ST AN2304 Application note
February 2007 Rev 4 1/
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
www.st.com
Contents AN2304
2/
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
AN2304 Schematic description
3/
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.
1.1 Application schematic
Figure 1. Typical application schematic
Table 1. Truth table selection mode
EN1 EN2 EN3 ILED
ILED_MAX
(mA)
MODE
00x 0 0 Shutdown
101 0 0 Shutdown+NTC
0
1
1
1
x
1
I
TORCH
= 160mV/(R
TORCH
+R
FLASH
) 250 Torch+NTC
100 I
MFLASH
= f(I
FLASH
, R
MFLASH
) 500 Medium Flash+NTC
110 I
FLASH
= 160mV/R
FLASH
600 Flash+NTC
Schematic description AN2304
4/
Figure 2. STCF02 Demo board electric schematic
4
Table 2. STCF02 Demo board BOM List
Symbol Parameter Type
Supplier & part
number
Typ. value Unit
Min Typ Max
R
FLASH
Sensing flash
current resistor
Thick film type
(0805)
ROHM
MCR10EZHFLOR27
0.27 Ohm
R
TORCH
Sensing torch
current resistor
Thick film type
(0603)
ROHM
MCR03EZPJ6R20
6.2 Ohm
R
MF
Setting Resistor for
middle flash mode
Thick film type
(0402)
ROHM
MCR01MZSF1K10
1100 Ohm
C
IN
Input filtering
capacitor
Ceramic type
(0603) 6.3 V
TDK
C1608X5ROJ106MT
10 2x10 µF
C
OUT
Output capacitor
Ceramic type
(0603) 6.3 V
TDK
C1608X5ROJ106MT
10 µF
Loading...