TS19376

1A Step-down High Brightness LED Driver

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Version: A12

Application Information (Continue)

Shutdown mode

Taking the DIM pin to a voltage below 0.3V will turn off the output and the supply current will fall to a low standby

level of 70µA nominal.

Capacitor selection

A low ESR capacitor should be used for input decoupling, as the ESR of this capacitor appears in series with the

supply source impedance and lowers overall efficiency. This capacitor has to supply the relatively high peak current

to the coil and smooth the current ripple on the input supply. A minimum value of 4.7uF is acceptable if the input

source is close to the device, but higher values will improve performance at lower input voltages, especially when

the source impedance is high. The input capacitor should be placed as close as possible to the IC.

For maximum stability over temperature and voltage, capacitors with X7R, X5R, or better dielectric are

recommended. Capacitors with Y5V dielectric are not suitable for decoupling in this application and should not be

used.

Inductor selection

Recommended inductor values for the TS19376 are in the range 47uH to 120uH.

Higher values of inductance are recommended at lower output current in order to minimize errors due to switching

delays, which result in increased ripple and lower efficiency. Higher values of inductance also result in a smaller

change in output current over the supply voltage range. The inductor should be mounted as close to the device as

possible with low resistance connections to the SW and VIN pins.

Diode selection

For maximum efficiency and performance, the rectifier (D1) should be a fast low capacitance Schottky diode with

low reverse leakage at the maximum operating voltage and temperature. They also provide better efficiency than

silicon diodes, due to a combination of lower forward voltage and reduced recovery time.

It is important to select parts with a peak current rating above the peak coil current and a continuous current rating

higher than the maximum output load current. It is very important to consider the reverse leakage of the diode

when operating above 85°C. Excess leakage will incr ease the power dissipation in the device and if close to the

load may create a thermal runaway condition.

The higher forward voltage and overshoot due to reverse recovery time in silicon diodes will increase the peak

voltage on the SW output. If a silicon diode is used, care should be taken to ensure that the total voltage appearing

on the SW pin including supply ripple, does not exceed the specified maximum value.

Reducing output ripple

Peak to peak ripple current in the LED(s) can be reduced, if required, by shunting a capacitor CLED across the

LED(s) as shown below:

A value of 1uF will reduce the supply ripple current by a factor three (approx.). Proportionally lower ripple can be

achieved with higher capacitor values. Note that the capacitor will not affect operating frequency or efficiency, but it

will increase start-up delay and reduce the frequency of dimming, by reducing the rate of rise of LED voltage.

By adding this capacitor the current waveform through the LED(s) changes from a triangular ramp to a more

sinusoidal version without altering the mean current value.