cd00087280

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/
www.st.com
Contents
AN2304
Contents
1
Schematic description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1
2
3
Application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
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
PCB design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1
PCB design rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4
Safety shutdown in flash mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2/
AN2304
1
Schematic description
Schematic description
The Flash LED driver is based on STCF02 device with all necessary external components.
High operational frequency (1.8MHz) small size external component usage, resulting a very
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.
1.1
Truth table selection mode
ILED_MAX
EN1
EN2
EN3
ILED
0
0
x
0
0
Shutdown
1
0
1
0
0
Shutdown+NTC
0
1
1
1
x
1
ITORCH= 160mV/(RTORCH+RFLASH)
250
Torch+NTC
1
0
0
IMFLASH = f(IFLASH, RMFLASH)
500
Medium Flash+NTC
1
1
0
IFLASH = 160mV/RFLASH
600
Flash+NTC
(mA)
MODE
Application schematic
Figure 1.
Typical application schematic
3/
Schematic description
Figure 2.
AN2304
STCF02 Demo board electric schematic
4
Table 2.
Symbol
STCF02 Demo board BOM List
Parameter
Type
Supplier & part
number
Typ. value
Min
Typ
Unit
Max
RFLASH
Sensing flash
current resistor
Thick film type
(0805)
ROHM
MCR10EZHFLOR27
0.27
Ohm
RTORCH
Sensing torch
current resistor
Thick film type
(0603)
ROHM
MCR03EZPJ6R20
6.2
Ohm
RMF
Setting Resistor for Thick film type
middle flash mode (0402)
ROHM
MCR01MZSF1K10
1100
Ohm
CIN
Input filtering
capacitor
Ceramic type
(0603) 6.3 V
TDK
C1608X5ROJ106MT
10
COUT
Output capacitor
Ceramic type
(0603) 6.3 V
TDK
C1608X5ROJ106MT
10
4/
2x10
µF
µF
AN2304
Table 2.
Symbol
L
Selection of external components
STCF02 Demo board BOM List (continued)
Parameter
4.5(L) x 4.7 (W) x
1.4 (L)
Type
Supplier & part
number
Low profile inductor
shielded
DCR
TDK VFL5014A-4R7
Isat
Typ. value
Min
Typ
Unit
Max
4.7
µF
0.12
Ohm
1.7
A
R1
Anti bump resistor
Thick film type
(0402)
ROHM
MCR03EZPJ10KR00
10
KOhm
R2
Anti bump resistor
Thick film type
(0402)
ROHM
MCR03EZPJ10KR00
10
KOhm
R3
Anti bump resistor
Thick film type
(0402)
ROHM
MCR03EZPJ10KR00
10
KOhm
R4
Resistor
Thick film type
(0402)
ROHM
MCR03EZPJ1R00
0
Ohm
R7
Safety shutdown
resistor
Thick film type
(0402)
ROHM
MCR03EZPJ1MR00
1
MOhm
C1
Filtering Capacitor
Ceramic (0603)
TDK C1608C0J1H470J
47
pF
C2
Safety shutdown
capacitor
Ceramic type
(0603) 6.3 V
TDK
C1608X5ROJ105MT
1
µF
D1
Schottky diode
STPS1L40M
D2
Schottky diode
STPS1L40M
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 recommended to use a ceramic capacitor with low 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 minimum value 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 RDCL<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 OUT ( V OUT – V in ) • V IN ⎞ V OUT
I PEAK = ⎜ ----------- + ----------------------------------------------------⎟ • -------------V IN
2• L• F• V
2
⎝ n
OUT ⎠
Where:
–
IPEAK Peak inductor current
–
IOUT Current sourced at the VOUT pin
–
n Efficiency of the STCF02
–
VOUT Output voltage at the pin Vout
–
VIN Input voltage at the pin Vin
–
L Inductance value of the inductor
–
F Switching frequency
Note:
See recommended components in Table 2.
2.3
LED selection
Any LED with forward voltage from 2.7 V to 5 V is feasible for use with device STCF02. 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
RFLASH selection
RFLASH resistor can be selected by equations RFLASH=160mV/IFLASH and
PRFLASH=RFLASH*IFLASH2, where PRFLASH is the dissipated power on RFLASH resistor. It is
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
RTORCH selection
RTORCH resistor can be selected by equations:
Equation 2
160mV – ( I TORCH • R FLASH )
R TORCH = ----------------------------------------------------------------------------I TORCH
Equation 3
P RTORCH = R TORCH • I
6/
2
TORCH
AN2304
Selection of external components
where PRTORCH is the dissipated power on RTORCH resistor. It is recommended to use thick
metal film resistor with 1% or 5% tolerance. Maximum torch LED current for 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 RMF resistor is defined by equation:
Equation 4
–5
13000 • R MF⎞
10
I MF = ----------- ⎛ 3000 + --------------------------------R FL ⎝
13000 + R MF⎠
where IMF is middle flash LED current. It is recommended to use thick film resistor in 0402
package size.
2.7
CCOMP selection
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.
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 power 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 defined area of copper on the PCB. Use the reference guide of each
LED to design the heatsink. Place the RFLASH resistor as close as possible 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 output current. The Led current should not flow through
this track! Sensing of the voltage 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 Rtorch resistor using this formula:
Equation 5
16mV
I TORCH = ------------------------------------------------R TORCH + R FLASH
8/
AN2304
Safety shutdown in flash mode
●
Medium Flash mode selection: pushing the Medium Flash button, we will have 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
10
I MF = --------------------------------------------------------------------------------13000 • R MF
⎛
⎞
--------------------------------R FL • 3000 +
⎝ 13000 + R MF⎠
●
Full Flash mode selection: pushing the Full Flash button, we will have 600 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 600 mA of full Flash current
calculating the Rflash resistor using this formula:
Equation 7
160mVI FLASH = -------------------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 stuck in a high 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 goes off and stays off waiting for EN1/EN2 pin resuming
at low level.
After the microcontroller reset, both EN pins will go at low level (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
Figure 4.
Safety shutdown RC circuit
Figure 5.
Safety shut down graphics
AN2304
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
5
Revision history
Revision history
Table 3.
Revision history
Date
Revision
26-Jan-2006
1
04-Apr-2006
2
25-May-2006
3
06-Feb-2007
4
Changes
First issue
- Pictures changed
- New values added in table “Bill of Material“
- Pictures changed
- Pictures changed
- BOM changed
- Minor text changes
11/
AN2304
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