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ISL97801 データシート(PDF) 12 Page - Renesas Technology Corp |
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ISL97801 データシート(HTML) 12 Page - Renesas Technology Corp |
12 / 20 page ISL97801 FN6428 Rev 1.00 Page 12 of 20 April 2, 2007 PWM dimming, an internal timer delays system shutdown via EN/PWM for 50ms. Bias Current Dimming Current in the LED load is determined by the value of the feedback resistor and the target feedback regulation voltage: With MODE tied to VDC, voltage across the feedback resistor is set by VLEVEL: The value of VFB should be limited to between 50mV and 450mV for linear operation. For minimum light output, VFB 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 ISL97801 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, VOUT 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 100 is recommended. FIGURE 31. OPERATION WITH ENL CONTROLLED FET FIGURE 32. OPERATION WITH NO ENL CONTROLLED FET ILED VFB RSENSE ----------------------- = (EQ. 1) VFB VLEVEL 5 ---------------------- = (EQ. 2) |
同様の部品番号 - ISL97801 |
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同様の説明 - ISL97801 |
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