ISL97801BOOSTEVALZ【PDF 12/19 页】EVAL BOARD FOR ISL97801

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ISL97801

further improve the linearity of PWM dimming, an internal

timer delays system shutdown via EN/PWM for 50ms.

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:

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 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

I LED = -----------------------

V FB

R SENSE

(EQ. 1)

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

With MODE tied to VDC, voltage across the feedback

resistor is set by V LEVEL :

capacitor and the value of output capacitor used.

The OVP threshold can be set to a lower value by using an

V FB = ----------------------

V LEVEL

5

12

(EQ. 2)

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 Ω is recommended.

FN6428.1

April 2, 2007

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