ST AN2304 Application note

AN2304
Application note
LED driver for single flash
Introduction
This application note is dedicated to the design of a flash LED driver using STCF02, which is a step-up/down current mode converter. Schematic and functional description, recommendations for PCB Layout and external components selection are also evaluated. This device is dedicated for driving a single LED with forward voltage range 2.7 to 5 V.
Photo of reference design board
February 2007 Rev 4 1/
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Contents AN2304
Contents
1 Schematic description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Selection of external components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1 Input and output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3 LED selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.4 RFLASH selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.5 RTORCH selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.6 RMF selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.7 CCOMP selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.8 NTC and Rx resistor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3 PCB design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1 PCB design rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4 Safety shutdown in flash mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2/
AN2304 Schematic description

1 Schematic description

The Flash LED driver is based on STCF02 device with all necessary external components. High operational frequency (1.8MHz) small size e xte rnal component usage, re sulting a v e ry compact design of the Flash LED device. The device can operate in five modes:
Shutdown mode
Shutdown mode with activated NTC LED temperature sensing
Torch mode
Middle flash mode
Flash mode
These modes can be selected by a combination of logic signals connected to enable pins.
Table 1. Truth table selection mode
EN1 EN2 EN3 ILED
00x 0 0 Shutdown 101 0 0 Shutdown+NTC 0
1
1
1 100 I 110 I
x 1
I
TORCH
MFLASH
= 160mV/(R
= f(I
FLASH

1.1 Application schematic

Figure 1. Typical application schematic
TORCH+RFLASH
, R
FLASH
= 160mV/R
MFLASH
FLASH
ILED_MAX
(mA)
) 250 Torch+NTC
) 500 Medium Flash+NTC
600 Flash+NTC
MODE
3/
Schematic description AN2304
Figure 2. STCF02 Demo board electric schematic
Table 2. STCF02 Demo board BOM List
Symbol Parameter Type
R
FLASH
R
TORCH
R
MF
C
IN
C
OUT
Sensing flash current resistor
Sensing torch current resistor
Setting Resistor for middle flash mode
Input filtering capacitor
Output capacitor
Thick film type (0805)
Thick film type (0603)
Thick film type (0402)
Ceramic type (0603) 6.3 V
Ceramic type (0603) 6.3 V
Supplier & part
number
ROHM MCR10EZHFLOR27
ROHM MCR03EZPJ6R20
ROHM MCR01MZSF1K10
TDK C1608X5ROJ106MT
TDK C1608X5ROJ106MT
4
Typ. value Unit
Min Typ Max
0.27 Ohm
6.2 Ohm
1100 Ohm
10 2x10 µF
10 µF
4/
AN2304 Selection of external components
Table 2. STCF02 Demo board BOM List (continued)
Symbol Parameter Type
Low profile inductor
L
R1 Anti bump resistor
R2 Anti bump resistor
R3 Anti bump resistor
R4 Resistor
R7
C1 Filtering Capacitor Ceramic (0603) TDK C1608C0J1H470J 47 pF
C2
D1 Schottky diode STPS1L40M D2 Schottky diode STPS1L40M
4.5(L) x 4.7 (W) x
1.4 (L)
Safety shutdown resistor
Safety shutdown capacitor
shielded DCR 0.12 Ohm Isat 1.7 A
Thick film type (0402)
Thick film type (0402)
Thick film type (0402)
Thick film type (0402)
Thick film type (0402)
Ceramic type (0603) 6.3 V
Supplier & part
number
TDK VFL5014A-4R7
ROHM MCR03EZPJ10KR00
ROHM MCR03EZPJ10KR00
ROHM MCR03EZPJ10KR00
ROHM MCR03EZPJ1R00
ROHM MCR03EZPJ1MR00
TDK C1608X5ROJ105MT
Typ. value Unit
Min Typ Max
4.7 µF
10 KOhm
10 KOhm
10 KOhm
0Ohm
1MOhm
F
LED Luxeon LED PWF1 LXCL-PW1

2 Selection of external components

2.1 Input and output capacitor selection

For input and output capacitors it is recommend ed to use a cer amic capacitor with lo w ESR. For a good stability of the device supplied by a low input voltage of 2.7 V at maximum ratings, it is recommended to use 10 uF/6. 3 V as a minim um v a lue of input capacitor and 10 uF/6.3 V as a minimum value of output capacitor.
Note: See recommended components in Table 2.

2.2 Inductor selection

Shielded thick inductor with low DC series resistance of wiring is recommended for this application. For good efficiency it is recommended to use an inductor with series DC resistance R
DCL<RD
/10, [Ω, Ω,1] where RD is dynamic resistance of LED.
5/
Selection of external components AN2304
For nominal operation, the peak inductor current can be calculated by this formula:
Equation 1
2
I
PEAK
⎛⎞
I
----------- -
⎜⎟ ⎝⎠
OUT
n
V
()V
OUTVin
--------------------------------------------------- -+
2L F V
OUT
IN
V
OUT
--------------
=
V
2
IN
Where:
–I –I
Peak inductor current
PEAK
Current sourced at the VOUT pin
OUT
n Efficiency of the STCF02 –V –V
Output voltage at the pin Vout
OUT
Input voltage at the pin Vin
IN
L Inductance value of the inductor – F Switching frequency
Note: See recommended components in Table 2.

2.3 LED selection

Any LED with forw ard voltage from 2.7 V to 5 V is f easib le for use with device STCF 02. LED forward voltage must include the voltage spread of this value. It is possible to set the LED current in the three different operating modes (torch, medium flash, high flash) through three external sensing resistors.
Note: See recommended components in Table 2.
2.4 R
R P
FLASH
FLASH RFLASH=RFLASH*IFLASH
selection
resistor can be selected by equations R
2
, where PRFLASH is the dissipated power on R recommended to use thick metal film resistor 0603 package size with 1% tolerance. Maximum flash LED current for STCF02 device is 600mA in battery voltage range from 2.7 V to 5.5 V.
2.5 R
R
TORCH
TORCH
selection
resistor can be selected by equations:
Equation 2
160mV I
R
TORCH
-----------------------------------------------------------------------------=
=160mV/I
FLASH
()
TORCHRFLASH
I
TORCH
FLASH
and
FLASH
resistor. It is
Equation 3
P
RTORCHRTORCH
6/
=
I2•
TORCH
AN2304 Selection of external components
where P
RTORCH
is the dissipated power on R metal film resistor with 1% or 5% tolerance. Maxim um torch LED current f or STCF02 device is 250 mA in voltage range from 2.7 V to 5.5 V.

2.6 RMF selection

With this resistor it is possible to set the LED current value in middle flash mode. The value of R
resistor is defined by equation:
MF
Equation 4
where I package size.
2.7 C
This component could optionally be used if the error amplifier bandwidth, which is set to 2kHz default, has to be decreased in case of unstable function of driver, which could cause the use of different inductor and output capacitor values. It is recommended to use a ceramic capacitor in 0402 package size.
COMP
is middle flash LED current. It is recommended to use thick film resistor in 0402
MF
selection
I
MF
resistor. It is reco mme nded t o use thick
TORCH
5–
10
⎛⎞
-----------
=
3000
⎝⎠
R
FL
13000 R
--------------------------------- -+
13000 R
+
MF MF

2.8 NTC and Rx resistor selection

The NTC resistor for sensing LED temperature and the Rx resistor create the voltage divider. Output of this divider is compared to the internal voltage reference 1.192 V. When the voltage of the voltage divider output is increased over the reference, the logic will switch off the power circuit.
The maximum current, which could flow through the NTC pin is 1mA. Voltage divider must be supplied from an external voltage source.
7/
PCB design AN2304

3 PCB design

3.1 PCB design rules

STCF02 is a powerful switched device, the PCB must be designed in line with rules for designing switched supplies. It is recommended to use at least four layers PCB to save the area on application PCB. The pow er wirings must be as short as possible and wide, because of large current. Place all external components close to the STCF02. High-energy switched loops should be as small as possible to reduce EMI. Most of LEDs need cooling, which could be done by defi ned area of copper on the PCB. Use t he ref erence guide of each LED to design the heatsink. Place the RFLASH resistor as close as po ssible to pin 8. When the change of PCB layer is needed, use enough vias. Place the NTC resistor as close as possible to LED for good temperature sensing. Direct connection of GND and PGND is needed to achieve correct value of outp ut current. The Led current should not flow through this track! Sensing of the v oltage on the Rflash resistor has to be done with a wire from pin 7 directly connected to Rflash resistor, no current flows through this track. Pin 6 and pin 7 have to be connected on the pin of Rflash resistor. Expose pad has to be connected to the PGND with a track as wide as possible.
Figure 3. PCB layout
+
-
Input supply range = 2.7 V to 5.5 V
Torch mode selection: pushing the Torch button, we will have 25 mA current flowing
through the WLed, no time limit is present. Is possible to reach up to 250 mA of torch current calculating the R
Equation 5
8/
resistor using this formula:
torch
I
TORCH
16mV
------------------------------------------------- -=
R
+
TORCHRFLASH
AN2304 Safety shutdown in flash mode
Medium Flash mode selection: pushing the Medium Flash button, we will ha ve 150 mA
current flowing through the WLed, safety shutdown is active and after maximum 0.5 sec. the WLed will be turned off. Is possible to reach up to 500 mA of medium Flash current calculating the Rmflash resistor using this formula:
Equation 6
5–
I
--------------------------------------------------------------------------------- -=
MF
R
FL
Full Flash mode selection: pushing the Full Flash button, we will have 600 mA current
10
13000 R
⎛⎞
3000
----------------------------------
+
⎝⎠
13000 R
MF
+
MF
flowing through the WLed, safety shutdown is active and after maximum 0.5 sec. the WLed will be turned off. Is possible to reach up to 600 mA of full Flash current calculating the Rflash resistor using this f ormula:
Equation 7
I
FLASH
160mV
-------------------- -=
R
FLASH

4 Safety shutdown in flash mode

Alternatively to NTC temperature sensing and protection, it is possible to properly drive all the STCF02 features with only the EN1 and EN2 pins and include the safety shutdown mode. This feature has to be active only for Mid Flash and Full Flash operating mode because in Torch mode there is no risk of burning the LED because of the low current flowing through it.
The purpose of this specific application is to avoid LED burning if the microcontroller does not work properly (this means EN1 and/or EN2 are pins stu c k in a hig h logic level). If EN2 is stuck at high level while EN1 still in low level, the selected mode is Torch, and the safety shutdown in not needed.
If EN1 is stuck at high level, the STCF02 is in Mid/Full Flash mode and the LED should be protected from long time operation at high peak current. In this condition, after an RC defined time, the LED automatically go es off and sta ys off wait ing fo r EN1/EN2 pin resuming at low level.
After the microcontroller reset, both EN pins will go at low lev el (device enters "All off" mode) and the capacitor is discharged. At this point, the STCF02 is ready to resume the normal operating mode.
In typical operating conditions safety shutdown time is about 500msec. This is possible by applying an RC filter with R=1 MOhm and C=0.47 uF.
9/
Safety shutdown in flash mode AN2304
Figure 4. Safety shutdown RC circuit
Figure 5. Safety shut down graphics
Operation for RC filter R=1M Ohm, C=1.0 uF; VLX1 switching voltage (dark blue), during flash mode with EN1 voltage in high level (light blue). The RX pin voltage (violet) increases and the output current (light green) goes down after one second (1.192 V threshold reached).
10/
AN2304 Revision history

5 Revision history

Table 3. Revision history
Date Revision
26-Jan-2006
04-Apr-2006
25-May-2006
06-Feb-2007
Changes
1
2
3
First issue
- Pictures changed
- New values added in table “Bill of Material“
- Pictures changed
- Pictures changed
4
- BOM changed
- Minor text changes
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AN2304
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