AN2827
Application note
Driver for double flash LED with I²C interface
Introduction
This application note is dedicated to the design of a flash LED driver using the STCF05
device, which is a boost current mode converter with an I²C interface and internal current
source. The schematic, functional description, recommendations for PCB layout and
external components selection are also discussed in this application note. The STCF05
device is designed to drive two LEDs in series with a total forward voltage from 5.3 V to
10.2 V.
Figure 1.
Demonstration board STCF05 v3: optimized for smallest PCB area
(24 mm²)
Figure 2.
Demonstration board STCF05 v2: optimized for best efficiency
June 2009
Doc ID 15047 Rev 1
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Contents
AN2827
Contents
1
2
Schematic description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Selection of external components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1
2.2
2.3
2.4
Input and output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
LED selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
NTC and RX resistor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3
PCB design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1
3.2
PCB design rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
PCB layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2.1
3.2.2
Four-layer PCB with 33.54 mm² application area using
VLF4014AT-1R0N2R2 coil 8
Four-layer PCB with 23.9 mm² application area using
VLS252012T-1R0N1R7 coil 11
4
5
Internal registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Operation modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.1
5.2
5.3
5.4
5.5
SHUTDOWN mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
SHUTDOWN mode with NTC feature activated . . . . . . . . . . . . . . . . . . . . 15
READY mode NTC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Torch mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Flash mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6
7
STATUS register and ATN pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Reading and writing to the STCF05 registers through the I²C bus . . . 20
7.1
7.2
7.3
7.4
Writing to a single register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Writing to multiple registers with incremental addressing . . . . . . . . . . . . 20
Reading from a single register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Reading from multiple registers with incremental addressing . . . . . . . . . 22
8
Examples of register setup for each mode . . . . . . . . . . . . . . . . . . . . . . 23
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Contents
8.1
8.2
8.3
8.4
8.5
Example 1: 400 mA flash with 700 ms duration . . . . . . . . . . . . . . . . . . . . 23
Example 2: 15 mA torch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Example 3: auxiliary LED running at 10 mA for 500 ms . . . . . . . . . . . . . . 25
Example 4: red-eye reduction (multiple short flashes) . . . . . . . . . . . . . . . 25
Example 5: flash pulse longer than 1.5 s . . . . . . . . . . . . . . . . . . . . . . . . . 27
9
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
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Schematic description
AN2827
1
Schematic description
The STCF05 flash LED driver has a high operational frequency (1.8 MHz) which allows the
use of small-sized external components.
Figure 3.
Typical application schematic
**: Connect to V
I
, GND, SDA or SCL to choose one of the four different I²C slave addresses.
***: Optional components to support auxiliary functions.
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Selection of external components
2
2.1
Selection of external components
Input and output capacitor selection
It is recommended to use ceramic capacitors with low ESR as input and output capacitors. It
is also recommended to use 10 µF/6.3 V as a minimum value for the input capacitor, and
1 µF/16 V as the optimal value for the output capacitor to achieve a good stability of the
device, for a supply range varying from a low input voltage (2.5 V) to the maximum ratings of
output power.
Note:
See recommended components in
Table 1.
2.2
Inductor selection
A thin shielded inductor with a low DC series resistance of winding is recommended for this
application. To achieve a good efficiency in step-up mode, we recommend using an inductor
with a DC series resistance R
DCL
= R
D
/ 10 [Ω;
Ω,
1], where R
D
is the dynamic resistance of
the LED [Ω;
Ω,
1].
For nominal operation, the peak inductor current can be calculated by the formula:
I
PEAK
= [(I
OUT
/
η
) + (V
OUT
- V
IN
) x V
IN²
) / (2 x L x F x V
OUT²
)] x V
OUT
/ V
IN
Where:
I
PEAK
Peak inductor current
I
OUT
Current sourced at the V
OUT
-pin
Efficiency of the STCF05
Input voltage at the V
BAT
-pin
Inductance value of the inductor
Switching frequency
η
V
OUT
Output voltage at the V
OUT
-pin
V
IN
L
F
Note:
See recommended components in
Table 1.
2.3
LED selection
Any string of LEDs with a cumulative forward voltage ranging from 5.3 to 10.2 V is
compatible with the STCF05. The total LED spread must be taken into account when
calculating the minimum and maximum voltage of the LEDs that must be inside the
5.3 –10.2 voltage range. It is possible to set the level of the LED current to flash mode and
torch mode by setting the corresponding registers through the I²C interface.
Note:
See recommended components in
Table 1.
2.4
NTC and R
X
resistor selection
Optionally, the STCF05 uses a negative thermistor (NTC) to sense the LED temperature, as
well as an R
X
resistor and an external voltage reference in order to use the NTC feature.
Refer to
Figure 3: Typical application schematic
for more details.
Doc ID 15047 Rev 1
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