PT4205 datasheet(11/18 Pages) POWTECH

11 / 18 page

30V, 1.2A Step-down HB LED Driver

China Resources Powtech (Shanghai) Limited

WWW.CRPOWTECH.COM

Page 11

PT4205_DS Rev EN_1.2

PT4205

APPLICATION NOTES

Setting nominal average output current with

external resistor RS

The nominal average output current is determined by

the value of the external current sense resistor (RS)

connected between VIN and CSN and is given by:

OUT



)

(





This equation is valid when DIM pin is float or applied

with a voltage higher than 2.5V (must be less than 5V).

Actually, RS sets the maximum average current which

can be adjusted to a less one by dimming.

Output current adjustment by external DC control

voltage

The DIM pin can be driven by an external dc voltage

(VDIM), as shown, to adjust the output current to a

value below the nominal average value defined by RS.

PT4205

VIN

CSN

DIM

GND

68uH

0.28Ω

LED

VIN

The average output current is given by:

DIM

OUT





0935

)

(

DIM 



071







DIM

OUT

)

(

DIM 



Note that 100% brightness setting corresponds to:

)

(

DIM 



Output current adjustment by PWM control

A Pulse Width Modulated (PWM) signal with duty

cycle PWM can be applied to the DIM pin, as shown

below, to adjust the output current to a value below the

nominal average value set by resistor RS:

OUT





)

100

(

pulse 





pulse

OUT





)

100

(

pulse 





PT4205

VIN

CSN

DIM

GND

68uH

0.28Ω

LED

VIN

PWM

dimming

provides

reduced

brightness

modulating the LED’s forward current between 0% and

100%. The LED brightness is controlled by adjusting

the relative ratios of the on time to the off time. A 25%

brightness level is achieved by turning the LED on at

full current for 25% of one cycle. To ensure this

switching process between on and off state is invisible

by human eyes, the switching frequency must be

greater than 100 Hz. Above 100 Hz, the human eyes

average the on and off times, seeing only an effective

brightness that is proportional to the LED’s on-time

duty cycle. The advantage of PWM dimming is that the

forward current is always constant, therefore the LED

color does not vary with brightness as it does with

analog dimming. Pulsing the current provides precise

brightness control while preserving the color purity.

The dimming frequency of PT4205 can be as high as 20

kHz.

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 130μA nominal.

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