ST AN2730 Application note

AN2730

Application note

High power LED driver for single Flash with I²C interface

Introduction

This application note is dedicated to the design of a Flash LED driver using the STCF06 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.

Figure 1. STCF06 demonstration board

C interface. The schematic,

April 2008 Rev 1 1/28

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Contents AN2730

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 D1 selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.5 RFL selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.6 RTR selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.7 NTC AND RX resistor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

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 55 mm2 for BGA package . . . . . 10

4 Internal registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.1 Accessing the internal registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5 Operation modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

5.1 Shutdown mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

5.2 Shutdown mode with the NTC feature activated . . . . . . . . . . . . . . . . . . . 14

5.3 Ready mode and NTC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

5.4 Torch mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

5.5 Flash mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

6 The status register and the ATN pin . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

6.1 The status register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

7 Reading and writing to the STCF06 registers through the I2C bus . . 19

7.1 Writing to a single register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

7.2 Writing to multiple registers with incremental addressing . . . . . . . . . . . . 19

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AN2730 Contents

7.3 Reading from a single register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

7.4 Reading from multiple registers with incremental addressing . . . . . . . . . 20

8 Examples of register setup for each mode . . . . . . . . . . . . . . . . . . . . . . 21

8.1 Example 1: 1066 mA Flash with 200 ms duration . . . . . . . . . . . . . . . . . . 21

8.2 Example 2: 48 mA torch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

8.3 Example 3: an auxiliary LED running at 10 mA for 500 ms . . . . . . . . . . . 23

8.4 Example 4: red-eye reduction (multiple short flashes) . . . . . . . . . . . . . . . 24

8.5 Example 5: a Flash pulse longer than 1.5 s . . . . . . . . . . . . . . . . . . . . . . . 25

9 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

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List of tables AN2730

List of tables

Table 1. Recommended components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Table 2. Accessibility of internal registers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Table 3. COMMAND register data to enter shutdown mode (version A) . . . . . . . . . . . . . . . . . . . . . 14

Table 4. COMMAND register data to enter shutdown mode (version B) . . . . . . . . . . . . . . . . . . . . . 14

Table 5. COMMAND register data to enter shutdown mode with NTC activated . . . . . . . . . . . . . . . 14

Table 6. COMMAND register data to enter ready mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Table 7. COMMAND register data to enter ready mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Table 8. COMMAND register data to enter torch mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Table 9. COMMAND register data to enter Flash mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Table 10. STATUS register bits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Table 11. Effect of the status register bits on the operation of the device . . . . . . . . . . . . . . . . . . . . . 18

Table 12. Torch mode and Flash mode DIMMING registers settings . . . . . . . . . . . . . . . . . . . . . . . . . 21

Table 13. COMMAND register data to enter Flash mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Table 14. DIMMING register data for the Flash mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Table 15. I

Table 16. COMMAND register data for the torch mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Table 17. DIMMING register data for the torch mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Table 18. I Table 19. I

Table 20. COMMAND register data for the AUX_LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Table 21. AUX_LED register data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Table 22. I Table 23. I

Table 24. COMMAND register data for Flash mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Table 25. DIMMING register data for Flash mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Table 26. I Table 27. I Table 28. 1 Table 29. 2 Table 30. 3

Table 31. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

C data packet for activating the Flash mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

C data packet for activate torch mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

C data packet for terminating the torch mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

C data packet for activating the ready mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

C data packet for activating the AUX_LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

C data packet for activating the Flash mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

st I2

C data packet to restart the Flash mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

nd I2

C data packet to restart of the Flash mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

rd I2

C data packet to restart of the Flash mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

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AN2730 List of figures

List of figures

Figure 1. STCF06 demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Figure 2. A typical application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 3. Top layer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 4. Middle layer 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 5. Middle layer 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 6. Bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 7. Top overlay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 8. Splitting the Flash pulse into several shorter pulses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 9. Writing to a single register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Figure 10. Writing to multiple registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Figure 11. Reading from a single register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Figure 12. Reading from multiple registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Figure 13. Multiple flashes handled by the TRIG pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Figure 14. I

C bus packets timing for a Flash lasting longer than FTIM max . . . . . . . . . . . . . . . . . . . 27

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Schematic description AN2730

1 Schematic description

The Flash LED driver STCF06 has a high operational frequency (1.8 MHz) which allows the usage of small external components.

1.1 Application schematic

Figure 2. A typical application schematic

(Note 2)

STCF06

(Note 1)

(Note 2)

Note 1: connect to V1, or GND or SDA or SCL to choose one of the 4 different I²C slave addresses

Note 2: optional components to support auxiliary functions

(Note 2)

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AN2730 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 2x10 µF/6.3 V as a minimum value for the input capacitor, and

4.7 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 (2.7 V) to the maximum ratings of output power.

Note: see recommended components in Tab le 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 R

DCL=RD

resistance of the LED.

For nominal operation, the peak inductor current can be calculated by the following formula:

/10 [ Ω; Ω ], where RD is the dynamic

Equation 1

PEAK

= ((I

/ η) + ((V

OUT

– VIN) x VIN ²) / (2 x L x f x V

OUT

Where:

● I

● η: Efficiency of the STCF06

● V

● L: Inductance value of the inductor

● f: Switching frequency

: Peak inductor current

PEAK

: Current sourced at the V

OUT

: Output voltage at the V

OUT

: Input voltage at the V

BAT

- pin, this is the LED current

OUT

- pin

OUT

- pin

Note: see recommended components in Tab le 1 .

2.3 LED selection

All LEDs with a forward voltage range ranging from 2.7 V to 5 V are compatible with STCF06. 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 maximum level of the LED current in Flash mode and Torch mode by setting the values of the corresponding sensing resistors, lower current values can be then set through the I LED with forward voltage as low as possible to get the best performance in terms of current consumption and efficiency

²)) x V

OUT

C port. It is strongly recommended to use the

OUT

/ V

Note: see recommended components in Tab le 1 .

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Selection of external components AN2730

2.4 D1 selection

All the Schottky diodes are compatible with the STCF06. It is recommended to use the Schottky diode with forward voltage as low as possible to get the best performance of the application in terms of input current consumption and efficiency of the application.

Note: see recommended components in Tab le 1 .

2.5 RFL selection

The value of the RFL resistor can be calculated by the following equations:

Equation 2

Equation 3

FL=VFB2/IFLASH(max)

RFLASH=RFL*IFLASH

where V

= 227 mV

FB2

where P

is the power dissipated on the RFL resistor. It is recommended to use a thin

RFL

metal film resistor in the 0606 package size and 1% tolerance. The maximum LED current in Flash mode for STCF06 is 1.5 A for a battery voltage ranging from 3.5 V to 5.5 V, 1.3 A from 3 V to 3.5 V and 1 A from 2.7 V to 3 V.

2.6 RTR selection

The value of the RTR resistor can be calculated by the following equations:

Equation 4

and

Equation 5

where P

RTORCH

equation R

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 370 mA for a battery voltage ranging from 2.7 V to

5.5 V.

is the power dissipated on the RTR resistor. It is recommended to follow the

=6.66* RFL to avoid any jump in the current DIMMING values.

= (V

-I

)/I

2.7 NTC AND RX resistor selection

STCF06 supports the usage of a negative thermistor (NTC) for sensing the LED temperature. In order to implement this functionality, the R reference are needed too. Once the NTC feature is activated, the internal switch connects the R

resistor to the NTC, and this creates a voltage divider supplied by the external

reference voltage connected to the NTC.

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resistor and an external voltage

AN2730 Selection of external components

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 R

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 R

and the NTC values in Tab le 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.

Table 1. Recommended components

Component Manufacturer Part number Value Size

L TDK VLF4014AT-1R5M1R8-1 1.5 µH 3.7 x 3.5 x 1.4 mm

NTC Murata NCP21WF104J03RA 100 kΩ 0805

LED Luxeon LED LXCL-PWF1 0805

D1 STMicroelectronics STPS1L20MF 3.8x1.9x0.85 mm

(1)

1. When the minimum application area is targeted, the BAT20J should be used, then the output current performance is decreased.

TDK C1608X5R0J106M 10 µF 0603 TDK C1608X5R0J475M 4.7 µF 0603

Tyco RL73K1JR15JTD 0.15 Ω 0603

Rohm CRG0805F1R0 1 Ω 0805

Rohm MCR01MZPJ15K 15 kΩ 0402

STMicroelectronics BAT20J-FILM 2.7x1.3x1.1 mm

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