ISL78171 Datasheet, PDF (16/29 Pages) Intersil Corporation – 6-Channel, 50mA Automotive LED Driver with Ultra-high Dimming Ratio and Phase Shift Control

ISL78171
properly, to ensure that multiple failures on one string do not
cause all other good channels to fault out. This condition could
arise if the increased forward voltage of the faulty channel
makes all other channels look as if they have LED shorts. See
Table 2 for details of responses to fault conditions.
OVP AND V_OUT
The Overvoltage Protection (OVP) pin has a function of setting the
overvoltage trip level as well as limiting the V_OUT regulation
range.
The ISL78171 OVP threshold is set by R_UPPER and R_LOWER such
that:
V_{OUT_OVP} = 1.22Vx^---^R----^U-----^P----^P--_R--^E---_LR---_O---+--_W---^R--_E--^L--_R-^O-----^W-----^E----^R-----^
(EQ. 14)
The output voltage V_OUT can regulate between 64% and 100% of
the V_{OUT_OVP} such that:
Allowable V_OUT = 64% to 100% of V_{OUT_OVP}
If R₁ and R₂ are chosen such that the OVP level is set at 40V,
then V_OUT is allowed to operate between 25.6V and 40V. If the
V_OUT requirement is changed to an application of six LEDs of 21V,
then the OVP level must be reduced. Users should follow the
V_OUT = (64% ~100%) OVP level requirement; otherwise, the
headroom control will be disturbed such that the channel voltage
can be much higher than expected. This can sometimes prevent
the driver from operating properly.
The resistances should be large, to minimize power loss. For
example, a 316kΩ R_UPPER and a 10kΩ R_LOWER sets OVP to 39.8V.
Large OVP resistors also allow C_OUT to discharge slowly during the
PWM Off time. Parallel capacitors should also be placed across
the OVP resistors such that R_UPPER/R_LOWER = C_LOWER/C_UPPER.
Using a C_UPPER value of 30pF is recommended. These capacitors
reduce the AC impedance of the OVP node, which is important
when using high-value resistors. For example, if
R_UPPER/R_LOWER = 33/1, then C_UPPER/C_LOWER = 1/33 with
C_UPPER = 100pF and C_LOWER = 3.3nF
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 is also switched off.
UNDERVOLTAGE LOCK-OUT
If the input voltage falls below the UVLO level, the device stops
switching and is reset. Operation restarts only when V_IN returns
to the normal operating range.
INPUT OVERCURRENT PROTECTION
During a normal switching operation, the current through the
internal boost power FET is monitored. If the current exceeds the
current limit, the internal switch is turned off. Monitoring occurs
on a cycle-by-cycle basis in a self-protecting way.
OVER-TEMPERATURE PROTECTION (OTP)
The ISL78171 includes two over-temperature thresholds. The lower
threshold is set to +130°C. When this threshold is reached, any
channel that is outputting current at a level significantly below the
regulation target is treated as “open circuit” and is disabled after a
time-out period. This time-out period is 800µs when it is above the
lower threshold. The lower threshold isolates and disables bad
channels 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 threshold is
reached, the boost stops switching, and the output current
sources switch off. Once the device has cooled to approximately
+100°C, the device restarts, with the DC LED current level
reduced to 75% of the initial setting. If dissipation persists,
subsequent hitting of the limit causes identical behavior, with the
current reduced in steps to 50% and finally 25%. Unless disabled
via the EN pin, the device stays in an active state throughout.
For complete details of fault protection conditions, see Figure 29
and Table 2.
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FN8602.0
June 15, 2015