ST AN2507 Application note
High-Power Camera Flash LED Driver with I2C™
Introduction
AN2507
Application note
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 I
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
2
C interface. The schematic,
Version A
April 2007 Rev 1 1/31
www.st.com
Contents AN2507
Contents
1 Schematic description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1 Application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2 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 R
2.5 R
2.6 NTC AND R
selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
FL
selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
TR
resistor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
X
3 PCB design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1 PCB design rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.2 PCB layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.2.1 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 mm
3.2.3 A four-layer PCB with application area 45.1 mm
version C 14
2
for QFN package . . . 13
2
for BGA package,
4 Internal registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.1 Accessing the internal registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5 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
6 The STATUS register and the ATN pin . . . . . . . . . . . . . . . . . . . . . . . . . . 21
6.1 The STATUS register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
7 Reading and writing to the STCF03 registers through the I2C bus . . 22
2/31
AN2507 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
8 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
9 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3/31
List of tables AN2507
List of tables
Table 1. Recommended components. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Table 2. Accessibility of internal registers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Table 3. COMMAND register data to enter SHUTDOWN mode (version B) . . . . . . . . . . . . . . . . . . 17
Table 4. COMMAND register data to enter SHUTDOWN mode (version A and C) . . . . . . . . . . . . . 17
Table 5. COMMAND register data to enter SHUTDOWN mode with NTC activated (version A and C)
18
Table 6. COMMAND register data to enter READY mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Table 7. COMMAND register data to enter READY mode with NTC ON. . . . . . . . . . . . . . . . . . . . . 18
Table 8. COMMAND register data to enter TORCH mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Table 9. COMMAND register data to enter FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Table 10. STATUS register bits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Table 11. Effect of the STATUS register bits on the operation of the device . . . . . . . . . . . . . . . . . . . 21
Table 12. TORCH mode and FLASH mode dimming registers settings. . . . . . . . . . . . . . . . . . . . . . . 24
Table 13. COMMAND register data to enter FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Table 14. DIMMING register data for the FLASH mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Table 15. I
Table 16. COMMAND register data for the TORCH mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Table 17. DIMMING register data for the TORCH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Table 18. I
Table 19. I
Table 20. COMMAND register data for the AUX_LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Table 21. COMMAND register data for the AUX_LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Table 22. I
Table 23. I
Table 24. COMMAND register data for FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Table 25. DIMMING register data for the FLASH mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Table 26. I
Table 27. I
Table 28. 1
Table 29. 2
Table 30. 3
Table 31. Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2
C data packet for activating the FLASH mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2
C data packet to activate TORCH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2
C data packet for terminating the TORCH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2
C data packet for activating the READY mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2
C data packet for activating the AUX_LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2
C data packet for activating the FLASH mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2
C data packet for activating the FLASH mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
st I2
C data packet to restart the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
nd I2
C data packet for restart of the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
rd I2
C data packet to restart the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4/31
AN2507 List of figures
List of figures
Figure 1. A typical application schematic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 2. NTC connection for versions with internal voltage reference . . . . . . . . . . . . . . . . . . . . . . . . 9
Figure 3. Top layer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Figure 4. Middle layer 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Figure 5. Middle layer 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Figure 6. Bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Figure 7. Top overlay. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Figure 8. Top layer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Figure 9. Bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Figure 10. Top overlay. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Figure 11. Top layer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Figure 12. Middle layer 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 13. Middle layer 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 14. Bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Figure 15. Top overlay. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Figure 16. Splitting the FLASH pulse into several shorter pulses . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Figure 17. Writing to a single register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Figure 18. Writing to multiple registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Figure 19. Reading from a single register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Figure 20. Reading from multiple registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Figure 21. Multiple flashes handled by the TRIG pin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Figure 22. I
2
C bus packets timing for a FLASH lasting longer than FTIM max . . . . . . . . . . . . . . . . . . 30
5/31
Schematic description AN2507
1 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 V
Addresses
Note: *** Optional components to support auxiliary functions
● 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
6/31
, or GND or SDA or SCL to choose one of the 4 different I2C Slave
I
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 f or t his
application. To achieve a good efficiency in step-up mode, it is recommended to use an
inductor with a DC series resistance R
resistance of the LED [Ω, Ω, 1].
For nominal operation, the peak inductor current can be calculated by the following formula:
= RD/10 [Ω, Ω, 1], where RD is the dynamic
DCL
Equation 1
I
PEAK
I
⎛⎞
⎛⎞
----------- -
⎜⎟
⎝⎠
⎝⎠
⎛⎞
OUT
+
⎜⎟
n
⎝⎠
Where:
● I
● I
● n: Efficiency of the STCF03
● V
● V
● L: Inductance value of the inductor
● F: Switching frequency
: Peak inductor current
PEAK
: Current sourced at the VOUT-pin
OUT
: Output voltage at the VOUT-pin
OUT
: Input voltage at the VBAT-pin
IN
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 b y 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.
–()V
V
OUTVIN
---------------------------------------------------- -
2L• F• V
•
IN
2
•
OUT
2
V
OUT
--------------
•=
V
IN
Note: see recommended components in Table 1.
7/31
Selection of external components AN2507
2.4 RFL selection
The value of the RFL resistor can be calculated by the following equations:
R
FL = VFB2/IFLASH
where V
power dissipated on the R
= 226 mV and P
FB2
resistor. It is recommended to use a thin metal film resistor with
FL
RFLASH
= R
FL
* I
FLASH
2
, where P
RFL
is the
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
REFITORCHRFL
⎛⎞
R
--------------------------------------------------------
=
TR
⎝⎠
I
TORCH
•–
and
P
RTORCH
RTRI
•=
2
TORCH
where P
RTORCH
equation R
is the power dissipated on the RTR resistor. It is reco mmended to follow the
= 6.66* RFL to avoid any jump in the current dimming values.
TR
It is recommended to use a thin metal film resistor with 1% or 5% tolerance. The maximum
LED current in TORCH mode f or SCTF03 is 200 mA f or a battery voltage r anging 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 w ell as
an R
resistor and an external voltage reference in order to use the NTC feature. Please
X
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 R
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 t he 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 aut omatically to the READY mode to av oid damaging th e
LED . This status is latched, until the microcontroller reads the STATUS register . Reading the
STATUS register clears the NTC_H bit.
resistor to the NTC,
X
The selection of the NTC and R
resistors values strongl y depend s on the po w er dissip ated
X
by the LED and all components surrounding the NTC-thermistor and on the cooling
capabilities of each specific application. The R
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
and the NTC values in Table 1 below work
X
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 fo r the NTC. This version requires a
different connection between the R
and NTC resistors. See Figure 2 below or
X
Figure 1 version B.
Figure 2. NTC connection for versions with internal voltage reference
NTC
STCF03
NTC thermistor
RX
resistor
R
X
To optional A/D converter
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
C
I
C
O
L TDK VLF4012AT-4R7M1R1 4.7 µH 3.7 x 3.5 x 1.2 mm
NTC Murata NCP21WF104J03RA 100 kΩ 0805
R
FL
R
TR
R
X
LED Luxeon LED LXCL-PW1
TDK C1608X5R0J106M 10 µF 0603
TDK C1608X5R0J105M 1 µF 0603
Tyco RL73K1JR27JTD 0.27 Ω 0603
Rohm MCR01MZPJ6R20 1.8 Ω 0402
Rohm MCR01MZPJ15K 15 kΩ 0402
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 po wer tr ac ks (or wires in demo-boar d) must be as short
as possible and wide enough, because of the high currents involved. It is recommended to
use a 4 lay ers PCB to get the best perf o rmance. All e xternal components must be pla ced as
close as possible to STCF03. All high-energ y s witched loo ps should be as small as possib le
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 R
ground pin of the COUT capacitor. In case a modification of any PCB lay er 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 betw een 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 R
and directly connected to the R
Pin FB2S must be connected to the R
the copper tracks (if used) must be 0.1 mm in diameter for BGA version. It is recommended
to use the filled vias.
resistor as close as possible to the PGND pins and the
FL
resistor must to done on a trac k from ball FB2
resistor . Again, no current should flow through this track.
FL
FL
resistor pin. Vias connecting the STCF03 pins to
FL
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. Top layer
10/31
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