DC923A - Demo Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 923
DUAL STEP-DOWN 1.5A LED DRIVER
LT3475
DESCRIPTION
Demonstration circuit 923 features the LT®3475 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 LEDs on
separate channels (one on each channel) from a wide
input voltage range. 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. Frequency foldback and thermal shutdown
provide additional protection. DC923A is specifically
designed for high power LED applications such as
automotive and avionic lighting, architectural detail
lighting and display backlighting. The LT3475 has a
few LED-dimming circuit options. See the datasheet
for more detailed information on LED brightness and
dimming control using both the PWM pin function
and the VADJ pin function. The PWM terminals can be
used to dim the LEDs with the addition of PWM
MOSFETS and the resistor-short connections from
LED- to GND removed.
NOTE: The input voltage is limited to 4-30V during
startup. During normal (non-startup) operation, the
input voltage can be as high as 36V.
Design files for this circuit board are available. Call
the LTC factory.
LT is a registered trademark of Linear Technology Corporation
QUICK START PROCEDURE
DC923A is easily set up for evaluating the LT3475
dual step-down 1.5A LED driver circuit. Follow the
test procedure outlined below and see Figure2.
1. Connect a bench supply with power OFF to
the VIN and GND terminals.
2. Connect a single white 1.5A LED to the LED1+
and LED1- terminals.
3. Connect a single white 1.5A LED to the LED2+
and LED2- terminals.
84
83
VLED1,2 = 3.8V
82
ILED1,2 = 1.5A
FSW = 600KHZ
81
EFFICIENCY (%)
80
79
78
4. Turn on the input power supply and set the
voltage above 6V and below 30V.
77
5. Observe the LEDs working properly at 1.5A
without looking straight at the LEDs. The
extreme brightness may be harmful to the
eye.
74
76
75
5
10
15
20
25
30
35
40
INPUT VOLTAGE (V)
Figure 1. DC923A Efficiency
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 923
DUAL STEP-DOWN 1.5A LED DRIVER
the PWM dimming MOSFETs are stuffed with their gate
pull-down resistors (R6, R7).
For best results, the C-R-C compensation network
shown in Figure 3 should be stuffed for each channel
when PWM dimming is used. The C-R-C network provides the most optimal LED current transient waveform
coming out of PWM dimming off-time. The C-R-C network also helps maintain switch current control during
startup with input voltages above 30V.
EXTENDED PWM DIMMING
Figure 2. Proper Measurement Equipment Setup
INPUT VOLTAGE RANGE
The operating input voltage range for the LT3475 IC is
4V to 36V with an absolute maximum rating of 40V. Under some conditions, it is not recommended to run the
IC up to 36V non-transient DC voltage. Please see the
datasheet for input voltage range calculations based on
switching frequency, LED output voltage, and compensation network topology during startup. As assembled,
the DC923A should not be started up above 30V DC input without first assembling the C-R-C compensation
network shown in Figure 3 or using a bigger inductor
with up to 3.2A capability in order to maintain control of
the switch current.
PWM DIMMING
DC923A is assembled to show a combination of the
most basic operation of the IC and the small solution
size. The optional PWM dimming MOSFETs (Q1, Q2) are
not stuffed on DC923A, but there are two SOT23 spaces
designated for these to be added to the board. The LEDto GND resistor-short connections (R2, R3) must be removed from the board to enable PWM dimming when
The PWM dimming range can be extended by adding the
components Q3, R13, and C7 to the circuit as shown in
Figure 4. 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
extended dimming range components. When these
components are added to the circuit, the PWM1 signal is
directly connected to the extended dimming 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.
LED STRINGS
DC923A is assembled with optimized components for
driving a single white LED at 1.5A on each of its two
channels. The LEDs are not assembled on the PCB and
should be attached to the LED terminals as shown in
Figure 2. If the LEDs are not attached, the DC923A will
protect itself and survive an open LED condition.
If more than one LED is attached in series to either output, the inductor size, switching frequency, 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 maximum LED output voltage is 13.5V as defined by
the clamp voltage in the datasheet. This limits the number of LEDs in series on the output.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 923
DUAL STEP-DOWN 1.5A LED DRIVER
If a single LED such as a red or amber LED with forward voltage below 2.7V is attached to the output, the boost circuit must be altered to supply a boost voltage above 3V from either the input, the other output channel, or from a
separate supply. There is a space on each channel on DC923A to accommodate an alternate boost supply or boost
diode as shown in Figure 5. If the alternate boost supply is not used in this case, the internal power switch will not
run in low VCE saturation and the part may heat up and run inefficiently.
VIN
5V-36V
CIN
4.7uF
50V
20k
PWM1
VIN VIN
SHDN
SHDN
PWM2
LT3475EFE
WHITE
1.5A
10uH
COUT1
2.2uF
PWM1
100Hz
800:1
1N4448W
LED1
LED2
OUT1
OUT2
BOOST1
BOOST2
0.22uF
SW1
Sumida 8D28
B240A
REF
Si2312BDS
VADJ1
C5
0.1uF
2.2uF
VADJ2
R11
VC1
C3
3300pF
600kHz
RT
GND GND
PWM2
Q2
R10
10k
WHITE
COUT2
Si2312BDS
REF
10k
PWM Dimming MOSFET
and Gate Resistor
Sumida 8D28
REF
B240A
1.5A
10uH
SW2
Q1
R6
1N4448W
0.22uF
10k
10k
VC2
C4
3300pF
R7
100Hz
800:1
C6
0.1uF
RT
24.3k
PWM Dimming C-RC Compensation
Figure 3. PWM Dimming MOSFETs, Gate Pull-down Resistors, and Optimal CRC Compensation
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 923
DUAL STEP-DOWN 1.5A LED DRIVER
VIN
5V-36V
CIN
4.7uF
50V
20k
PWM1
VIN VIN
SHDN
SHDN
PWM2
LT3475EFE
WHITE
1.5A
10uH
COUT1
100Hz
1200:1
LED2
OUT1
OUT2
BOOST1
BOOST2
0.22uF
0.22uF
B240A
Sumida 8D28
B240A
REF
VADJ1
10k
0.1uF
R13
VC1
GND GND
3300pF
VC2
COUT2
2.2uF
PWM2
10k
10k
3300pF
100Hz
1200:1
0.1uF
1.0M
Q3
C7
VADJ2
RT
WHITE
Si2312BDS
REF
10k
1.5A
10uH
REF
Si2312BDS
2N7002
1N4448W
SW2
SW1
Sumida 8D28
2.2uF
PWM1
1N4448W
LED1
220pF
600kHz
RT
24.3k
Extended Dimming Range
Figure 4. Extended Dimming Range Components Q3, R13, and C7
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 923
DUAL STEP-DOWN 1.5A LED DRIVER
4V-36V*
3k
CIN
4.7uF
2222A 50V
CMHZ5231
5.1V
4V Boost from VIN Over Wide VIN Range
VIN
20k
VIN VIN
PW M1
OUT2
BOOST1
RED
10uH
COUT1
2.2uF
0.22uF
SW 1
MMSD4148
SW 2
B240A
REF
RED
10uH
Sum ida 8D 28
REF
B240A
1.5A
BOOST2
0.22uF
Sum ida 8D 28
CMHZ5231
5.1V
LED 2
OUT1
MMSD4148
2222A
SH DN
PW M 2
LT3475EFE
LED1
1.5A
3k
SHDN
COUT2
2.2uF
REF
VADJ1
Red Vf from 2.2V to 3.2V
VC1
VADJ2
GNDGND
RT
0.1uF
VC 2
0.1uF
600kHz
RT
24.3k
Figure 5. Single Red LED Driver With Alternate Boost Supply From VIN
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 923
DUAL STEP-DOWN 1.5A LED DRIVER
VIN
8V-36V
Higher String Voltage Must
Remain Under 13.5V
CIN
4.7uF
50V
20k
PWM1
VIN VIN
SHDN
PWM2
LT3475EFE
LED1
1.5A
SHDN
OUT1
WHITE
15uH
WHITE
COUT1
2.2uF
1N4448W
LED2
BOOST1
RED
BOOST2
0.22uF
1N4448W
0.22uF
SW1
Sumida 104R
1.5A
OUT2
15uH
SW2
Sumida 104R
REF
B240A
B240A
REF
RED
COUT2
2.2uF
REF
VADJ1
VADJ2
Bigger L for Higher LED String Voltage
VC1
RT
GND GND
0.1uF
VC2
0.1uF
600kHz
RT
24.3k
Figure 6. LED strings up to 13.5V require larger inductors to avoid subharmonic oscillations at high duty cycles.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 923
DUAL STEP-DOWN 1.5A LED DRIVER
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