DEMO CIRCUIT 1036
LT3477
QUICK START GUIDE
LT3477
Multiple-string LED Driver with Wide
PWM Dimming
WARNING!
DO NOT LOOK AT OPERATING LED.
This circuit produces light that can damage eyes.
DESCRIPTION
Demonstration circuit 1036 is a M ultiple-string LED
driver w ith w ide PW M dimming featuring the LT3477.
The demo circuit boosts VIN, betw een 8V and 16V, to the
required VO U T in order to generate a total of 330mA output current for a load of up to 3 LED strings. The 330mA
output current is evenly shared among the LED strings.
In the case of 3 LED strings, the nominal current in each
string w ill be 110mA. If one or more LED strings are
open, the current in the remaining LED string(s) w ill be
reduced to allow leaping-home operation. If any LED
string is open, the output voltage w ill be limited to 29V
by the demo circuit, protecting the circuit from over voltage damage. U sing current mirror ballast offers high efficiency and good current matching. The demo circuit operates at 2M H z, offering small solution size w hile avoiding the frequency band of the AM radio. Pulling the
SH DN terminal to ground (G ND) w ill disable the LT3477.
Fast LED current turn–on and –off is made possible by
adding D1, Q 1 and Q 2. This allow s a w ide PW M dimming
ratio. PW M dimming offers better LED light output integrity than DC dimming w ith the same dimming ratio. In
another w ord, less color shift. A 300H z PW M frequency
w ith 1% dimming ratio can easily be achieved w ith this
demo circuit.
O ther current levels and/or load configurations can be
achieved w ith a few simple modifications. Please consult
the datasheet or the factory for customization details.
The LT3477 datasheet gives a complete description of
the part, operation and application information. The datasheet must be read in conjunction w ith this quick start
guide for w orking on or modifying the demo circuit 1036.
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 TA = 25°C
SYM BO L
VIN
VPW M
Fs
N
M **
IOUT
VOPEN
η
PAR AM ETER
Input Supply Range
PW M Voltage
Sw itching Frequency
Number of LED Strings
Number of LEDs per String
Total Output Current
Open Circuit Output Voltage
Efficiency
FPW M
PW M Signal Frequency
CO NDITION S
M IN
8
4.5
TYP
12
5
2
M AX
16
5.5
3
UN ITS
V
V
M Hz
N/A
6
330
29
81
No Load
VIN = 12V
3 LED strings, 7 LEDs per string
100
mA
V
%
300
Hz
1
LT3477
PERFORMANCE SUMMARY
Specifications are at TA = 25°C
SYM BO L PAR AM ETER
RatioPW M PW M Dimming Ratio
CO NDITION S
M IN
1
TYP
M AX
100
UN ITS
%
**:The minimum number of LEDs per string is limited by the minimum duty cycle of the LT3477 at VIN=16V and Fs=2M Hz
QUICK START PROCEDURE
Demonstration circuit 1036 is easy to set up to evaluate
the performance of the LT3477. Refer 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 VO U T and GND terminals. See Figure 2 for proper scope probe
technique.
1.
W ith pow er off, connect the input pow er supply to VIN
and G ND.
2.
Connect the PW M turret to a 5V supply.
3.
Connect up to three (3) LED strings betw een LED+
and LED1- to LED3- respectively.
2
4.
Turn on the pow er at the input.
NOTE. M ake sure that the input voltage does not exceed 16V.
5.
Check for the proper voltages and currents.
NOTE. If the LEDs do not light up, check all the connections.
6.
O nce the proper operations are established, adjust the
input w ithin the operating range and observe the parameters of interest.
7.
To test PW M dimming, apply a PW M signal to the
PW M turret and observe the relationship betw een the
LED mean current and the dimming ratio. The PW M
signal should have a peak of less than 5V.
LT3477
Figure 1. Proper M easurem ent Equipm ent Setup
GND
VIN
Figure 2. M easuring Input or Output Ripple
3
LT3477
4