AD8240 datasheet(7/12 Pages) AD

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AD8240

Rev. 0 | Page 7 of 12

CONTROLLING THE LED LAMP

The LEDs are turned on and off depending on the CMOS

compatible digital voltage level present at the PWM pin (Pin 3).

This voltage can be continuous for a simple on/off function, or

PWM for dimming control. The PWM frequency should be less

than 500 Hz with a range from 5% to 100%. Typical values are

5% for running and 95% for braking.

USING/EVALUATING THE VSENSE OUTPUT

Important: The output impedance of VSENSE is approximately

10 kΩ. Because of this, it may be necessary to buffer the output

in order to drive a load of less than 100 kΩ. An oscilloscope,

micro-controller A/D converter, or DVM may be used to

accurately measure the voltage at the VSENSE pin.

The VSENSE output is used to detect a partial LED failure, or an

overcurrent condition. The voltage present at VSENSE is propor-

tional to the current through the load with the equation

ILOAD = (VSENSE/24)/RSHUNT

Selection of the shunt resistor can be found by manipulating this

equation.

For example

VSENSE = ILOAD

× RSHUNT

Expected Load

= 500 mA

RSHUNT

= 0.4 Ω

To determine if the load is correct, the voltage at VSENSE should

be as follows during full power operation:

500 mA × 0.4 Ω × 24 V/V = 4.8 V

If there is a partial LED failure, VSENSE drops in proportion to the

quantity of the failure. For example, if 25% of the LEDs fail, the

voltage drops by 25%.

If there is a short to ground, VSENSE is near 0 V because the

output is latched off and no current is flowing.

Using/Evaluating the Short-Circuit Protection Feature

If there is a short or an overload condition, the voltage at

VSENSE falls close to zero, and the output shuts down

(the transistor driver shuts off). This resets when the PWM

voltage is brought low and then high again. If the condition

persists, the AD8240 attempts to drive the output to 12 V and

then immediately shuts down. If a PWM voltage is used, the

AD8240 attempts to start after each PWM cycle.

This can be simulated by increasing the load so the voltage at

VSENSE slightly exceeds 5 V. When this happens, the output shuts

down, and the VSENSE voltage is close to 0 V.

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