DESCRIPTION
Demonstration circuit 905 is a LED Driver in SC70 W ith
Built-in Schottky featuring the LT3491. The LT3491 is a
fixed frequency step-up current mode DC/DC converter.
There are tw o circuits on demonstration board 905. The
“Small Size” circuit generates 15mA LED current at the
smallest possible solution size. The “High Efficiency”
circuit generates 20mA LED current at higher efficiency.
The circuit outputs share 6 LEDs. At any given time, de-
pending on the positions of jumpers, only one circuit is
in operation. O ther features of the LT3491, such as the
internal schottky diode, the high sw itching frequency and
the internal open circuit protection, allow tiny compo-
nents for both circuits. The input range of the demo
board is from 3V to 5V. Different dimming controls can
be implemented on the demo board. Please refer to the
quick start procedure. The high side current sensing fea-
ture of the LT3491 allow s “one w ire” current source, i.e.
the low side of the LED string can return to ground any-
w here.
The LT3491 datasheet gives a complete description of
the part, operation and application information. The data-
sheet must be read in conjunction w ith this quick start
guide for w orking on or modifying demo circuit 905.
Design files for this circuit board are available. Call
the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corporation. ThinSOT and
Pow erPath are trademarks of Linear Technology Corporation.
PERFORMANCE SUMMARY
Specifications are at T
A
= 25°C
SYM BOL PAR AM ETER CO NDITION S M IN TYP M AX UN ITS
V
IN
Input Supply Range **
3 5 V
I
OUT
Output Current Small Size circuit
High Efficiency circuit
14.25
19
15
20
15.75
21
mA
V
OUT
Output Voltage Small Size circuit
High Efficiency circuit
20.3
20.6
V
η
Efficiency V
IN
= 4.2V, Small Size circuit
V
IN
= 4.2V, High Efficiency circuit
73.6
75.5
%
V
OPEN
CAP pin Over-voltage Protection LED Open 26 27 28 V
I
Q
Quiescent Current V
IN
= 4.2V, CTRL = ON
4 mA
I
QSH DN
Quiescent Current W hile in Shutdow n V
IN
= 4.2V, CTRL = OFF
10 µA
Fs
Sw itching Frequency 1.8 2.3 2.8 M Hz
** The Input Supply Range of demo circuit 905 is limited by the input capacitors. The input voltage absolute maximum
rating of the LT3491 is 12V.
LED Driver in SC70 With Built-
WARNING!
DO NOT LOOK AT OPERATING LED.
This circuit produces light that can damage eyes.
