AN2129 APPLICATION NOTE

4/9

Figure 6. PCB layout

Table 1. Bill of materials

The calculation of the resistor current feedback network can look relatively complicated, but

with few simplifications it becomes easy to take in. First assumption is that all the current flows

fortunately this calculation will give uncommon values (e.g. for 350mA it gives 0.2857

Ω) thus

the nearest higher standard (e.g. E24 series) value for Rsense should be selected (e.g. 0.33

Ω)

and then the difference between ideal and standard value is compensated by R4, R5 and R6

to receive precise output current.

The application is shifting between two limit states with dimming; maximum current (zero dim-

ming voltage) and zero current (full dimming voltage). In Figure 7, the dimming network with

grounded dimming input (Equation 1 describes the circuit) is shown, it means when the current

flowing through the LED is on its maximum (i.e. 350mA on this board).

Type

Reference

Part

Supplier Order Code

Ceramic Capacitor

C1

10uF

N/A

Tantal Capacitor

C2

10µF; 35V

N/A

Capacitor SMD 0805

C3

22nF

N/A

Capacitor SMD 0805

C4

220pF

N/A

Schotky Diode

D1

STPS340U

STMicroelectronics

Connector

J1

8 -24V

N/A

Connector

J2

Output

N/A

Connector

J3

3.3V Vref

N/A

Connector

J4

Dimming Input

N/A

Connector

J6

GND

N/A

Coil

L1

100µH; 1.2A; 0.33

Ω

Würth Elektronik 744 562 0

Resistor SMD 2010

Rsense

0.33

N/A

Resistor SMD 0805

R1

9k1

N/A

Resistor SMD 0805

R2

5k1

N/A

Resistor SMD 0805

R3

510

N/A

Resistor SMD 0805

R4

1k

N/A

Resistor SMD 0805

R5

8k2

N/A

Resistor SMD 0805

R6

27k

N/A

Converter

U1

L6902D

STMicroelectronics