cd00274990

AN3223
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 STCF04
device, which is a buck-boost converter with an I²C interface dedicated to charging a supercapacitor. The schematic, functional description, recommendations for PCB layout, and
external component selection are also covered. This device is designed for driving four
LEDs. A detailed functional description can be found in Figure 1 below.
Figure 1.
Picture of the demonstration board and the external transistor with TDK
EDLC
AM05038v1
February 2012
Doc ID 17553 Rev 2
1/29
www.st.com
Contents
AN3223
Contents
1
Schematic description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1
2
3
Application schematic with external transistor . . . . . . . . . . . . . . . . . . . . . . 4
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
PCB design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1
PCB design rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2
PCB layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2.1
4
Internal registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1
5
An example of the 3-layer PCB with the external transistor STL8NH3LL 7
Accessing the internal registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Operation modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1
Shutdown mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.2
Monitoring mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.3
Idle mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.4
NTC feature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.5
Torch mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.6
Flash mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6
The STATUS register and the ATN pin . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7
READY pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
8
2/29
7.1
Function of the READY pin in Monitoring mode and Torch mode (fixed) . 18
7.2
Function of the READY pin in Flash mode . . . . . . . . . . . . . . . . . . . . . . . . 18
7.3
Function of the READY pin in Torch mode (optimized) . . . . . . . . . . . . . . 19
The light sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Doc ID 17553 Rev 2
AN3223
9
10
11
Contents
Reading and writing to the STCF04 registers through the I²C bus . . . 22
9.1
Writing to a single register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
9.2
Writing to multiple registers with incremental addressing . . . . . . . . . . . . 22
9.3
Reading from a single register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
9.4
Reading from multiple registers with incremental addressing . . . . . . . . . 23
Examples of register setup for each mode . . . . . . . . . . . . . . . . . . . . . . 25
10.1
Example 1: 10 A FLASH with 30 ms duration . . . . . . . . . . . . . . . . . . . . . 25
10.2
Example 2: 60 mA Torch with 10 s duration . . . . . . . . . . . . . . . . . . . . . . . 25
10.3
Example 3: An Auxiliary LED running at 10 mA for 500 ms . . . . . . . . . . . 26
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
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Schematic description
1
AN3223
Schematic description
The Flash LED driver STCF04 has a high operational frequency (1.8 MHz) which allows the
use of small-sized external components.
1.1
Application schematic with external transistor
Figure 2.
Typical application schematic
AM05040v1
**: Connect to VI, GND, SDA, or SCL to choose one of the four different I²C slave addresses.
Blue rectangle: optional components to support auxiliary functions.
4/29
Doc ID 17553 Rev 2
AN3223
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 for the input capacitor, and 10 µF
/ 6.3 V as the optimal value for the output capacitor to achieve a good stability of the device,
for a supply range varying from low input voltage (2.5 V) to the maximum ratings of output
power.
Note:
See recommended components in Table 1.
2.2
Inductor selection
The STCF04 device works with the switching algorithm ILIM-ZCOM. It charges the inductor
until the current crosses the threshold for the ILIM function and then it discharges the energy
in the inductor to the output until it reaches the zero current value. Therefore, it is
recommended to use a 1 µH inductor as the minimum value, which guarantees a proper
function with the used algorithm and speed of used components in the silicon design.
Note:
See recommended components in Table 1.
2.3
LED selection
All LEDs with a forward voltage range from 2.5 V to 4.5 V are compatible with the STCF04.
The forward voltage spread of any selected LED must, however, lay within this range (2.5 V
to 4.5 V). It is possible to set the level of the LED current in Flash mode and Torch mode by
setting the dimming registers. The maximum level of the LED current in Flash mode can be
set by changing the external Flash resistor.
Note:
See recommended components in Table 1.
2.4
NTC and RX resistor selection
The STCF04 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 2 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 automatically goes to Ready mode to avoid damaging the LED.
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Selection of external components
AN3223
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 resistor 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 demonstration board presented in this application note. A real 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: Monitoring mode.
Table 1.
Recommended components
Component
Manufacturer
Part number
Value
Size
Murata
LQM2HPN1R0MJC
1 µH / 1.5 A
2.5 x 2.0 x 1.1 mm
TDK
VLS252012T-1R0N1R7
1 µH / 1.7 A
2.5 x 2.0 x 1.2 mm
CIN,COUT
TDK
C1608X5R0J106MT
10 µF / 6.3 V
0603
Rx
Rohm
MCR01MZPJ15K
15 kΩ
0402
NTC
Murata
NCP21WF104J03RA
100 kΩ
0805
Murata
DME2W5R5K404M
400 mF / 5.5 V
20.5 x 18.5 x 3 mm
EDLC152344
550 mF / 5.5 V
44 x 23 x 1.5 mm
EDLC272020
500 mF / 5.5 V
20 x 20 x 2.7 mm
CAP-xx
GS 2 19F
1.6 F / 5 V
40 x 17 mm
LED MODUL
Luxeon
4x LXCL-PWF4
White LED
0805
TFL
STMicroelectronics
STL8NH3LL
8 A / 12 mΩ
3.3 x 3.3 x 0.9 mm
RFL
Tyco
TL2BR01FTE
0R01
1206
10 µF / 6.3 V
0402
L
CSUP
CINT
(1)
TDK
TDK
RLIGHT
(1)
Tyco
TFOTO
(1)
Vishay
0402
TEMT6000
AUXLED
Red LED
0603
CR
100 nF
0402
1. Optional components for the auxiliary light sensor feature.
6/29
4 x 2 x 1 mm
Doc ID 17553 Rev 2
AN3223
PCB design
3
PCB design
3.1
PCB design rules
The STCF04 is a powerful switching device working from low input voltages and high duty
cycle, where the PCB must be designed in line with switched supplies 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 3 to 4-layer
PCB to obtain the best performance. All external components must be placed as close as
possible to the STCF04. All high-energy switched loops should be as small as possible to
reduce EMI. Most of the LEDs need efficient cooling, which may be done by using a
dedicated copper area on the PCB. Please report to the selected LED's reference guide to
design the heatsink. If a modification to any PCB layer is required, it is highly recommended
to use an adequate number of 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!
Vias connecting the STCF04 pins to the copper tracks (if used) must be 0.1 mm in diameter
for the BGA version.
It is recommended to connect the LEDs close to the VOUT and ILED pins for a stable
operation margin. The impedance of this connection should be lower than 0.4 Ω.
3.2
PCB layout
3.2.1
An example of the 3-layer PCB with the external transistor STL8NH3LL
Figure 3.
Top layer
AM05042v1
Doc ID 17553 Rev 2
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PCB design
AN3223
Figure 4.
Middle layer 1
AM05043v1
Figure 5.
Bottom layer
SUPERCAP LEADS – BOTTOM SIDE
AM05044v1
8/29
Doc ID 17553 Rev 2
AN3223
PCB design
Figure 6.
Top overlay
ADDRESS SELECTION - VBAT - GND - SDA - SCL (use zero ohm resistor for the setting)
AM05045v1
Doc ID 17553 Rev 2
9/29
Internal registers
AN3223
4
Internal registers
4.1
Accessing the internal registers
There are six internal registers in the STCF04 (which are the COMMAND, FLASH,
AUX_LED, STATUS, FEATURE, and TORCH registers). The STATUS register is read-only.
The COMMAND and FEATURE registers can be accessed in any operation mode. All the
other registers can be accessed in any mode, except in Shutdown, Shutdown + NTC, and
Monitoring mode. When the device enters Shutdown mode, the FLASH, AUX_LED,
STATUS, and TORCH registers are cleared. The COMMAND and FEATURE register values
remain untouched when entering Shutdown mode, however reading their value gives 0
when the bit PWR_ON = 0. Table 2 shows the accessibility of each register in all operation
modes.
In other words, whenever the PWR_ON bit in the COMMAND register is set to zero, then
only the COMMAND and FEATURE registers can be accessed. It is necessary to set the
PWR_ON bit to 1 to access all the registers.
Table 2.
Accessibility of internal registers
Mode
Shutdown
value
Power-ON
reset value
Read / Write
Untouched
Cleared
Inaccessible
Read / Write
Cleared
Cleared
02
Inaccessible
Read / Write
Cleared
Cleared
STATUS
03
Inaccessible
Read only
Cleared
Cleared
FEATURE
04
Read / Write
Read / Write
Untouched
Cleared
TORCH
05
Inaccessible
Read / Write
Cleared
Cleared
Register
Address
COMMAND
Shutdown and
Monitoring
Idle, Charging, Flash,
Torch and AUX LED
00
Read / Write
FLASH
01
AUX_LED
10/29
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AN3223
Operation modes
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. In this mode, only the I2C interface is live. The only
action which can be performed in Shutdown mode is to access the COMMAND and
FEATURE registers. Entering Shutdown mode by writing to the COMMAND register aborts
any running operation and clears the values of the FLASH, AUX_LED, STATUS, and
TORCH registers. The COMMAND and FEATURE register values are not affected by
entering Shutdown mode, but an attempt to read their value always gives 0 when the bit
PWR_ON = 0.
The following data must be written to the COMMAND register to enter Shutdown mode.
Table 3.
COMMAND register data for entering Shutdown mode
PWR_ON FLASH_ON
CMD_REG
0
TCH_ON
NTC_ON
TCHV_H
CHRG
MONTR
N/A
x
x
x
x
0
0
x
MSB
5.2
LSB
Monitoring mode
The super-capacitor voltage is monitored by a comparator in this mode. The comparator is
the only analog circuit which is enabled and that is why the consumption of the STCF04 is
minimized in this mode. Information about the super-capacitor voltage is given by the logic
level on the READY pin. If the voltage is higher than VDCTHRESHOLD, the READY pin is low.
When the voltage falls by 200 mV below the VDCTHRESHOLD, the READY pin goes high. The
level of the VDCTHRESHOLD can be set by the VDC_0 and VDC_1 bits in the FEATURE
register.
Table 4.
VDCTHRESHOLD voltage setup
VDC_1
VDC_0
VDCTHRESHOLD
0
0
4.5 V
0
1
5.0 V
1
0
5.5 V
Monitoring mode can be entered from Shutdown mode by setting the MONTR bit in the
COMMAND register to 1.
Table 5.
COMMAND register data for entering Monitoring mode
PWR_ON FLASH_ON
CMD_REG
0
x
TCH_ON
NTC_ON
TCHV_H
CHRG
MONTR
N/A
x
x
x
x
1
0
MSB
LSB
Doc ID 17553 Rev 2
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Operation modes
AN3223
Note:
Monitoring mode can be also entered from Idle mode, but the device has a greater power
consumption in this case.
5.3
Idle mode
Idle mode allows accessing of 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 Idle
mode.
Table 6.
COMMAND register data for entering Idle mode
PWR_ON FLASH_ON
CMD_REG
1
0
TCH_ON
NTC_ON
TCHV_H
CHRG
MONTR
N/A
0
x
x
0
0
0
MSB
5.4
LSB
NTC feature
The NTC feature can be used in all modes. The NTC is activated automatically in the Flash
and Torch mode regardless of the value of the NTC_ON bit. NTC must be activated
manually in all the other modes.
The NTC feature is activated by setting the NTC_ON bit in the COMMAND register to 1.
Table 7.
COMMAND register data for activation of the NTC feature
PWR_ON FLASH_ON
CMD_REG
x
x
TCH_ON
NTC_ON
TCHV_H
CHRG
MONTR
N/A
x
1
x
x
x
0
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. The bits NTC_W and NTC_H of the STATUS
register are not properly set if there is no external reference voltage connected to the NTC.
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 Flash and Torch mode, leaving the RX pin
floating could lead to unwanted interruptions of the light due to non-defined voltage on the
RX pin.
If the NTC feature is activated and the PWR_ON bit in the COMMAND register is zero, the
bits NTC_W and NTC_H in the STATUS register are not set properly, because the
comparators which determine their values are not enabled in this case. But it is still possible
to measure the voltage on the NTC pin through the A/D converter.
Table 8 summarizes the NTC feature possibilities.
12/29
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AN3223
Operation modes
Table 8.
NTC feature possibilities
Operation mode
Way of activation
Voltage on the Rx-NTC divider
NTC_W, NTC_H bits
Shutdown
Manual
Available
Not set
Monitoring
Manual
Available
Not set
Idle
Manual
Available
Set
Charging
Manual
Available
Set
Flash
Automatic
Available
Set
Torch
Automatic
Available
Set
Aux LED
Manual
Available
Set
5.5
Torch mode
This mode is intended to be used for low light intensities. The LED current in Torch mode
can be adjusted in a range from 15 mA up to 320 mA.
Torch mode is activated by writing the following data to the COMMAND register.
Table 9.
COMMAND register data for entering Torch mode
PWR_ON FLASH_ON
CMD_REG
1
0
TCH_ON
NTC_ON
TCHV_H
CHRG
MONTR
N/A
1
x
x
x
x
0
MSB
LSB
The TORCH DIMMING register value (TDIM) must also be set, unless it has already been
set during a previous operation. If the TDIM register is not set, then the default output
current value is at the minimum.
It is also possible to set the safety timeout for Torch mode through the TTRCH1 and
TTRCH0 bits.
If 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 Shutdown mode, then the TDIM value must be set again during
restart of Torch mode, because entering Shutdown mode clears the TDIM and TTRCH
values.
As soon as Torch mode is activated, the NTC feature is automatically activated too in order
to protect the LED against overheating. The NTC feature is activated even if the NTC_ON bit
in the COMMAND register is set to zero.
5.6
Flash mode
This mode is intended to be used for high light intensities. The LED current in Flash mode
can be adjusted up to 12 A with the input voltage ranging from 2.7 V up to 5.5 V with
recommended external components.
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Operation modes
Table 10.
AN3223
COMMAND register data for entering Flash mode
PWR_ON FLASH_ON
CMD_REG
1
1
TCH_ON
NTC_ON
TCHV_H
CHRG
MONTR
N/A
0
x
x
x
x
0
MSB
LSB
The FLASH register value must also be set.
The activation of Flash mode requires the FLASH pin to be high. Flash mode is active only
when the FLASH_ON bit in the COMMAND register is set to 1 and the FLASH pin is high.
This gives the user the possibility to choose between a soft and a hard triggering of the
Flash mode.
Soft triggering is done by writing data to the internal registers only, while the FLASH pin is
permanently kept high, e.g. 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.
Hard triggering of the Flash mode requires the microcontroller to manage the FLASH pin.
The COMMAND and the FLASH registers are loaded with data before the FLASH pin is set
to high. This allows the user to avoid the I²C bus latency. Flash mode then starts as soon as
the FLASH pin is set to high. It takes typically about 0.3 ms to ramp up the LED current to
the adjusted value. When the FLASH 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 11 µs to inform the microcontroller that the STATUS register was updated
and that the FLASH is over. If the FLASH pin is set to low before the internal timer reaches
zero, Flash mode is interrupted and can be restarted by setting the FLASH pin to high again.
The internal timer is stopped while the FLASH 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 7 shows the
case for FTIM = 17 (130 ms FLASH time). The cumulative time when the FLASH pin is high
is 150 ms (5 pulses 30 ms long), but the last FLASH pulse is only 10 ms long. The reason is
that the internal FLASH timer reaches zero and the FLASH_ON bit is set to 0.
14/29
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AN3223
Figure 7.
Operation modes
Splitting the FLASH pulse into several shorter pulses
AM07801v1
Hard triggering therefore allows a smooth setting of the FLASH duration. The resolution is
about 8.8 µs. The minimum FLASH duration is limited by the ramp-up time of the LED
current and the maximum is limited by the FTIM value.
Note:
When performing multiple flashes, it is necessary to make sure that the super-capacitor
contains enough energy to cover all the required flashes in the burst. If the super-capacitor
voltage falls below 4.2 V during the burst, the internal FLASH timer is stopped and the
device waits until the super-capacitor is recharged to the VDCTHRESHOLD value. Then the
burst can continue. See Figure 8 for more details.
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Operation modes
Figure 8.
AN3223
Burst of flashes with insufficient energy in the super-capacitor
AM07802v1
16/29
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The STATUS register and the ATN pin
6
The STATUS register and the ATN pin
Table 11.
STATUS register bits
Bit name
N/A
F_RUN
FL_R
NTC_W
NTC_H
OT_F
FL_OVR
MSB
LTH
LSB
A detailed description of each bit is given in the STCF04 datasheet.
Table 12.
Effect of the STATUS register bits on the operation of the device
Bit name
F_RUN
STAT_REG
FL_R
NTC_W
STAT_REG STAT_REG
NTC_H
STAT_REG
OT_F
STAT_REG
FL_OVR
STAT_REG
LTH
STAT_REG
Default
value
0
0
0
0
0
0
0
Latched (1)
NO
NO
YES
YES
YES
NO
NO
Forces
Ready mode
when set
NO
NO
NO
YES
YES
NO
NO
Sets ATN
LOW when
set
NO
YES
YES
YES
YES
YES
NO
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 FLASH_ON 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 polled fast enough so as not to miss the 11 µs pulse; e.g., by a programming
loop which is entered after starting Flash mode. This loop runs until the ATN pin gets low. It
is recommended to make a timeout of such a loop.
The ATN pin is an open drain output and an external pull-up resistor should be connected to
it. The ATN pin is capable of sinking a maximum 3 mA current. This is why the minimum
value of the pull-up resistor connected to it should not be lower than 1.8 kΩ.
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READY pin
7
AN3223
READY pin
It is an open drain output, which provides information about the voltage on the supercapacitor. The signal is active low. The behavior of this pin depends on the mode of
operation.
7.1
Function of the READY pin in Monitoring mode and Torch
mode (fixed)
The threshold for the transition from high to low level is defined by the VDCTHRESHOLD
voltage in this case. This voltage is set by the VDC_0 and VDC_1 bits in the FEATURE
register. The comparator works with a fixed hysteresis of 200 mV in this case, so the
transition from low to high level occurs when the voltage of the super-capacitor falls to
VDCTHRESHOLD - 0.2 V.
Figure 9.
READY pin behavior in Monitoring mode and Torch mode (fixed)
VDCTHRESHOLD
200 mV
VSUPERCAP
READY pin level
AM07803v1
7.2
Function of the READY pin in Flash mode
The threshold for the transition from high to low level is defined by the VDCTHRESHOLD
voltage again. Unlike Monitoring mode, the transition from low to high occurs when the
super-capacitor voltage falls to 4.2 V. This threshold is fixed and cannot be changed by any
settings of the registers.
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READY pin
Figure 10. READY pin behavior in Flash mode
VDCTHRESHOLD
VSUPERCAP
4.2 V
READY pin level
AM07804v1
7.3
Function of the READY pin in Torch mode (optimized)
The threshold for the transition from high to low level is 4.2 V in this case. It is a fixed
threshold, which cannot be changed by any settings of the registers. The transition from low
to high occurs when the voltage on the LEDIN pin falls to 300 mV, which is the optimum
value from an efficiency point of view.
Figure 11. READY pin behavior in Torch mode optimized
4.2 V
VSUPERCAP
VFWDLED + 300 mV
READY pin level
AM07805v1
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READY pin
AN3223
The READY pin is an open drain output, which is capable of sinking a maximum 3 mA
current. That is why the minimum value of the pull-up resistor connected to it should not be
lower than 1.8 kΩ.
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8
The light sensor
The light sensor
The light sensor is an optional feature which optimizes the FLASH duration according to the
light conditions in the flashed scene. This feature requires three external components to be
connected to the STCF04 according to Figure 12. It is recommended to connect the
collector of the phototransistor to VREF = 1.8 V. The integrating capacitor CINT is discharged
before every FLASH pulse. This reset takes approximately 200 µs. During the FLASH the
voltage on this capacitor increases according to the amount of light in the scene and TFOTO,
RLIGHT, and CINT parameters. The values of these components must be selected according
to the final application purpose.
Figure 12. Optional light sensor feature
AM07806v1
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Reading and writing to the STCF04 registers through the I²C bus
AN3223
9
Reading and writing to the STCF04 registers through
the I²C bus
9.1
Writing to a single register
Writing to a single register starts with a START bit followed by the 7-bit device address of the
STCF04. The 8th 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 the STCF04. The 8-bit
address of the desired register is sent afterwards to the STCF04. It is also followed by an
acknowledge pulse. The last transmitted byte is the data which is going to be written into the
register. It is followed again by an acknowledge pulse from STCF04. Then the master
generates a STOP bit and the communication is over. See Figure 13 below.
Figure 13. Writing to a single register
DEVICE
ADDRESS
7 bits
S M
T S
A B
R
T
W
R
I
T
E
ADDRESS OF
REGISTER
L R A M
S / C S
B W K B
DATA
L A M
S C S
B K B
L A S
S C T
B K O
P
SDA LINE
AM07807v1
9.2
Writing to multiple registers with incremental addressing
It would not be practical to send the device address and the address of the register when
writing to multiple registers several times. The STCF04 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 14 below.
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Reading and writing to the STCF04 registers through the I²C bus
Figure 14. Writing to multiple register
DEVICE
ADDRESS
7 bits
S M
T S
A B
R
T
W
R
I
T
E
ADDRESS OF
REGISTER i
L R A M
S / C S
B W K B
DATA i
L A M
S C S
B K B
DATA i+1
L A M
S C S
B K B
DATA i+2
DATA i+2
L A M
S C S
B K B
L A M
S C S
B K B
DATA i+n
L A M
S C S
B K B
L A S
S C T
B K O
P
SDA LINE
AM07808v1
9.3
Reading from a single register
The reading operation starts with a START bit followed by the 7-bit device address of the
STCF04. The 8th bit is the R/W-bit, which is 0 in this case. STCF04 confirms receipt of the
address + R/W bit by an acknowledge pulse. The address of the register which should be
read is sent and confirmed by an acknowledge pulse from the STCF04 again. Then the
master generates a START bit again and sends the device address followed by the R/W bit,
which is now 1. The STCF04 confirms receipt 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 Figure 15 below.
Figure 15. Reading from a single register
DEVICE
ADDRESS
7 bits
S M
T S
A B
R
T
W
R
I
T
E
L R A M
S / C S
B WK B
ADDRESS
OF
REGISTER
DEVICE
ADDRESS
7 bits
L A S
S C T
B K A
R
T
R
E
A
D
R A
/ C
WK
DATA
L N S
S O T
B
O
A P
C
K
SDA LINE
AM07809v1
9.4
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
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Reading and writing to the STCF04 registers through the I²C bus
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reading from the next register can start immediately without having to once again send the
device and the register addresses. The last acknowledge pulse before the STOP-bit is not
required. See Figure 16 below.
Figure 16. Reading from multiple registers
DEVICE
ADDRESS
7 bits
S M
T S
A B
R
T
W
R
I
T
E
L R A M
S / C S
B W K B
DEVICE
ADDRESS
7 bits
ADDRESS OF
REGISTER i
L A S
S C T
B K A
R
T
R
E
A
D
R A
/ C
W K
DATA i
DATA i+1
L A M
S C S
B K B
DATA i+2
L A M
S C S
B K B
DATA i+2
L A M
S C S
B K B
DATA i+n
L A M
S C S
B K B
L N S
S O T
O
B
A P
C
K
SDA LINE
AM07810v1
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Examples of register setup for each mode
10
Examples of register setup for each mode
10.1
Example 1: 10 A FLASH with 30 ms duration
The value of FDIM (3 bits) must be set to 0x7. The value of FTIM (5 bits) must be set to 0x6.
Bit PWR_ON must be set to 1.
Bit FLASH_ON must be set to 1.
Bit TCH_ON must be set to 0.
Bit NTC_ON can be set to any value, because NTC is automatically ON when Flash mode is
active. Setting this bit to 0 does not switch off the NTC.
Table 13.
COMMAND register data for entering Flash mode
PWR_ON FLASH_ON
CMD_REG
1
TCH_ON
NTC_ON
TCHV_H
CHRG
MONTR
N/A
0
0
0
0
0
0
1
MSB
Table 14.
LSB
FLASH register data
FL_REG
FTIM_4
FTIM_3
FTIM_2
FTIM_1
FTIM_0
FDIM_2
FDIM_1
FDIM_0
0
0
1
1
0
1
1
1
MSB
LSB
It is necessary to write 4 bytes to the STCF04 to make a FLASH.
Table 15.
I²C data packet for activation of Flash mode
Byte
Hex
1
62
0
1
1
0
0
0
0
0
Device address + R/W bit
2
00
0
0
0
0
0
0
0
0
COMMAND register address
3
C1
1
1
0
0
0
0
0
0
Data of the COMMAND register
4
37
0
0
1
1
0
1
1
1
Data of the FLASH register
10.2
Binary
Comment
Example 2: 60 mA Torch with 10 s duration
The value of TDIM (4 bits) must be set to 0x4 to setup the current source to 60 mA.
Bit PWR_ON must be set to 1.
Bit FLASH_ON must be set to 0.
Bit TCH_ON must be set to 1.
Bit NTC_ON 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.
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Examples of register setup for each mode
Table 16.
AN3223
COMMAND register data for entering Torch mode
PWR_ON FLASH_ON
CMD_REG
1
TCH_ON
NTC_ON
TCHV_H
CHRG
MONTR
N/A
1
0
0
0
0
0
0
MSB
Table 17.
LSB
TORCH register data
DIM_REG
TTRCH1
TTRCH0
TDIM_3
TDIM_2
TDIM_1
TDIM_0
N/A
N/A
1
0
0
1
0
0
0
0
MSB
LSB
The following packet sets the TORCH register.
Table 18.
I²C data packet for setting the TORCH register
Byte
Hex
Binary
Comment
1
60
0
1
1
0
0
0
0
0
Device address + R/W bit
2
05
0
0
0
0
0
1
0
1
TORCH register address
3
90
1
0
0
1
0
0
0
0
Data of the TORCH register
The following packet sets the COMMAND register.
Table 19.
I²C data packet for setting the COMMAND register
Byte
Hex
Binary
Comment
1
60
0
1
1
0
0
0
0
0
Device address + R/W-bit
2
00
0
0
0
0
0
0
0
0
COMMAND register address
3
A0
1
0
1
0
0
0
0
0
Data of the COMMAND register
The TORCH pin must be high to enter Torch mode.
10.3
Example 3: An Auxiliary LED running at 10 mA for 500 ms
The auxiliary LED can be activated from Idle mode only.
A 10 mA output current is reached when AUXI is set to 0x2.
AUXT must be set to 0x5 to have 500 ms duration of the auxiliary LED lighting.
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Examples of register setup for each mode
Table 20.
COMMAND register data for the AUX_LED
PWR_ON FLASH_ON
CMD_REG
1
TCH_ON
NTC_ON
TCHV_H
CHRG
MONTR
N/A
0
0
0
0
0
0
0
MSB
Table 21.
LSB
AUX_LED register data
AUX_LED
AUXI_3
AUXI_2
AUXI_1
AUXI_0
AUXT_3
AUXT_2
AUXT_1
AUXT_0
0
0
1
0
0
1
0
1
MSB
LSB
Writing the 3 bytes in Table 22 to STCF04 puts it into Idle mode. This can be skipped if it is
already in Idle mode.
Table 22.
I²C data packet for activating Idle mode
Byte
Hex
Binary
Comment
1
60
0
1
1
0
0
0
0
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 the STCF04 activates the auxiliary LED for the desired time.
Table 23.
I²C data packet for activating the AUX_LED
Byte
Hex
Binary
Comment
1
60
0
1
1
0
0
0
0
0
Device address + R/W bit
2
02
0
0
0
0
0
0
1
0
Auxiliary LED register address
3
25
0
0
1
0
0
1
0
1
Data of the Auxiliary LED register
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Revision history
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11
Revision history
Table 24.
Document revision history
Date
Revision
27-Aug-2010
1
Initial release.
13-Feb-2012
2
Modified title in cover page.
Removed references to part number STCS44.
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Changes
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