PR4101 datasheet(13/24 Pages) PREMA

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LED Driver PR4101

Application Notes

SELECTION OF THE INPUT CAPACITOR Cin

The input capacitor is necessary in case of AC

supply voltages to smooth the supply voltage. A

value between 100 µF and 4700 µF for normal

AC and of 470 µF up to 10000 µF for phase-cut

supply voltages is recommended.

In parallel a 100 nF capacitor should be placed

close to the IC supply pins.

Step-down regulators draw current from the

input supply in pulses with very fast rise and

fall times. The input capacitor is also required to

reduce the resulting voltage ripple at the PR4101

input and to force this switching current into a

tight local loop, minimizing EMI. The input

capacitor must have a low impedance at the

switching frequency to do this effectively, and it

should have an adequate ripple current rating.

SELECTION OF INDUCTOR AND CLED

Selection of the inductor value depends a lot on

the supply voltage, the number of connected

LEDs, but also on the allowed current ripple and

the desired efficiency. For a smaller LED current

higher values above 660 µH should be used. In

case that a minimized board size is desired

inductor values around 100 µH may be selected

but efficiency and LED current ripple are not

optimized in this case. The saturation current of

the inductors must be higher than the LED peak

current. A low DC resistance of the coil avoids

additional loss of efficiency.

A capacitor value of CLED between 22 µF and

1000 µF in parallel to the LED is recommended

to reduce the LED current ripple and avoid

exceeding the LED current rating.

SELECTION OF EXTERNAL MOSFET

The n-channel MOSFET must have a gate

threshold voltage of less than 3 V and a low ON

resistance. A recommended transistor is the

International Rectifier IRLL024N. To improve the

behaviour of the module, long lines between

the IC and the transistor should be avoided.

SELECTION OF THE EXTERNAL DIODE

A Schottky diode with fast recovery is needed

to reduce the voltage drop. The diode must be

able to carry the LED current flowing during the

OFF time of the driver. The reverse voltage of

the diode should be higher than the input

voltage.

DELAY START FEATURE (only PR4101A)

A delayed start is possible by connecting the

pin Delay to Gnd. Within the delay start period

the output current is switched off. After the

delay start period the output current rises to

100 % of the nominal current. The delay start

period is fixed and cannot be changed by

external components.

SELECTION OF RVSENSE AND RSENSE

The input VSENSE needs a series resistor

RVSENSE = 1 kΩ while the LED current is defined by

the selection of RSENSE. The nominal value of the

current sense resistor can be calculated with

the following formula:

SENSE=

SENSE

LED

The value of VSENSE can be found in

„Electrical Characteristics“.

For example: With an LED current of 1 A and

VSENSE =200 mV, RSENSE has a value of 200 mΩ.

The following table gives some resistor values

LED Current

RSENSE

350 mA

0.571 Ω

700 mA

0.286 Ω

1000 mA

0.200 Ω

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