cd00152097

AN2507
Application note
High-Power Camera Flash LED Driver with I2C™
Introduction
This application note explains the design of a FLASH LED driver using the STCF03 device,
which is a Buck-Boost current mode converter with an I2C interface. The schematic,
functional description, recommendations for PCB Layout and external components
selection are also discussed in this application note. This device is designed for driving a
single LED with a forward voltage range from 2.7 to 5 V. A detailed functional description
can be found below.
Package and demo board top view
Version B - Version C
April 2007
Version A
Rev 1
1/31
www.st.com
Contents
AN2507
Contents
1
Schematic description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1
2
3
4
Selection of external components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1
Input and output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2
Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3
LED selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.4
RFL selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.5
RTR selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.6
NTC AND RX resistor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
PCB design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1
PCB design rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.2
PCB layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6
2/31
A four-layer PCB with application area 45.1 mm2 for BGA package,
version B 10
3.2.2
A two-layer PCB with application area 72.4 mm2 for QFN package . . . 13
3.2.3
A four-layer PCB with application area 45.1 mm2 for BGA package,
version C 14
Accessing the internal registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Operation modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.1
SHUTDOWN mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.2
SHUTDOWN mode with the NTC-feature activated . . . . . . . . . . . . . . . . . 17
5.3
READY mode and NTC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.4
TORCH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.5
FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
The STATUS register and the ATN pin . . . . . . . . . . . . . . . . . . . . . . . . . . 21
6.1
7
3.2.1
Internal registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.1
5
Application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
The STATUS register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Reading and writing to the STCF03 registers through the I2C bus . . 22
AN2507
8
9
Contents
7.1
Writing to a single register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.2
Writing to multiple registers with incremental addressing . . . . . . . . . . . . 22
7.3
Reading from a single register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.4
Reading from multiple registers with incremental addressing . . . . . . . . . 23
Examples of register setup for each mode . . . . . . . . . . . . . . . . . . . . . . 24
8.1
Example 1: 600 mA FLASH with 700 ms duration . . . . . . . . . . . . . . . . . . 24
8.2
Example 2: 25 mA TORCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
8.3
Example 3: an Auxiliary LED running at 10 mA for 500 ms . . . . . . . . . . . 26
8.4
Example 4: Red-eye reduction (multiple short flashes) . . . . . . . . . . . . . . 27
8.5
Example 5: A FLASH pulse longer than 1.5 s . . . . . . . . . . . . . . . . . . . . . 28
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3/31
List of tables
AN2507
List of tables
Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Table 6.
Table 7.
Table 8.
Table 9.
Table 10.
Table 11.
Table 12.
Table 13.
Table 14.
Table 15.
Table 16.
Table 17.
Table 18.
Table 19.
Table 20.
Table 21.
Table 22.
Table 23.
Table 24.
Table 25.
Table 26.
Table 27.
Table 28.
Table 29.
Table 30.
Table 31.
4/31
Recommended components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Accessibility of internal registers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
COMMAND register data to enter SHUTDOWN mode (version B) . . . . . . . . . . . . . . . . . . 17
COMMAND register data to enter SHUTDOWN mode (version A and C) . . . . . . . . . . . . . 17
COMMAND register data to enter SHUTDOWN mode with NTC activated (version A and C)
18
COMMAND register data to enter READY mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
COMMAND register data to enter READY mode with NTC ON . . . . . . . . . . . . . . . . . . . . . 18
COMMAND register data to enter TORCH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
COMMAND register data to enter FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
STATUS register bits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Effect of the STATUS register bits on the operation of the device . . . . . . . . . . . . . . . . . . . 21
TORCH mode and FLASH mode dimming registers settings. . . . . . . . . . . . . . . . . . . . . . . 24
COMMAND register data to enter FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
DIMMING register data for the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
I2C data packet for activating the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
COMMAND register data for the TORCH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
DIMMING register data for the TORCH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
I2C data packet to activate TORCH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
I2C data packet for terminating the TORCH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
COMMAND register data for the AUX_LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
COMMAND register data for the AUX_LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
I2C data packet for activating the READY mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
I2C data packet for activating the AUX_LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
COMMAND register data for FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
DIMMING register data for the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
I2C data packet for activating the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
I2C data packet for activating the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
1st I2C data packet to restart the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2nd I2C data packet for restart of the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3rd I2C data packet to restart the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
AN2507
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
A typical application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
NTC connection for versions with internal voltage reference . . . . . . . . . . . . . . . . . . . . . . . . 9
Top layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Middle layer 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Middle layer 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Top overlay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Top layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Top overlay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Top layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Middle layer 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Middle layer 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Top overlay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Splitting the FLASH pulse into several shorter pulses . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Writing to a single register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Writing to multiple registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Reading from a single register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Reading from multiple registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Multiple flashes handled by the TRIG pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
I2C bus packets timing for a FLASH lasting longer than FTIM max . . . . . . . . . . . . . . . . . . 30
5/31
Schematic description
1
AN2507
Schematic description
The FLASH LED driver STCF03 has a high operational frequency (1.8 MHz) which allows
the usage of small-sized external components. The three versions (A, B and C) differ in the
way the NTC feature is supported.
1.1
Application schematic
Figure 1.
A typical application schematic
Note:
** Connect to VI, or GND or SDA or SCL to choose one of the 4 different I2C Slave
Addresses
Note:
*** Optional components to support auxiliary functions
6/31
●
Version A: STCF03PNR - QFN package, external reference for NTC protection
●
Version B: STCF03ITBR - BGA package, internal reference for NTC protection
●
Version C: STCF03TBR - BGA package, external reference for NTC protection
AN2507
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 recommended to use 10 µF/6.3 V as a minimum value of input capacitor and 1 µF/ 6.3 V
as an optimal value of output capacitor to achieve good stability of the device supplied from
low input voltage (2.7 V) at 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, it is recommended to use an
inductor with a DC series resistance RDCL = RD/10 [Ω, Ω, 1], where RD is the dynamic
resistance of the LED [Ω, Ω, 1].
For nominal operation, the peak inductor current can be calculated by the following formula:
Equation 1
2
⎛ I OUT
⎛ ( V OUT – V IN ) • V IN ⎞ ⎞ V OUT
-⎟ ⎟ • -------------I PEAK = ⎜ ⎛ ------------⎞ + ⎜ ---------------------------------------------------⎝
⎠
V IN
⎝ n
⎝ 2 • L • F • V OUT 2 ⎠ ⎠
Where:
●
IPEAK: Peak inductor current
●
IOUT: Current sourced at the VOUT-pin
●
n: Efficiency of the STCF03
●
VOUT: Output voltage at the VOUT-pin
●
VIN: Input voltage at the VBAT-pin
●
L: Inductance value of the inductor
●
F: Switching frequency
Note:
see recommended components in Table 1.
2.3
LED selection
All LEDs with a forward voltage range ranging from 2.7 V to 5 V are compatible with
STCF03. The forward voltage spread of any selected LED must however lay within this
range (2.7 V to 5 V). It is possible to set the level of the LED current in FLASH mode and
TORCH mode by setting the values of the corresponding sensing resistors. The level of the
LED current in FLASH mode can be set by changing the external FLASH resistor.
Note:
see recommended components in Table 1.
7/31
Selection of external components
2.4
AN2507
RFL selection
The value of the RFL resistor can be calculated by the following equations:
RFL = VFB2/IFLASH where VFB2 = 226 mV and PRFLASH = RFL * IFLASH2, where PRFL is the
power dissipated on the RFL resistor. It is recommended to use a thin metal film resistor with
0805 package size and 1% tolerance. The maximum LED current in FLASH mode for
STCF03 is (800 mA) for a battery voltage ranging from 2.7 V to 5.5 V in VQFPN version.
2.5
RTR selection
The value of the RTR resistor can be calculated by following equations:
Equation 2
V REF – I TORCH • R FL
R TR = ⎛ --------------------------------------------------------⎞
⎝
⎠
I TORCH
and
P RTORCH = R TR • I TORCH
2
where PRTORCH is the power dissipated on the RTR resistor. It is recommended to follow the
equation RTR = 6.66* RFL to avoid any jump in the current dimming values.
It is recommended to use a thin metal film resistor with 1% or 5% tolerance. The maximum
LED current in TORCH mode for SCTF03 is 200 mA for a battery voltage ranging from 2.7 V
to 5.5 V
2.6
NTC AND RX resistor selection
a)
A, C versions without an internal reference voltage for the NTC feature.
STCF03 requires a negative thermistor (NTC) for sensing the LED temperature, as well as
an RX resistor and an external voltage reference in order to use the NTC feature. Please
refer to the typical application schematic in Figure 1 VER A,C for more details.
Once the NTC feature is activated, the internal switch connects the RX resistor to the NTC,
and this creates a voltage divider supplied by the external reference voltage connected to
the NTC.
If the temperature of the NTC-thermistor rises due to 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 goes automatically to the 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 RX resistors values strongly depends on the power dissipated
by the LED and all components surrounding the NTC-thermistor and on the cooling
capabilities of each specific application. The RX and the NTC values in Table 1 below work
well in the demo board presented in this application note. A real application may require a
different type of NTC-thermistor to achieve optimal thermal protection.
8/31
AN2507
Selection of external components
The procedure to activate the NTC-feature is described in Section 5.2.
b)
Versions with an internal reference voltage for the NTC. This version requires a
different connection between the RX and NTC resistors. See Figure 2 below or
Figure 1 version B.
Figure 2.
NTC connection for versions with internal voltage reference
NTC
STCF03
NTC thermistor
To optional A/D converter
RX
RX resistor
Note:
Versions with internal reference voltage do not support the SHUTDOWN+NTC mode,
because the internal reference voltage is off in SHUTDOWN mode.
Table 1.
Recommended components
Component
Manufacturer
Part number
Value
Size
CI
TDK
C1608X5R0J106M
10 µF
0603
CO
TDK
C1608X5R0J105M
1 µF
0603
L
TDK
VLF4012AT-4R7M1R1
4.7 µH
3.7 x 3.5 x 1.2 mm
NTC
Murata
NCP21WF104J03RA
100 kΩ
0805
RFL
Tyco
RL73K1JR27JTD
0.27 Ω
0603
RTR
Rohm
MCR01MZPJ6R20
1.8 Ω
0402
RX
Rohm
MCR01MZPJ15K
15 kΩ
0402
LED
Luxeon LED
LXCL-PW1
9/31
PCB design
AN2507
3
PCB design
3.1
PCB design rules
STCF03 is a powerful switching device where the PCB must be designed in line with
switched supplies design rules. The power tracks (or wires in demo-board) must be as short
as possible and wide enough, because of the high currents involved. It is recommended to
use a 4 layers PCB to get the best performance. All external components must be placed as
close as possible to STCF03. All high-energy switched loops should be as small as possible
to reduce EMI. Most of LEDs need efficient cooling, which could be done by using a
dedicated copper area on the PCB. Please refer to the selected LED's reference guide to
design the heatsink. Place the RFL resistor as close as possible to the PGND pins and the
ground pin of the COUT capacitor. In case a modification of any PCB layer is required, it is
highly recommended to use enough vias. Place the NTC resistor as close as possible to the
LED for good temperature sensing. Direct connection between GND and PGND is
necessary in order to achieve correct output current value. No LED current should flow
through this track! Voltage sensing on the RFL resistor must to done on a track from ball FB2
and directly connected to the RFL resistor. Again, no current should flow through this track.
Pin FB2S must be connected to the RFL resistor pin. Vias connecting the STCF03 pins to
the copper tracks (if used) must be 0.1 mm in diameter for BGA version. It is recommended
to use the filled vias.
3.2
PCB layout
3.2.1
A four-layer PCB with application area 45.1 mm2 for BGA package,
version B
(for version C is layout exactly same except the NTC connection, see Figure 1)
Figure 3.
10/31
Top layer
AN2507
PCB design
Figure 4.
Middle layer 1
Figure 5.
Middle layer 2
11/31
PCB design
12/31
AN2507
Figure 6.
Bottom layer
Figure 7.
Top overlay
AN2507
3.2.2
PCB design
A two-layer PCB with application area 72.4 mm2 for QFN package
Figure 8.
Top layer
Figure 9.
Bottom layer
13/31
PCB design
AN2507
Figure 10. Top overlay
3.2.3
A four-layer PCB with application area 45.1 mm2 for BGA package,
version C
Figure 11. Top layer
14/31
AN2507
PCB design
Figure 12. Middle layer 1
Figure 13. Middle layer 2
15/31
Internal registers
AN2507
Figure 14. Bottom layer
Figure 15. Top overlay
4
Internal registers
4.1
Accessing the internal registers
There are 4 internal registers in STCF03 (which are the COMMAND, DIMMING, AUX_LED
and STATUS registers). The STATUS register is read-only. The COMMAND register can be
accessed in any operation mode. All the other registers can be accessed in any mode,
except in SHUTDOWN mode. When the device enters SHUTDOWN mode, the DIMMING,
AUX_LED and STATUS registers are cleared. The COMMAND register value remains
untouched when entering SHUTDOWN mode. Table 2 shows the accessibility of each
register in all operation modes.
16/31
AN2507
Table 2.
Operation modes
Accessibility of internal registers
Mode
Register
Address
SHUTDOWN
READY
TORCH
FLASH
SHUTDOWN
value
PowerON
reset value
COMMAND
00
Read / Write
Read / write
Read / write
Read / write
Untouched
Cleared
DIMMING
01
Inaccessible
Read / write
Read / write
Read / write
Cleared
Cleared
AUX_LED
02
Inaccessible
Read / write
Read / write
Read / write
Cleared
Cleared
STATUS
03
Inaccessible
Read only
Read only
Read only
Cleared
Cleared
5
Operation modes
5.1
SHUTDOWN mode
SHUTDOWN mode is entered after the Power-ON reset. This mode is mainly used to
decrease the power consumption of the device. During this mode, only the I2C interface is
alive. The only thing which can be done in SHUTDOWN mode is to access the COMMAND
register. Entering SHUTDOWN mode by writing to the COMMAND register aborts any
running operation and clears the values of the DIMMING, AUX_LED and STATUS registers.
The COMMAND register value is not affected by entering SHUTDOWN mode.
The following data must be written to the COMMAND register to enter SHUTDOWN mode.
Table 3.
CMD_REG
COMMAND register data to enter SHUTDOWN mode (version B)
PWR_ON
TRIG_EN
TCH_ON
NTC_ON
FTIM_3
FTIM_2
FTIM_1
FTIM_0
0
x
x
x
x
x
x
x
MSB
Table 4.
CMD_REG
LSB
COMMAND register data to enter SHUTDOWN mode (version A and C)
PWR_ON
TRIG_EN
TCH_ON
NTC_ON
FTIM_3
FTIM_2
FTIM_1
FTIM_0
0
x
x
0
x
x
x
x
MSB
5.2
LSB
SHUTDOWN mode with the NTC-feature activated
This mode is supported only in version A, which does not have any internal voltage
reference for the NTC feature. When this operation mode is activated, the microcontroller
can still monitor the NTC voltage through its A/D converter, while STCF03 remains in
SHUTDOWN mode and therefore saves power.
The following data must be written to the COMMAND register to enter SHUTDOWN mode +
NTC.
17/31
Operation modes
Table 5.
CMD_REG
AN2507
COMMAND register data to enter SHUTDOWN mode with NTC activated (version A
and C)
PWR_ON
TRIG_EN
TCH_ON
NTC_ON
FTIM_3
FTIM_2
FTIM_1
FTIM_0
0
x
x
1
x
x
x
x
MSB
5.3
LSB
READY mode and NTC
The READY mode allows the user to access all the internal registers. The NTC feature can
be activated in this mode and the temperature of the LED can be sensed by the A/D
converter of the microcontroller. The following data must be written to the COMMAND
register to enter READY mode.
Table 6.
CMD_REG
COMMAND register data to enter READY mode
PWR_ON
TRIG_EN
TCH_ON
NTC_ON
FTIM_3
FTIM_2
FTIM_1
FTIM_0
1
0
0
0
x
x
x
x
MSB
LSB
The following data must be written to the COMMAND register to activate the NTC feature.
Table 7.
CMD_REG
COMMAND register data to enter READY mode with NTC ON
PWR_ON
TRIG_EN
TCH_ON
NTC_ON
FTIM_3
FTIM_2
FTIM_1
FTIM_0
1
0
0
1
x
x
x
x
MSB
LSB
As soon as the NTC feature is activated, the internal switch connects the NTC resistor to the
RX resistor, thereby creating a voltage divider. The voltage on this divider can be, if desired,
monitored by the A/D converter of the microcontroller. An external voltage reference must be
connected to the NTC to use this feature (only in version A and C). The bits NTC_W and
NTC_H of the STATUS register will not be properly set if there is no external reference
voltage connected to the NTC (only in version A and C).
If the NTC feature is not going to be used, neither the negative thermistor, nor the external
reference needs to be connected. In this case, it is recommended to ground the RX pin. As
the NTC feature is automatically activated during the FLASH and TORCH mode, leaving the
RX pin floating could lead to unwanted interruptions of the light due to non-defined voltages
on the RX pin
5.4
TORCH mode
This mode is intended to be used for low light intensities. The LED current in the TORCH
mode can be adjusted in a range from 15 mA up to 200 mA.
The TORCH mode is activated by writing the following data to the COMMAND register.
18/31
AN2507
Table 8.
CMD_REG
Operation modes
COMMAND register data to enter TORCH mode
PWR_ON
TRIG_EN
TCH_ON
NTC_ON
FTIM_3
FTIM_2
FTIM_1
FTIM_0
1
0
1
x
x
x
x
x
MSB
LSB
The DIMMING register value (TDIM) must be set as well, unless it has already been set
during a previous operation. If TDIM register is not set, then the default output current value
will be at the minimum.
There is no internal timer which controls the TORCH duration. Therefore, as soon as the
TORCH mode is activated, it remains active until a new mode is entered by writing a new
data to the COMMAND register.
If the TORCH mode was terminated by entering READY or FLASH mode, it can be restarted
again by writing the corresponding data to the COMMAND register only, because entering
any of the READY and FLASH modes does not influence the TDIM value. If the TORCH
mode was terminated by entering into SHUTDOWN mode, then the TDIM value must be set
again during the restart of the TORCH, because entering the SHUTDOWN mode clears the
TDIM value.
As soon as the TORCH mode is activated, the NTC feature is automatically activated too in
order to protect the LED against overheating. The NTC feature will be activated even if the
NTC_ON bit in the COMMAND register is set to zero.
5.5
FLASH mode
This mode is intended to be used for high light intensities. The LED current in the FLASH
mode can be adjusted up to 800 mA with the input voltage ranging from 3.3 V up to 5.5 V.
The FLASH mode is activated by writing the following data to the COMMAND register.
Table 9.
CMD_REG
COMMAND register data to enter FLASH mode
PWR_ON
TRIG_EN
TCH_ON
NTC_ON
FTIM_3
FTIM_2
FTIM_1
FTIM_0
1
1
x
x
x
x
x
x
MSB
LSB
The DIMMING register value (FDIM) must be set as well, unless it has already been set
during a previous operation.
The activation of the FLASH mode requires the TRIG pin to be High. The FLASH mode is
active only when the TRIG_EN bit in the COMMAND register is set to 1 and the TRIG pin is
High. This gives the user the possibility to choose between a soft and a hard triggering of
the FLASH mode.
The soft triggering is done by writing data to the internal registers only, while the TRIG pin is
permanently kept High, that is, by connecting it to VBAT. This saves one pin of the
microcontroller, which can be used for a different purpose, but this way of triggering is less
accurate than the hard one. The second disadvantage of this solution is that the FLASH
duration can only be set in discrete steps of the internal timer (1 step = approx. 100 ms).
19/31
Operation modes
AN2507
Hard-triggering of the FLASH mode requires the microcontroller to manage the TRIG pin.
The COMMAND and the DIMMING registers are loaded with data before the TRIG pin is set
to High. This allows the user to avoid the I2C-bus latency. FLASH mode then starts as soon
as the TRIG pin is set to High. It takes typically about 0.7 ms to ramp-up the LED current to
the adjusted value. This time may vary according to the LED current value and the battery
voltage. When the TRIG pin is kept High long enough, the internal timer reaches zero and
the FLASH mode is over. As soon as the FLASH is timed out, the ATN pin is pulled down for
11µs to inform the microcontroller that the STATUS register was updated and that the flash
is over. If the TRIG pin is set to Low before the internal timer reaches zero, the FLASH mode
will be interrupted and can be restarted by setting the TRIG pin to High again. The internal
timer is stopped while the TRIG pin is Low. This means that the user can split the FLASH
into several pulses of a total length equal to the FTIM value. Figure 16 below shows splitting
of the FLASH into several shorter pulses. The cumulative length of all the pulses is
determined by the FTIM value. Figure 16 shows the case for FTIM = 9 (900 ms FLASH
time). The cumulative time when the TRIG pin is High is 1000 ms (5 pulses 200 ms long).
The last FLASH pulse will be100ms long only. The reason is that the internal FLASH timer
reaches zero and the TRIG_EN bit is set to 0.
Figure 16. Splitting the FLASH pulse into several shorter pulses
1300 ms
200
ms
Time when the
internal flash
timer reaches 0
100
ms
2
I C bus packet
TRIG_EN bit
TRIG pin
LED current
Internal Flash timer values
9
8
7
6
5
4
3
2
1
0
Hard triggering allows therefore a smooth setting of the FLASH duration. The resolution is
about 8.8µs. The minimum FLASH duration is limited by the ramp-up time of the LED
current and the maximum is limited by the FTIM value. If it is necessary to make a FLASH
pulse longer than the maximum allowed by FTIM, then it is necessary to reload the
COMMAND register before the internal timer reaches zero (start a new FLASH before the
previous one elapses). See Section 8.5 - Example 5 for more details.
20/31
AN2507
The STATUS register and the ATN pin
6
The STATUS register and the ATN pin
6.1
The STATUS register
Table 10.
Bit name
STATUS register bits
N/A
F_RUN
LED_F
NTC_W
NTC_H
OT_F
N/A
MSB
VOUTOK_N
LSB
A detailed description of each bit is stated in the datasheet.
Table 11.
Effect of the STATUS register bits on the operation of the device
Bit name
Default value
F_RUN
LED_F
NTC_W
NTC_H
OT_F
(STAT_REG) (STAT_REG) (STAT_REG) (STAT_REG) (STAT_REG)
VOUTOK_N
(STAT_REG)
0
0
0
0
0
0
NO
YES
YES
YES
YES
YES
Forces READY
mode when set
NO
YES
NO
YES
YES
YES
Sets ATN LOW when
set
NO
YES
YES
YES
YES
YES
Latched
(1)
1. YES means that the bit is set by internal signals and is reset to its default value by an I2C-read operation of STAT_REG; NO
means that the bit is set and reset by internal signals in real-time
When the status register is latched, reading and writing to the registers is still possible, but the bits
TRIG_EN and TCH_ON in the COMMAND register and AUXL register cannot be changed, until the
device is unlatched. It is necessary to read the STATUS register to unlatch the device.
The ATN pin is also pulled down when the internal timer reaches zero in FLASH mode. In this case the
ATN pin is pulled down for 11 µs only. It is recommended to connect the ATN pin to the interrupt input of
the microcontroller. If it is not connected to the interrupt input, the ATN pin should be pulled fast enough
not to miss the 11 µs pulse, that is, by a programming loop which is entered after start of the FLASH
mode. This loop runs until the ATN pin gets Low. It is recommended to make a time-out of such a loop.
21/31
Reading and writing to the STCF03 registers through the I2C bus
AN2507
7
Reading and writing to the STCF03 registers through
the I2C bus
7.1
Writing to a single register
Writing to a single register starts with a START-bit followed by the 7-bit device address of
STCF03. The 8-th bit is the R/W bit, which is 0 in this case. R/W = 1 means a Reading
operation. Then the master awaits an acknowledgement from STCF03. The 8-bit address of
the desired register is sent afterwards to STCF03. It will also be followed by an acknowledge
pulse. The last transmitted byte is the data that is going to be written into the register. It is
followed again by an acknowledge pulse from STCF03. Then the master generates a STOPbit and the communication is over. See Figure 17 below.
Figure 17. Writing to a single register
DEVICE
ADDRESS
7 bits
W
R
I
T
E
ADDRESS OF
REGISTER
DATA
SDA LINE
S M
T S
A B
R
T
7.2
L R A M
S / C S
B W K B
L A M
S C S
B K B
L A S
S C T
B K O
P
Writing to multiple registers with incremental addressing
It would be unpractical to send several times the device address and the address of the
register when writing to multiple registers. STCF03 supports writing to multiple registers with
incremental addressing. When data is written to a register, the register address is
automatically incremented (by one), and therefore the next data can be sent without sending
again the device address and the register address. See Figure 18 below.
Figure 18. Writing to multiple registers
DEVICE
ADDRESS
7 bits
W
R
I
T
E
ADDRESS OF
REGISTER i
DATA i
DATA i+1
DATA i+2
DATA i+2
DATA i+n
SDA LINE
S M
T S
A B
R
T
22/31
L R A M
S / C S
B W K B
L A M
S C S
B K B
L A M
S C S
B K B
L A M
S C S
B K B
L A M
S C S
B K B
L A M
S C S
B K B
L A S
S C T
B K O
P
AN2507
7.3
Reading and writing to the STCF03 registers through the I2C bus
Reading from a single register
The reading operation starts with a START-bit followed by 7 bit device address of STCF03.
The 8-th bit is the R/W bit, which is 0 in this case. STCF03 confirms the receiving of the
address + R/W bit by an acknowledge pulse. The address of the register which should be
read is sent after and confirmed by an acknowledge pulse from STCF03 again. Then the
master generates a START-bit again and sends the device address followed by the R/W-bit,
which is 1 now. STCF03 confirms the receiving of the address + R/W-bit by an acknowledge
pulse, and starts to send data to the master. No acknowledge pulse from the master is
required after receiving the data. Then the master generates a STOP-bit to terminate the
communication. See the Figure 19 below.
Figure 19. Reading from a single register
DEVICE
ADDRESS
7 bits
W
R
I
T
E
DEVICE
ADDRESS
7 bits
ADDRESS OF
REGISTER
R
E
A
D
DATA
SDA LINE
S M
T S
A B
R
T
7.4
L R A M
S / C S
B W K B
L A S
S C T
B K A
R
T
R A
/ C
W K
L N S
S O T
O
B
A P
C
Reading from multiple registers with incremental addressing
Reading from multiple registers starts in the same way as reading from a single register. As
soon as the first register is read, the register address is automatically incremented. If the
master generates an acknowledge pulse after receiving the data from the first register, then
reading from the next register can start immediately without having to send once more the
device and the register addresses. The last acknowledge pulse before the STOP-bit is not
required. See Figure 20 below.
Figure 20. Reading from multiple registers
DEVICE
ADDRESS
7 bits
W
R
I
T
E
DEVICE
ADDRESS
7 bits
ADDRESS OF
REGISTER i
R
E
A
D
DATA i
DATA i+1
DATA i+2
DATA i+2
DATA i+n
SDA LINE
S M
T S
A B
R
T
L R A M
S / C S
B W K B
L A S
S C T
B K A
R
T
R A
/ C
W K
L A M
S C S
B K B
L A M
S C S
B K B
L A M
S C S
B K B
L A M
S C S
B K B
L N S
S O T
O
B
A P
C
K
23/31
Examples of register setup for each mode
8
AN2507
Examples of register setup for each mode
A device address 0x62 is used in all the example below. The STCF03 is configured to this
device address, if the ADD pin is connected to VBAT pin. In the demoboard the device
address is 0x60 because the ADD pin is connected to GND.
Table 12.
T_DIM
(hex)
0
TORCH mode and FLASH mode dimming registers settings
1
2
3
4
5
6
7
F_DIM
(hex)
8
9
A
B
C
D
E
F
0
1
2
3
4
5
6
7
8
9
A
B
C
D
E
F
LED
current
[mA]
16
19
23
27
32
39
46
55
65
77
92
109
124 147 175 209 248 296 352 418 498 592 705 840
Internal
step
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
VREF1
[mV]
33
40
47
56
67
80
95
113
134
160
190
227
33
40
47
56
67
79
95
113 134 160 190 227
Sense
Resist.
RFL RFL RFL RFL RFL RFL RFL RFL RFL RFL
+
+
+
+
+
+
+
+
+
+
RTR RTR RTR RTR RTR RTR RTR RTR RTR RTR
RFL RFL
+
+
RFL
RTR RTR
RFL RFL RFL RFL
21
22
23
24
RFL RFL RFL RFL RFL RFL RFL
Note:
LED current values refer to RFL= 0.27 Ohm, RTR = 1.8 Ohm
8.1
Example 1: 600 mA FLASH with 700 ms duration
Let's suppose that RFL = 0.27 Ω. The targeted value of the FLASH current is 600 mA and the
FLASH duration should be 700 ms.
The reference voltage must be set to 160 mV to achieve a 600 mA FLASH current with a
0.27 Ω sensing resistor. The value of FDIM (4 bits) must be set to 0xD to set up the
reference voltage to 160 mV. (See Table 12)
The FLASH duration timer can be set to 100ms up to 1500 ms in 100ms increments. If the
desired FLASH duration is 700 ms the value FTIM (4 bits) must be set to 0x7.
Bit PWR_ON of the command register must be set to 1.
Bit TRIG_EN of the command register must be set to 1.
Bit TCH_ON of the command register must be set to 0.
Bit NTC_ON of the command register can be set to any value, because NTC is
automatically ON when the FLASH mode is active. Setting this bit to 0 will not switch off the
NTC.
Table 13.
CMD_REG
COMMAND register data to enter FLASH mode
PWR_ON
TRIG_EN
TCH_ON
NTC_ON
FTIM_3
FTIM_2
FTIM_1
FTIM_0
1
1
x
x
0
1
1
1
MSB
24/31
LSB
AN2507
Table 14.
Examples of register setup for each mode
DIMMING register data for the FLASH mode
TDIM_3
TDIM_2
TDIM_1
TDIM_0
FDIM_3
FDIM_2
FDIM_1
FTIM_0
0
0
0
0
1
1
0
1
DIM_REG
MSB
LSB
It is necessary to write 4 bytes to STCF03 to make a FLASH.
Table 15.
8.2
I2C data packet for activating the FLASH mode
Byte
Hex
Binary
Comment
1
62
0
1
1
0
0
0
1
0
Device address + R/W bit
2
00
0
0
0
0
0
0
0
0
Command register address
3
D7
1
1
0
1
0
1
1
1
Data of the command register
4
0D
0
0
0
0
1
1
0
1
Data of the dimming register
Example 2: 25 mA TORCH
Let's suppose that RFL = 0.27 Ω, RTR = 1.8 Ω and the targeted value of the TORCH current is
25 mA.
The reference voltage must be set to 56 mV to achieve 25 mA in TORCH mode with the
resistor values mentioned above. The value of TDIM (4 bits) must be set to 0x3 to set up the
reference voltage to 56 mV.
Bit PWR_ON of the command register must be set to 1.
Bit TRIG_EN of the command register must be set to 0.
Bit TCH_ON of the command register must be set to 0.
Bit NTC_ON of the command register can be set to any value, because NTC is
automatically ON, when TORCH mode is active. Setting this bit to 0 does not switch off the
NTC.
Table 16.
CMD_REG
COMMAND register data for the TORCH mode
PWR_ON
TRIG_EN
TCH_ON
NTC_ON
FTIM_3
FTIM_2
FTIM_1
FTIM_0
1
0
1
1
0
0
0
0
MSB
Table 17.
DIM_REG
LSB
DIMMING register data for the TORCH mode
TDIM_3
TDIM_2
TDIM_1
TDIM_0
FDIM_3
FDIM_2
FDIM_1
FDIM_0
0
0
1
1
0
0
0
0
MSB
LSB
25/31
Examples of register setup for each mode
AN2507
It is necessary to write 4 bytes to the STCF03 to run the TORCH mode.
Table 18.
I2C data packet to activate TORCH mode
Byte
Hex
Binary
Comment
1
62
0
1
1
0
0
0
1
0
Device address + R/W bit
2
00
0
0
0
0
0
0
0
0
Command register address
3
B0
1
0
1
1
0
0
0
0
Data of the command register
4
30
0
0
1
1
0
0
0
0
Data of the dimming register
The duration of the TORCH mode is "unlimited". TORCH mode is terminated by setting the
TCH_ON bit in the COMMAND register to 0.
Termination of the TORCH mode can be done by writing the following data to STCF03.
Table 19.
I2C data packet for terminating the TORCH mode
Byte
Hex
Binary
Comment
1
62
0
1
1
0
0
0
1
0
Device address + R/W bit
2
00
0
0
0
0
0
0
0
0
Command register address
3
80
1
0
0
0
0
0
0
0
Data of the command register
This puts the STCF03 into READY mode.
8.3
Example 3: an Auxiliary LED running at 10 mA for 500 ms
STCF03 must be into READY mode (both bits TRIG_EN and TCH_ON are 0) to activate the
Auxiliary LED.
A 10 mA output current is reached when AUXI is set to 0x8.
AUXT must be set to 0x5 to have a 500 ms duration of the Auxiliary LED lighting.
Table 20.
CMD_REG
COMMAND register data for the AUX_LED
PWR_ON
TRIG_EN
TCH_ON
NTC_ON
FTIM_3
FTIM_2
FTIM_1
FTIM_0
1
0
0
0
0
0
0
0
MSB
Table 21.
AUX_LED
COMMAND register data for the AUX_LED
AUX_3
AUX_2
AUX_1
AUX_0
AUX_3
AUX_2
AUX_1
AUX_0
1
0
0
0
0
1
0
1
MSB
26/31
LSB
LSB
AN2507
Examples of register setup for each mode
Writing the 3 bytes below to STCF03 puts it into READY mode. This can be skipped if it
already is in READY mode.
Table 22.
I2C data packet for activating the READY mode
Byte
Hex
Binary
Comment
1
62
0
1
1
0
0
0
1
0
Device address + R/W bit
2
00
0
0
0
0
0
0
0
0
Command register address
3
80
1
0
0
0
0
0
0
0
Data of the command register
Writing the following 3 bytes to STCF03 will activate the Auxiliary LED for the desired time.
Table 23.
8.4
I2C data packet for activating the AUX_LED
Byte
Hex
Binary
Comment
1
62
0
1
1
0
0
0
1
0
Device address + R/W bit
2
02
0
0
0
0
0
0
1
0
Auxiliary LED register address
3
85
1
0
0
0
0
1
0
1
Data of the auxiliary LED
register
Example 4: Red-eye reduction (multiple short flashes)
There are two ways to manage this task. The first one is to use hardware triggering of the
flashes through the TRIG pin. This is the most suitable and recommended solution, as it
reduces the usage of the I2C bus and the length of each FLASH pulse can be adjusted
continuously. The second solution is to use the software triggering feature, which means a
periodical reloading of the COMMAND register. This however increases traffic on the I2C
bus and the flashes can only have length, adjustable in 100 ms increments only.
Let's suppose that RFL = 0.27 Ω and the targeted value of the FLASH current is 600 mA. The
task is to make 5 flashes of 200 ms duration with 100 ms pause between them. The setting
of the reference voltage is identical to the one in Section 8.1. The FLASH timer (FTIM) is set
to 0xF, which represents 1.5 s.
Table 24.
CMD_REG
COMMAND register data for FLASH mode
PWR_ON
TRIG_EN
TCH_ON
NTC_ON
FTIM_3
FTIM_2
FTIM_1
FTIM_0
1
1
0
1
1
1
1
1
MSB
Table 25.
DIM_REG
LSB
DIMMING register data for the FLASH mode
TDIM_3
TDIM_2
TDIM_1
TDIM_0
FDIM_3
FDIM_2
FDIM_1
FDIM_0
0
0
0
0
1
1
0
1
MSB
LSB
27/31
Examples of register setup for each mode
AN2507
The data packet which has to be sent is in the table below.
Table 26.
I2C data packet for activating the FLASH mode
Byte
Hex
Binary
Comment
1
62
0
1
1
0
0
0
1
0
Device address + R/W bit
2
00
0
0
0
0
0
0
0
0
Command register address
3
DF
1
1
0
1
1
1
1
1
Data of the command register
4
0D
0
0
0
0
1
1
0
1
Data of the dimming register
The picture below shows the TRIG pin and the I2C bus timings.
Figure 21. Multiple flashes handled by the TRIG pin
2
I C bus packet
TRIG_EN bit
TRIG pin
200
ms
100
ms
1400 ms
8.5
Example 5: A FLASH pulse longer than 1.5 s
Let's suppose that RFL = 0.27 Ω and the targeted value of the FLASH current is 600 mA. The
task is to make a single FLASH pulse with a 4 seconds duration.
It is necessary to reload FTIM in the COMMAND REGISTER before the internal FLASH
timer reaches zero. This guarantees that the FLASH does go on and does not stop after
1.5 sec.
The first packet must contain also the DIMMING REGISTER data, if they are different from
those which were used in the previous operation.
●
Packet 1
Sets FLASH mode with 1.5 s duration and the proper dimming.
Table 27.
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I2C data packet for activating the FLASH mode
Byte
Hex
Binary
Comment
1
62
0
1
1
0
0
0
1
0
Device address + R/W bit
2
00
0
0
0
0
0
0
0
0
Command register address
3
DF
1
1
0
1
1
1
1
1
Data of the command register
4
0D
0
0
0
0
1
1
0
1
Data of the dimming register
AN2507
Examples of register setup for each mode
●
Packet 2
Sets FLASH mode with 1.5 s duration. Dimming is not set again as it is same as before.
Table 28.
1st I2C data packet to restart the FLASH mode
Byte
Hex
1
62
0
1
1
0
0
0
1
0
Device address + R/W bit
2
00
0
0
0
0
0
0
0
0
Command register address
3
DF
1
1
0
1
1
1
1
1
Data of the command register
●
Binary
Comment
Packet 3
Sets FLASH mode with 1.5 s duration. Dimming remains untouched.
Table 29.
2nd I2C data packet for restart of the FLASH mode
Byte
Hex
1
62
0
1
1
0
0
0
1
0
Device address + R/W bit
2
00
0
0
0
0
0
0
0
0
Command register address
3
DF
1
1
0
1
1
1
1
1
Data of the command register
●
Binary
Comment
Packet 4
Sets FLASH mode with 1 s duration. Dimming remains untouched.
Table 30.
3rd I2C data packet to restart the FLASH mode
Byte
Hex
Binary
Comment
1
62
0
1
1
0
0
0
1
0
Device address + R/W bit
2
00
0
0
0
0
0
0
0
0
Command register address
3
DA
1
1
0
1
1
0
1
0
Data of the command register
Please refer to Figure 22 for more details about the I2C-bus packets timing.
The solution described above is using a software termination of the FLASH pulse. (It is
timed out by the internal timer.) The FLASH pulse could be also terminated by setting the
TRIG pin to low after 4 seconds. In this case, the fourth packet could be the same as
packets Packet 2 and Packet 3, because the timing of the FLASH is done by the TRIG pin
and it is not necessary to change the value of FTIM in the COMMAND REGISTER.
This way of periodical reloading of the COMMAND REGISTER can be used to achieve a
continuous FLASH light. In this case, it is very strongly recommended to guarantee an
efficient cooling of both the LED and the chip, otherwise the light can be interrupted by
activation of the thermal protections.
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Revision history
AN2507
Figure 22. I2C bus packets timing for a FLASH lasting longer than FTIM max
Timeout of the
first Flash
Timeout of the
second Flash
1.5s
Timeout of the
third Flash
1.5s
Timeout of the
fourth Flash –
ending of the
whole Flash
pulse
1.5s
1.0s
1.0s
1.0s
1.0s
2
I C bus packets
TRIG_EN bit
TRIG pin
4.0s
9
Revision history
Table 31.
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Revision history
Date
Revision
19-Apr-2007
1
Changes
Initial release
AN2507
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