STEVAL-ILL008V1 STMicroelectronics, STEVAL-ILL008V1 Datasheet - Page 8

• 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

AN1941 APPLICATION NOTE
Figure 11.
3.3
The input ranges from 20 volts to 55 volts. The switching frequency is set by
where R
The minimum voltage for the L4971 is 8 volts but the regulator-reference U3 needs a minimum
of 20 volts to stay in regulation. A higher breakdown voltage regulator can be used to achieve
a wider range of input. U3 provides power to the LM393 and a reference for the comparator
input. This voltage is compared to the voltage drop across Rs to maintain it at the same voltage
set by the potentiometer R8. The voltage drop across the resistor is proportional to the current
following through it by:
The output of the LM393 turns on and off to adjust the voltage at the slow start pin. The slow
start voltage is directly related to the output regulation thus achieving a constant current out-
put. The L4971 regulates by adjusting the duty cycle to maintain a constant output. R9 sets
the gain of the loop by controlling the discharge rate. L1 and C8 form the output filter to smooth
out the current. The inductor required is calculated at the worse case which is max input line
and minimum LEDS. This gives the minimum duty cycle and maximum time that the inductor
has to supply current to the load.
8/15
I
o
is the current ripple set by the application, usually 10% of the max current.
Circuit description:
osc
and C
D
max
osc
=
represent R2 and C2.
-----------------------------
V
in min
V
o
+
V
+
F
f
V
SW
f
=
D
Iout = V(U2Apin3)/Rs.
min
------------------------------------------------------------------------------ -
R
osc
=
-----------------------------
V
C
in min
osc
V
o
+
ln
V
+
 
 
1
6
-- -
5
f
V
f
+
100 C
L
o
=
os c
V
o
+
V
f
-------------------------- -
1 D
I
–
o
f
min
sw