LT3491
1
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.
DEMO CIRCUIT
905
QUICK START GUIDE
LT3491
LED Driver in SC70 With Built-
WARNING!
DO NOT LOOK AT OPERATING LED.
This circuit produces light that can damage eyes.
LT3491
2
QUICK START PROCEDURE
Demonstration circuit 905 is easy to set up to evaluate
the performance of the LT3491. R efer to Figure 1 for
proper measurement equipment setup and follow the
procedure below :
NOTE.
W hen measuring the input or output voltage ripple, care must be
taken to avoid a long ground lead on the oscilloscope probe. M easure the
input or output voltage ripple by touching the probe tip directly across the
Vin or Vout and GND terminals. See Figure 2 for proper scope probe
technique.
1.
Place jumpers in the follow ing positions to test the
“SM ALL SIZE” circuit:
ON
OFF
SIZE
SIZE
2.
Place jumpers in the follow ing positions to test the
“HIG H EFFICIENCY” circuit:
OFF
ON
EFFICIEN CY
EFFICIEN CY
3.
Prior to connection, preset the input pow er supply to
the desired voltage (3V to 5V). W ith pow er off, con-
nect the input pow er supply to VIN
and GN D.
4.
Turn on the pow er at the input.
NOTE.
M ake sure that the input voltage does not exceed 5V.
5.
Check for the proper voltages and currents.
NOTE.
If the LEDs do not light up, check all the jumper settings.
6.
Once proper operation is established, adjust the input
w ithin the operating range and observe the parameters
of interest.
7.
To test the filtered PW M dimming or the direct DC
dimming, remove the jumper JP1 or JP2, apply the
PW M or the DC signal to the CTRL1 or CTRL2 and
observe the brightness of the LED output.
8.
To test the direct PW M dimming, follow the Direct
PW M Dimming section in the datasheet.
JP1
JP2
JP3
JP4
JP1
JP2
JP3
JP4
LT3491
3
Figure 1.
Proper M easurem ent Equipm ent Setup
VIN
GND
Figure 2.
M easuring Input or Output Ripple
LT3491
4