ISL78100 Datasheet, PDF (12/19 Pages) Intersil Corporation – High Power LED Driver

ISL78100
.
FIGURE 31. OPERATION WITH ENL CONTROLLED FET
FIGURE 32. OPERATION WITH NO ENL CONTROLLED FET
Bias Current Dimming
Current in the LED load is determined by the value of the
feedback resistor and the target feedback regulation voltage
as shown in Equation 1:
I_LED
=
-------^V----^F----^B--------
R_SENSE
(EQ. 1)
With MODE tied to VDC, voltage across the feedback
resistor is set by V_LEVEL as shown in Equation 2:
V_FB
=
V_{-----L---E----V----E----L-}
5
(EQ. 2)
The value of V_FB should be limited to between 50mV and
450mV for linear operation. For minimum light output, V_FB
may be set below 50mV. With MODE tied to GND, voltage
across the feedback resistor is set at ~400mV via an internal
reference. In either operating mode, if LED temperature
control is enabled, the value of VFB will be reduced when
maximum LED temperature is exceeded.
Input Overvoltage
For automotive applications, an external high voltage NFET
driven by the FAULT pin disconnects the device from the
input supply in response to voltage spikes on the input
supply. During start-up, an internal charge pump drives the
FAULT pin above the input voltage, ensuring the NFET is
fully enhanced and powering up the device. In normal
operation, the switching node of the boost regulator or the
floating supply of the buck regulator is used to pump FAULT
above VIN. On detection of an overvoltage, the FAULT pin is
discharged to GND. The gate to source voltage of the
NDMOS is internally limited to ±15V to prevent voltage
stress.
Fault Protection
The external NFET is also used as a fault protection switch,
disconnecting the input supply if a fault occurs for more than
50ms. The system monitors feedback voltage regulation,
output overvoltage and input overvoltage. For applications
not requiring input voltage or fault protection, connect VBAT
and VIN directly together. All faults except input supply
overvoltage latch the ISL78100 into an off-state that can be
cleared by either power cycling the input supply or the
EN/PWM pin. Connecting the TMAX pin to VDC disables the
fault latch function (LED over-temperature control is also
disabled).
Output Overvoltage Protection (OVP)
If the FB pin is shorted to ground or an LED fails open circuit,
output voltage in BOOST mode can increase to potentially
damaging voltages. An optional overvoltage protection
circuit can be enabled by connection of the OVP pin to the
output voltage. The device will stop switching if the output
voltage exceeds OVPH and re-start when the output voltage
falls below OVPL. During sustained OVP fault conditions,
V_OUT will saw-tooth between the upper and lower threshold
voltages at a frequency determined by the magnitude of
current available to discharge the output capacitor and the
value of output capacitor used.
The OVP threshold can be set to a lower value by using an
external zener diode and resistor, as shown in Figure 33. R₁
should be adjusted to minimize offset in the FB voltage due
to FB pin input current. A value of 100Ω is recommended.
12
FN6626.0
December 17, 2007