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/
www.st.com
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/
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