Linear Technology 1315 User manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1315
DUAL STEP-DOWN 1.5A LED DRIVER
1
LT3475-1
DESCRIPTION
Demonstration circuit 1315 features the
LT®3475-1 dual step-down 1.5A LED driver in a
16-lead TSSOP thermally enhanced package.
The demonstration circuit is designed to drive
two 1.5A LED strings (one on each channel)
from a high input voltage. The high switching
frequency scheme permits the use of a small
inductor and ceramic capacitors to save space
and cost. The current mode PWM control
scheme provides fast transient response and
cycle-by-cycle current limiting. DC1315 is
specifically designed for high power LED
applications such as automotive and avionic
lighting, architectural detail lighting and display
backlighting. See the LT3475-1 datasheet for
more detailed information on LED brightness
and dimming control using both the PWM pin
function and the V
ADJ
pin function. The PWM
terminals can be used to dim the LEDs with the
addition of a PWM MOSFET and the resistor-
short connection from LED- to GND removed.
The efficiency of DC1315A with 20V input and 4
white LEDs on each 1.5A channel is 91.5%.
NOTE: The input voltage is limited to 30V during
startup. During normal (non-startup) operation,
the input voltage can be as high as 36V.
DC1315 allows LED string voltages up to 23V.
However, some combinations of VIN, VLED,
and ILED can result in the LT3475-1 getting hot.
Please consult the datasheet for thermal calcu-
lations and details. The voltages, the LED cur-
rent, and the switching frequency can be ad-
justed for optimal thermal performance.
Design files for this circuit board are avail-
able. Call the LTC factory.
LT is a registered trademark of Linear Technology Corporation
QUICK START PROCEDURE
DC1315 is easily set up for evaluating the
LT3475-1 dual step-down 1.5A LED driver
circuit. Follow the test procedure outlined below
and see Figure 1.
1. Connect a bench supply with power OFF
to the V
IN
and GND terminals.
2. Connect a 1.5A LED string with forward
voltage >13V to the LED1+ and LED1-
terminals.
3. Connect a 1.5A LED string with forward
voltage >13V to the LED2+ and LED2-
terminals.
NOTE: The boost circuitry on DC1315 is de-
signed to work best with VLED >13V since
there is a 9V zener diode limiting the boost
voltage. If lower voltage LED strings are re-
quired, please lower the voltage of the zener
diode or use another boost method. Please
see the datasheet for details.
4. Turn on the input power supply and set
the voltage above the LED string voltage
and below 30V.
5. Observe the LEDs working properly at
1.5A without looking straight at the LEDs.
The brightness is harmful to the eye.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1315
DUAL STEP-DOWN 1.5A LED DRIVER
2
Figure 1. Proper Measurement Equipment
Setup
INPUT VOLTAGE RANGE
The operating input voltage range for the LT3475-
1 IC is 4V to 36V with an absolute maximum rat-
ing of 40V. Under some conditions, it is not rec-
ommended to run the IC up to 36V non-transient
DC voltage. Please see the datasheet for input
voltage range calculations based on switching fre-
quency, LED output voltage, and compensation
network topology during startup. As assembled,
the DC1315 should not be started up above 30V
DC input without first assembling the C-R-C com-
pensation network as shown in the datasheet or
using a bigger inductor with up to 3.2A capability
in order to maintain control of the switch current.
PWM DIMMING
DC1315 is assembled to show a combination of
the most basic operation of the IC and the small
solution size. The optional PWM dimming MOS-
FETs (Q1, Q2) are not stuffed on DC1315, but
there are two SOT23 spaces designated for these
to be added to the board. The LED- to GND resis-
tor-short connections (R2, R3) must be removed
from the board to enable PWM dimming when the
PWM dimming MOSFETs are stuffed with their
gate pull-down resistors (R6, R7).
EXTENDED PWM DIMMING
The PWM dimming range can be extended by
adding the components Q3, R13, and C7 to the
circuit as shown in the datasheet. Q3 is located on
the back of the PCB near the PWM terminal. The
datasheet describes the calculation of maximum
PWM dimming range with and without the ex-
tended dimming range components. When these
components are added to the circuit, the PWM1
signal is directly connected to the extended dim-
ming range components. If both channel 1 and
channel 2 are being used in PWM dimming mode
and the extended dimming range circuit is used,
the PWM2 signal must be synchronized with the
PWM1 signal. The rising edges must occur at the
same time although their duty cycles may vary.
Failure to do this may result in significant jitter on
the second channel or limited PWM dimming
range. Please note that a C-R-C compensation
network may also help provide the highest pwm
dimming range as shown in the datasheet.
LED STRINGS
DC1315 is assembled with optimized components
for driving four white LEDs at 1.5A on each of its
two channels from an input voltage of 20V. The
LEDs are not assembled on the PCB and should
be attached to the LED terminals as shown in Fig-
ure 1. If the LEDs are not attached, DC1315 will
protect itself and survive an open LED condition.
If a different number of LEDs is attached in series
to either output, the inductor size, switching fre-
quency, and/or input voltage range may need to
be adjusted for proper operation. Consult the
datasheet for proper sizing of the inductor for
given input and output voltage conditions. The

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1315
DUAL STEP-DOWN 1.5A LED DRIVER
3
maximum LED output voltage is 23V as defined by
the external clamp voltage. This limits the number
of LEDs in series on the output and sets the open
LED maximum output voltage.
Please note the zener diode in the boost path. The
9V zener diode reduces the boost voltage by 9V
from the LED (output) voltage. A minimum of 3V is
required on the boost voltage capacitor for proper
operation. Please consult the datasheet for details
regarding the boost circuit in relation to the input
voltage, output voltage, thermals, and maximum
ratings.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1315
DUAL STEP-DOWN 1.5A LED DRIVER
4
23V
U1
LT3475EFE-1
6
5
1 6
2 1
20
3 1 4 2
17 1
1 8
11 8 10 7 12
1 3
1 5
1 4
V I N
V I N
SH DN
GN D
PW M 1 BOOST1 OU T1 SW 1 LED1 REF VADJ1
V C 1
PW M 2
BOOST2
OU T2
SW 2 LED2
VADJ2
V C 2
GN D
RT
E2
VIN
L1 47uH
E11
LED2-
Q3
OPT
2N7002
R13 1.0M
OPT
E5
PWM2
E1
SH DN
D1
1N4448W
C7
220pF
O P T
R1 1k 1%
UP TO 36V
D2
1N4448W
E7
GND
C4 0.1uF
C2
0.22uF 25V
D8 22V
Q2 OPT
Si2318DS
1.5A
D6 9.1V
COUT1
2.2uF 50V
VC2
Q4
MMBT3904
1
32
R18 100k
R9 OPT
23V
REF
600kHz
R5 OPT
R6 OPT
R15 100k
R16 10k
VC1
R3
0
E8
LED1-
E3
GND
EXTENDED DIMMING RANGE
Q1 OPT
Si2318DS
R14
OPT
Q5
FMMT3904
1
32
OPTIONAL
D5 9.1V
D3
B240A
R2
0
COUT2 2.2uF 50V
VC2
R20
1k 1%
E6
GND
VC1
D4
B24 0A
R4
OPT
CIN1
4.7uF
50V
R17 10k
D7 22V
E10
LED2+
UP TO 1.5A
R7 OPT
L2 47uH
UP TO
R12 24.3k 1%
E4
PWM1
R8
OPT
C3
0.1uF
REF
E9
LED1+
R1
20k
C1
0.22uF
25V

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