STEVAL-ILL008V1 STMicroelectronics, STEVAL-ILL008V1 Datasheet - Page 3

• Manufacturer Part Number: STEVAL-ILL008V1
• Description: EVAL BOARD CREE LED FLASHLIGHT
• Manufacturer: STMicroelectronics

Specifications of STEVAL-ILL008V1

• Mfg Application Notes: Low Voltage LED Driver AppNote
• Design Resources: STEVAL-ILL008V1 Gerber Files STEVAL-ILL008V1 Schematic STEVAL-ILL008V1 Bill of Materials
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 3.5V
• Features: 1V Start up Input Voltage
• Voltage - Input: 1 ~ 5.5 V
• Utilized Ic / Part: L6920
• Product: Display Modules
• Core Chip: L6920DA
• Output Voltage: 3.5V
• Kit Contents: L6920D, L4971, L6902D Reference Board
• Development Tool Type: Hardware - Eval/Demo Board
• Tool / Board Applications: LED Flashlight
• Mcu Supported Families: L6920DA
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output / Channel: -
• Lead Free Status / Rohs Status: Details
• Other names: 497-5516

2.2 Circuit Description
The circuit shown in figure 3 is a constant current control to provide constant luminosity from
the LED. A current sensing resistor is in series with the white LED is used to provide the current
feedback. The feedback reference voltage for the controller is 1.23V. If this voltage level is di-
rectly feedback from the current sensing resistor, the loss in the resistor will be too high. The
circuit uses a low value sense resistor, R1 to reduce the dissipation and an op-amp to amplify
the current sense voltage back up to the required 1.23V level.
Figure 3. Schematic of L6920D LED driver
From the circuit, the control rule is: I
where I
the OP AMP, and Vref is the reference voltage.
Therefore, the LED current will be
In the reference circuit, there are two gains. When J7 and J8 are shorted, K1=1+R3/R4. When
J7 and J8 are open, K2=1+(R3+R2)/R4.
In the circuit, R1 = 0.33 ; R2 = 100 k ; R3 = 12 k ; R4 = 1 k . the current level of the LED
can be I
Following are some typical waveforms at Vin=2.5 V.
J6
CON1
1
J2
LED
LED1
+
CON1
47uF
C2
6.3V
is the current through the LED; R1 is the current sensing resistor, K is the gain of
= 280mA or I
L1
10uH
C4
.1uF
7
5
2
4
LED2
LX
SHDN
LBI
REF
U1
L6920
= 32 mA.
GND
OUT
LBO
FB
I
LED
LED
8
1
3
6
·R1·K = Vref
=
--------------- -
R1 K
V
REF
C3
.47uF
J7
CON1
.01uF
1
C5
+
R2
100K
1/8W
47uF
C1
6.3V
OPAMP
U2 TS951ILT
OUT
J8
CON1
2
+
AN1941 APPLICATION NOTE
-
1
R3
12K
1/8W
2
.33 Ohm
R1
1/4W
1K
1/8W
R4
1
1
J4
CON1
J5
CON1
D1
LED
3/15