EC4304

1A, Hysteretic, High Brightness

LED Driver with Internal Switch

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4B25N-Rev.F005

Functional Description

The EC4304 is a simple high-efficiency, continuous mode inductive step-down converter. The device operates with an

input voltage range from 7V to 40V and delivers output current 1A. A high- side current-sense resistor sets the output

current and a dedicated PWM dimming input enables pulsed LED dimming over a wide range of brightness levels. A high-

side current-sensing scheme and an on-board current- setting circuitry minimize the number of external components which

is required while delivering LED current with +5% accuracy, using a 1% sense resistor.

Adjusting Output Current

The device contains a low pass filter between the ADJ pin and the threshold comparator and an internal current limiting

resistor between ADJ and the internal reference voltage. This allows the ADJ pin to be overdriven with either DC or pulse

signals to change the VSENSE switching threshold and adjust the output current. Details of the different modes of

adjusting output current are given in the applications section by:

The value of the output current is 1A at 0.1

(0.5A at 0.2 ) and this is a calculated output current when the ADJ terminal is

1.25V floating.

Shutdown Mode

Taking the ADJ pin to a voltage below 0.2V for more than approximately 100µs will turn off the output, and supply current

will fall to a low standby level of 20µA nominal.

Soft-Start

The device has inbuilt soft-start action due to the delay through the PWM filter. An external capacitor from the ADJ pin to

ground will provide additional soft-start delay, by increasing the time taken for the voltage on this pin to rise to the turn-on

threshold and by slowing down the rate of rise of the control voltage at the input of the comparator.

With no external capacitor, the time taken for the output to reach 90% of its final value is approximately 500µs. Adding

capacitance increases this delay by approximately 0.5ms/ nF.

Inherent open-circuit LED protection

If the connection to the LED(s) is open-circuited, the coil is isolated from the LX pin of the chip, so the chip will not be

damaged. Unlike in many boost converters, where the back EMF may damage the internal switch by forcing the drain

above its breakdown voltage.