ISL78171 Datasheet, PDF (15/29 Pages) Intersil Corporation – 6-Channel, 50mA Automotive LED Driver with Ultra-high Dimming Ratio and Phase Shift Control

ISL78171
Switching Frequency
The default switching frequency is 600kHz but it can be selected
to 600kHz or 1.2MHz if the SMBus/I²C communications is used.
The switching frequency select bit is accessible in the SMBus/I²C
Configuration Register 0x08 bit 2. Alternatively 800kHz can be
selected by writing value 0x80 only to register 0x7F. The default
value for register 0x7F should otherwise be set to 0x00 in order
to use the 600kHz or 1.2MHz.
5V Low Dropout Regulator
An internal 5V low dropout (LDO) regulator supplies the
necessary low-voltage needed for the chip’s internal control
circuitry. VDC is the output of this LDO regulator which requires a
bypass capacitor of 1µF or more for the purpose of filtering &
regulation. The VDC pin can be used as a coarse reference as
long as it is sourcing 20mA or less.
IC Protection Features and Fault
Management
ISL78171 has several protection and fault management features
that improve system reliability. The following sections describe
them in more detail.
INRUSH CONTROL AND SOFT-START
The ISL78171 has separate, built-in, independent inrush control
and soft-start functions. The inrush control function is built
around an external short-circuit protection P-channel FET in
series with VIN. At start-up, the fault protection FET is turned on
slowly due to a 21µA 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 low-current FET
before it becomes fully enhanced. This results in a low inrush
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
fully, it assumes that inrush is complete. At this point, the boost
regulator begins to switch, and the current in the inductor ramps
up. The current in the boost power switch is monitored, and
switching is terminated in any cycle in which the current exceeds
the current limit. The ISL78171 includes a soft-start feature in
which this current limit starts at a low value (275mA). This value
is stepped up to the final 2.2A current limit in seven additional
steps of 275mA each. These steps happen over at least 8ms and
are extended at low LED PWM frequencies if the LED duty cycle is
low. This extension allows the output capacitor to charge 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 inrush
current flows towards C_OUT when V_IN is applied. The inrush
current is determined by the ramp rate of V_IN and the values of
C_OUT and L.
FAULT PROTECTION AND MONITORING
The ISL78171 features extensive protection functions to cover all
perceivable failure conditions.
The failure mode of an LED can be either an open circuit or 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 results only 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 ISL78171 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 2 for details.
A fault condition that results in an input current that exceeds the
boost FET current limit of 2A (typ.) will result in a shutdown of all
output channels.
All LED faults are reported via the SMBus/I²C interface to
Register 0x02 (Fault/Status register). The controller is able to
determine which channels has 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.
SHORT-CIRCUIT PROTECTION (SCP)
The short-circuit detection circuit monitors the voltage on each
channel and disables faulty channels that are above
approximately 7.5V (this action is described in “Protections Table”
on page 17).
OPEN CIRCUIT PROTECTION (OCP)
When one of the LEDs becomes an open circuit, it can behave as
either an infinite resistance or as a gradually increasing finite
resistance. The ISL78171 monitors the current in each channel
such that any string that reaches the intended output current is
considered “good.” Should the current subsequently fall below the
target, the channel is considered an “open circuit.” Furthermore,
should the boost output of the ISL78171 reach the OVP limit, or
should the lower over-temperature threshold be reached, all
channels that are not good are immediately considered to be open
circuit. Detection of an open circuit channel results in a time-out
before the affected channel is disabled. 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 special types of LEDs that have a Zener
diode structure in parallel with the LED. This configuration
provides ESD enhancement and enables open circuit operation.
When this type of LED is open circuited, the effect is as if the LED
forward voltage has increased but the lighting level has not
increased. Any affected string will not be disabled, unless the
failure results in the boost OVP limit being reached, which allows
all other LEDs in the string to remain functional. In this case, care
should be taken that the boost OVP limit and SCP limit are set
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FN8602.0
June 15, 2015