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PDF LT3475-1 Data sheet ( Hoja de datos )
| Número de pieza | LT3475-1 | |
| Descripción | Dual Step-Down l 1.5A LED Driver | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LT347ww5w/.DLatTaS3he4et47U.c5om-1
Dual Step-Down
1.5A LED Driver
FEATURES
■ True Color PWMTM Delivers Constant Color with
3000:1 Dimming Range
■ Wide Input Range: 4V to 36V Operating, 40V
Maximum
■ Accurate and Adjustable Control of LED Current
from 50mA to 1.5A
■ High Side Current Sense Allows Grounded Cathode
LED Operation
■ Open LED (LT3475) and Short Circuit Protection
■ LT3475-1 Drives LED Strings Up to 25V
■ Accurate and Adjustable 200kHz to 2MHz
Switching Frequency
■ Anti-Phase Switching Reduces Ripple
■ Uses Small Inductors and Ceramic Capacitors
■ Available in the Compact 20-Lead TSSOP Thermally
Enhanced SurfacUe Mount Package
APPLICATIO S
■ Automotive and Avionic Lighting
■ Architectural Detail Lighting
■ Display Backlighting
■ Constant-Current Sources
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners. Patents Pending.
DESCRIPTIO
The LT®3475/LT3475-1 are dual step-down DC/DC
converters designed to operate as a constant-current
source. An internal sense resistor monitors the output
current allowing accurate current regulation ideal for
driving high current LEDs. The high side current sense al-
lows grounded cathode LED operation. High output current
accuracy is maintained over a wide current range, from
50mA to 1.5A, allowing a wide dimming range. Unique
PWM circuitry allows a dimming range of 3000:1, avoid-
ing the color shift normally associated with LED current
dimming.
The high switching frequency offers several advantages,
permitting the use of small inductors and ceramic capaci-
tors. Small inductors combined with the 20 lead TSSOP
surface mount package save space and cost versus
alternative solutions. The constant switching frequency
combined with low-impedance ceramic capacitors result
in low, predictable output ripple.
With its wide input range of 4V to 36V, the LT3475/LT3475-1
regulate a broad array of power sources. A current mode
PWM architecture provides fast transient response and
cycle-by-cycle current limiting. Frequency foldback and
thermal shutdown provide additional protection.
TYPICAL APPLICATIO
Dual Step-Down 1.5A LED Driver
VIN
5V TO 36V
4.7μF
0.22μF
10μH
VIN
BOOST1
SHDN
BOOST2
LT3475
SW1 SW2
0.22μF
10μH
2.2μF
*DIMMING
CONTROL
0.1μF
1.5A LED
CURRENT
OUT1
LED1
OUT2
LED2
PWM1
VC1
REF
VADJ1
GND
PWM2
VC2
RT
VADJ2
DIMMING*
CONTROL
24.3k
0.1μF
1.5A LED
CURRENT
2.2μF
*SEE APPLICATIONS SECTION FOR DETAILS
fSW = 600kHz
3475 TA01
Efficiency
95 VIN = 12V
90
TWO SERIES CONNECTED
WHITE 1.5A LEDS
85
80
SINGLE WHITE 1.5A LED
75
70
65
60
55
0
0.5 1
LED CURRENT (A)
1.5
3475 TA01b
3475fb
1
1 page  
LT347ww5w/.DLatTaS3he4et47U.c5om-1
TYPICAL PERFOR A CE CHARACTERISTICS
Boost Pin Current
35
TA = 25°C
30
25
20
15
10
5
0
0 0.5 1.0 1.5 2.0
SWITCH CURRENT (A)
3475 G10
Reference Voltage
1.28
1.27
1.26
1.25
1.24
1.23
1.22
–50 –25
0 25 50 75
TEMPERATURE (˚C)
100 125
3475 G13
Quiescent Current
7
TA = 25°C
6
5
4
3
2
1
0
0 10
20 30 40
VIN (V)
3475 G11
Minimum Input Voltage, Single
1.5A White LED
6
TA = 25°C
5 TO START
4
TO RUN
3
LED VOLTAGE
2
1
0
0 0.5 1 1.5
LED CURRENT (A)
3475 G14
Open-Circuit Output Voltage and
Input Current
50 TA = 25°C
45
40
INPUT CURRENT
LT3475-1
14
12
35 10
30 LT3475
8
25 LT3475-1
20
6
15
OUTPUT VOLTAGE
4
10 LT3475
2
5
00
0 10 20 30 40
VIN (V)
3475 G12
Minimum Input Voltage, Two Series
Connected 1.5A White LEDs
10
TA = 25°C
9
8 TO START
7
TO RUN
6
LED VOLTAGE
5
0 0.5 1 1.5
LED CURRENT (A)
3475 G15
PI FU CTIO S
OUT1, OUT2 (Pins 1, 10): The OUT pin is the input to the
current sense resistor. Connect this pin to the inductor
and the output capacitor.
LED1, LED2 (Pins 2, 9): The LED pin is the output of
the current sense resistor. Connect the anode of the LED
here.
VIN (Pins 5, 6): The VIN pins supply current to the internal
circuitry and to the internal power switches and must be
locally bypassed.
SW1, SW2 (Pins 4, 7): The SW pin is the output of the
internal power switch. Connect this pin to the inductor,
switching diode and boost capacitor.
BOOST1, BOOST2 (Pins 3, 8): The BOOST pin is used to
provide a drive voltage, higher than the input voltage, to
the internal bipolar NPN power switch.
GND (Pins 15, Exposed Pad Pin 21): Ground. Tie the GND
pin and the exposed pad directly to the ground plane. The
exposed pad metal of the package provides both electrical
contact to ground and good thermal contact to the printed
circuit board. The exposed pad must be soldered to the
circuit board for proper operation. Use a large ground plane
and thermal vias to optimize thermal performance.
3475fb
5
5 Page 
LT347ww5w/.DLatTaS3he4et47U.c5om-1
APPLICATIONS INFORMATION
The current in the inductor is a triangle wave with an average
value equal to the load current. The peak switch current
is equal to the output current plus half the peak-to-peak
inductor ripple current. The LT3475 limits its switch cur-
rent in order to protect itself and the system from overload
faults. Therefore, the maximum output current that the
LT3475 will deliver depends on the switch current limit,
the inductor value, and the input and output voltages.
When the switch is off, the potential across the inductor
is the output voltage plus the catch diode drop. This gives
the peak-to-peak ripple current in the inductor
ΔIL
=
(1– DC)(VOUT
(L • f)
+
VF
)
where f is the switching frequency of the LT3475 and L
is the value of the inductor. The peak inductor and switch
current is
IS W (P K )
=
IL (P K )
=
IOUT
+
ΔIL
2
To maintain output regulation, this peak current must be
less than the LT3475’s switch current limit ILIM. ILIM is at
least 2.3A at low duty cycles and decreases linearly to 1.8A
at DC = 0.9. The maximum output current is a function of
the chosen inductor value:
IO U T (M A X )
=
IL IM
–
ΔIL
2
=
2 . 3A • (1 – 0 . 2 5 •DC)
–
ΔIL
2
Choosing an inductor value so that the ripple current is
small will allow a maximum output current near the switch
current limit.
One approach to choosing the inductor is to start with the
simple rule given above, look at the available inductors,
and choose one to meet cost or space goals. Then use
these equations to check that the LT3475 will be able to
deliver the required output current. Note again that these
equations assume that the inductor current is continu-
ous. Discontinuous operation occurs when IOUT is less
than ΔIL/2.
Input Capacitor Selection
Bypass the input of the LT3475 circuit with a 4.7μF or
higher ceramic capacitor of X7R or X5R type. A lower
value or a less expensive Y5V type will work if there is
additional bypassing provided by bulk electrolytic capaci-
tors or if the input source impedance is low. The following
paragraphs describe the input capacitor considerations in
more detail.
Step-down regulators draw current from the input supply
in pulses with very fast rise and fall times. The input ca-
pacitor is required to reduce the resulting voltage ripple at
the LT3475 input and to force this switching current into a
tight local loop, minimizing EMI. The input capacitor must
have low impedance at the switching frequency to do this
effectively, and it must have an adequate ripple current rat-
ing. With two switchers operating at the same frequency
but with different phases and duty cycles, calculating the
input capacitor RMS current is not simple. However, a
conservative value is the RMS input current for the channel
that is delivering most power (VOUT • IOUT):
CINRMS = IOUT •
VOUT (VIN – VOUT ) < IOUT
VIN 2
and is largest when VIN = 2VOUT (50% duty cycle). As the
second, lower power channel draws input current, the
input capacitor’s RMS current actually decreases as the
out-of-phase current cancels the current drawn by the
higher power channel. Considering that the maximum
load current from a single channel is ~1.5A, RMS ripple
current will always be less than 0.75A.
The high frequency of the LT3475 reduces the energy
storage requirements of the input capacitor, so that the
capacitance required is less than 10μF. The combination
of small size and low impedance (low equivalent series
resistance or ESR) of ceramic capacitors makes them the
preferred choice. The low ESR results in very low voltage
ripple. Ceramic capacitors can handle larger magnitudes
of ripple current than other capacitor types of the same
value. Use X5R and X7R types.
3475fb
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| PDF Descargar | [ Datasheet LT3475-1.PDF ] | |
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