regulators
A 5V LDO regulator is used to provide the low voltage supply
needed to drive internal circuits. The output of this LDO is the VDC
pin. A decoupling capacitor of 1µF or more is required between
this pin and AGND for correct operation. Similarly, a 2.4V LDO
regulator is present at the VLOGIC pin, and also requires a 1µF
decoupling capacitor. Both pins can be used as a coarse voltage
reference, or as a supply for other circuits, but can only support a
load of up to ~10mA and should not be used to power noisy
circuits that can feed significant noise onto their supply.
Soft-Start and Boost Current Limit
The boost current limit should be set by using a resistor from CS
to PGND. The typical current limit can be calculated as:
I_LIMIT
=
0_----._-1_----7_--
R_CS
(EQ. 5)
The CS resistor should be chosen based on the maximum load
that needs to be driven. Typically, a limit of 30~40% more than is
required under DC conditions is sufficient to allow for necessary
overshoots during load transients. Values of 20~100mΩ are
supported.
It is important that PGND pin 14 (QFN)/18 (SOIC) is connected
directly to the base of the sense resistor, with no other
connection to the ground system, except via this path. This is
because this pin is used as a ground reference for the CS pin.
Connecting it here gives the maximum noise immunity and the
best stability characteristics.
The ISL97687 uses a digital current limit based soft start. The
initial limit level is set to one ninth of the full current limit, with
eight subsequent steps increasing this by a ninth of the final
value every 2ms until it reaches the full limit. 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), the step will be delayed until the LEDs are
conducting again.
If the LEDs are off for more than 120ms, making the converter
go into sleep mode, soft-start will be restarted when the LEDs are
re-enabled.
Fault Protection and Monitoring
The ISL97687 features extensive protection functions to cover all
perceivable failure conditions. The failure mode of an LED can be
either open or short circuit. The behavior of an open circuit LED
can additionally take the form of either infinite or very high
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 LED failure 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 significant step changes in LED
duty cycle, or a change in LED current caused by CSEL switching)
can transiently look like LED fault modes. The ISL97687 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 strings to fault out.
See Figure 26 and Table 2 for more details.
Short Circuit Protection (SCP)
The short circuit detection circuit monitors the voltage on each
channel and disables faulty channels which are detected to be
more than the short circuit threshold, 8V above the lowest CH
pin, following a timeout period.
Open Circuit Protection (OCP)
When any of the LEDs become open circuit during the operation,
that channel will be disabled after a timeout period, and the part
will continue to drive the other channels. The ISL97687 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 ISL97687 reach the OVP limit, all channels which
are not “good” will be timed out.
Unused CH pins should be grounded, which will disable them
from start-up. This will prevent V_OUT having to ramp to OVP at
start-up, in order to determine that they are open.
Undervoltage Lock-out
If the input voltage falls below the UVLO level of 2.8V, the device
will stop switching and reset. Operation will restart, with all
digital settings will be returned to their default states, once the
input voltage is back in the normal operating range.
Over-Temperature Protection (OTP)
The OTP threshold is set to +150°C. When this is reached, the
boost will stop switching and the output current sources will be
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FN7714.0
September 15, 2011