ST AN2827 Application note
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 1/30
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Contents AN2827
Contents
1 Schematic description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2 Selection of external components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1 Input and output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3 LED selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.4 NTC and RX resistor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3 PCB design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1 PCB design rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2 PCB layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2.1 Four-layer PCB with 33.54 mm² application area using
VLF4014AT-1R0N2R2 coil 8
3.2.2 Four-layer PCB with 23.9 mm² application area using
VLS252012T-1R0N1R7 coil 11
4 Internal registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5 Operation modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.1 SHUTDOWN mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.2 SHUTDOWN mode with NTC feature activated . . . . . . . . . . . . . . . . . . . . 15
5.3 READY mode NTC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.4 Torch mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.5 Flash mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6 STATUS register and ATN pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
7 Reading and writing to the STCF05 registers through the I²C bus . . . 20
7.1 Writing to a single register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.2 Writing to multiple registers with incremental addressing . . . . . . . . . . . . 20
7.3 Reading from a single register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.4 Reading from multiple registers with incremental addressing . . . . . . . . . 22
8 Examples of register setup for each mode . . . . . . . . . . . . . . . . . . . . . . 23
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AN2827 Contents
8.1 Example 1: 400 mA flash with 700 ms duration . . . . . . . . . . . . . . . . . . . . 23
8.2 Example 2: 15 mA torch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
8.3 Example 3: auxiliary LED running at 10 mA for 500 ms . . . . . . . . . . . . . . 25
8.4 Example 4: red-eye reduction (multiple short flashes) . . . . . . . . . . . . . . . 25
8.5 Example 5: flash pulse longer than 1.5 s . . . . . . . . . . . . . . . . . . . . . . . . . 27
9 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Doc ID 15047 Rev 1 3/30
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 VI, GND, SDA or SCL to choose one of the four different I²C slave addresses.
***: Optional components to support auxiliary functions.
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AN2827 Selection of external components
2 Selection of external components
2.1 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 Ta bl e 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
the LED [Ω; Ω, 1].
For nominal operation, the peak inductor current can be calculated by the formula:
= RD / 10 [Ω; Ω, 1], where RD is the dynamic resistance of
DCL
I
PEAK
= [(I
OUT
/ η) + (V
- VIN) x V
OUT
) / (2 x L x F x V
IN²
Where:
I
Peak inductor current
PEAK
I
Current sourced at the V
OUT
OUT
-pin
η Efficiency of the STCF05
V
Output voltage at the V
OUT
V
Input voltage at the V
IN
BAT
OUT
-pin
-pin
L Inductance value of the inductor
F Switching frequency
Note: See recommended components in Ta bl e 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 Ta bl e 1 .
OUT²
)] x V
OUT
/ V
IN
2.4 NTC and RX resistor selection
Optionally, the STCF05 uses a negative thermistor (NTC) to sense the LED temperature, as
well as an R
resistor and an external voltage reference in order to use the NTC feature.
X
Refer to Figure 3: Typical application schematic for more details.
Doc ID 15047 Rev 1 5/30
Selection of external components AN2827
Once the NTC feature is activated through the I²C, the internal switch connects the RX
resistor to the NTC; this creates a voltage divider supplied by the external reference voltage
connected to the NTC.
If the temperature of the NTC thermistor rises as a result of the heat dissipated by the LED,
the voltage on the NTC pin increases. When this voltage exceeds 0.56 V, the NTC_W bit in
the STATUS register is set to High, and the ATN pin is set to Low to inform the
microcontroller that the LED is becoming hot. The NTC_W bit is cleared by reading the
STATUS register.
If the voltage on the NTC pin rises further and exceeds 1.2 V, the NTC_H bit in the STATUS
register is set to High, and the ATN pin is set to Low to inform the microcontroller that the
LED is too hot, and the device switches automatically to READY mode to avoid damaging
the LED. This status is latched until the microcontroller reads the STATUS register. Reading
the STATUS register clears the NTC_H bit.
The selection of the NTC and R
resistor values strongly depends on the power dissipated
X
by the LED and all the components surrounding the NTC thermistor, and on the cooling
capabilities of each specific application. The R
and NTC values in Tab le 1 work well for the
X
demonstration board presented in this application note. A real-life application may require a
different type of NTC thermistor to achieve optimal thermal protection.
The procedure to activate the NTC feature is described in Section 5.2: SHUTDOWN mode
with NTC feature activated.
Table 1. List of components
Component Manufacturer Part number Value Size
1. Used in v2 version to achieve best efficiency.
2. Used in v3 version, when low application area down to 23.9 mm² is preferred.
C
I
C
O
LTDK
TDK C1608X5R0J106M 10 µF 0603
TDK C1608X5R1A105M 1 µF 0603
VLF4014AT-1R0N2R2
VLS252012T-1R0N1R7
(1)
(2)
1 µH 3.7 x 3.5 x 1.2 mm
1 µH 2.5 x 2 x 1.2 mm
NTC Murata NCP21WF104J03RA 100 kΩ 0805
R
X
Rohm MCR01MZPJ15K 15 kΩ 0402
LED Luxeon LED 2x LXCL-PWF1 0805
AUXLED Rohm SML-210VT 0805
6/30 Doc ID 15047 Rev 1
AN2827 PCB design
3 PCB design
3.1 PCB design rules
The STCF05 is a powerful switching device that operates with low input voltages and a high
duty cycle. The PCB must be designed in line with switched mode power supply design
rules. The power tracks (or wires on the demonstration board) must be as short as possible
and wide enough, because of the large currents involved. It is recommended to use a
4-layer PCB to get the best performance. All external components must be placed as close
as possible to STCF05. All high-energy switched loops should be as small as possible to
reduce EMI.
Most of the LEDs need to be cooled efficiently. This can be achieved by using a dedicated
copper area on the PCB. Refer to the selected LED's reference guide to design the proper
heatsink. In case a modification of any PCB layer should be required, it is highly
recommended to use enough vias. Place the NTC resistor as close as possible to the LED
for good temperature sensing. A direct connection between GND and PGND is necessary to
achieve a correct output current value. No LED current should flow through this track. Vias
connecting the STCF05 pins to the copper tracks (if used) must be 0.1 mm in diameter. It is
recommended to use the filled vias.
It is possible to route the STCF05 device with a total PCB area of 23.9 mm² using the TDK
inductor VLS252012 1 µH value. When using the VLF4014AT, the application area is
increased by 9.6 mm², and the efficiency of the application is improved up to 85%.
Doc ID 15047 Rev 1 7/30
PCB design AN2827
3.2 PCB layout
3.2.1 Four-layer PCB with 33.54 mm² application area using
VLF4014AT-1R0N2R2 coil
Figure 4. Top layer
Figure 5. Middle layer 1
8/30 Doc ID 15047 Rev 1
AN2827 PCB design
Figure 6. Middle layer 2
Figure 7. Bottom layer
Doc ID 15047 Rev 1 9/30
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