Preliminary Datasheet EL7801 (Intersil) - 7

EL7801
Figure 6. The load disconnect switch improves PWM The value of VFB should be limited to between 50mV and dynamic range, linearity and color temperature control. To 450mV for linear operation and is internally limited to 500mV. further improve the linearity of PWM dimming, an internal LEVEL voltages above 2.5V will have no effect on LED timer delays system shutdown via EN/PWM for 50ms. current. 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 ±10V to prevent voltage stress.
FIGURE 5. OPERATION WITH ENL 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 over-voltage and input over-voltage. In camera-flash mode, the fault switch disconnects the input supply when EN/PWM is held high longer than the 16ms flash time-out period. For applications not requiring input voltage or fault protection, connect VBAT and VIN directly together. All faults except input supply over-voltage latch the EL7801 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
FIGURE 6. OPERATION WITH NO ENL
protection circuit can be enabled by connection of the OVP
Bias Current Dimming
pin to the output voltage. The device will stop switching if the Current in the LED load is determined by the value of the output voltage exceeds VFAULTU and re-start when the feedback resistor and the target feedback regulation voltage: output voltage falls below VFAULTL. During sustained OVP fault conditions, VOUT will saw-tooth between the upper and V lower threshold voltages at a frequency determined by the I FB LED = ------------ R magnitude of current available to discharge the output SENSE capacitor and the value of output capacitor used. With MODE tied to VDC, voltage across the feedback The OVP threshold can be set to a lower value by using an resistor is set by V external zener diode and resistor, as shown in Figure 7. R1 LEVEL: should be adjusted to minimize offset in the FB voltage due VLEVEL to FB pin input current. A value of 100Ω is recommended. VFB = ----------- 5 7 FN7354.0 December 22, 2005