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LT3475IFE#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
LT3475/LT3475-1
7
3475fb
The LT3475 is a dual constant frequency, current mode
regulator with internal power switches capable of gen-
erating constant 1.5A outputs. Operation can be best
understood by referring to the Block Diagram.
If the SHDN pin is tied to ground, the LT3475 is shut
down and draws minimal current from the input source
tied to V
IN
. If the SHDN pin exceeds 1V, the internal bias
circuits turn on, including the internal regulator, reference
and oscillator. The switching regulators will only begin to
operate when the SHDN pin exceeds 2.6V.
The switcher is a current mode regulator. Instead of directly
modulating the duty cycle of the power switch, the feedback
loop controls the peak current in the switch during each
cycle. Compared to voltage mode control, current mode
control improves loop dynamics and provides cycle-by-
cycle current limit.
A pulse from the oscillator sets the RS fl ip-fl op and turns
on the internal NPN bipolar power switch. Current in the
switch and the external inductor begins to increase. When
this current exceeds a level determined by the voltage at
V
C
, current comparator C1 resets the fl ip-fl op, turning
off the switch. The current in the inductor fl ows through
the external Schottky diode and begins to decrease. The
cycle begins again at the next pulse from the oscillator.
In this way, the voltage on the V
C
pin controls the current
through the inductor to the output. The internal error
amplifi er regulates the output current by continually
adjusting the V
C
pin voltage. The threshold for switching
on the V
C
pin is 0.8V, and an active clamp of 1.8V limits
the output current.
The voltage on the V
ADJ
pin sets the current through the
LED pin. The NPN, Q3, pulls a current proportional to the
voltage on the V
ADJ
pin through the 100Ω resistor. The gm
amplifi er servos the V
C
pin to set the current through the
0.067Ω resistor and the LED pin. When the voltage drop
across the 0.067Ω resistor is equal to the voltage drop
across the 100Ω resistor, the servo loop is balanced.
Tying the REF pin to the V
ADJ
pin sets the LED pin current
to 1.5A. Tying a resistor divider to the REF pin allows the
programming of LED pin currents of less than 1.5A. LED
pin current can also be programmed by tying the V
ADJ
pin
directly to a voltage source.
An LED can be dimmed with pulse width modulation
using the PWM pin and an external NFET. If the PWM
pin is unconnected or is pulled high, the part operates
nominally. If the PWM pin is pulled low, the V
C
pin is dis-
connected from the internal circuitry and draws minimal
current from the compensation capacitor. Circuitry draw-
ing current from the OUT pin is also disabled. This way,
the V
C
pin and the output capacitor store the state of
the LED pin current until the PWM is pulled high again.
This leads to a highly linear relationship between pulse
width and output light, allowing for a large and accurate
dimming range.
The R
T
pin allows programming of the switching frequency.
For applications requiring the smallest external components
possible, a fast switching frequency can be used. If low
dropout or very high input voltages are required, a slower
switching frequency can be programmed.
During startup V
OUT
will be at a low voltage. The NPN,
Q3, can only operate correctly with suffi cient voltage
of ≈1.7V at V
OUT
, A comparator senses V
OUT
and forces
the V
C
pin high until V
OUT
rises above 2V, and Q3 is op-
erating correctly.
The switching regulator performs frequency foldback
during overload conditions. An amplifi er senses when
V
OUT
is less than 2V and begins decreasing the oscillator
frequency down from full frequency to 15% of the nominal
frequency when V
OUT
= 0V. The OUT pin is less than 2V
during startup, short circuit, and overload conditions.
Frequency foldback helps limit switch current under these
conditions.
The switch driver operates either from V
IN
or from the
BOOST pin. An external capacitor and Schottky diode
are used to generate a voltage at the BOOST pin that
is higher than the input supply. This allows the driver
to saturate the internal bipolar NPN power switch for
effi cient operation.
OPERATION
LT3475/LT3475-1
8
3475fb
Open Circuit Protection
The LT3475 has internal open-circuit protection. If the LED
is absent or is open circuit, the LT3475 clamps the voltage
on the LED pin at 14V. The switching regulator then oper-
ates at a very low frequency to limit the input current. The
LT3475-1 has no internal open circuit protection. With the
LT3475-1, be careful not to violate the ABSMAX voltage of
th BOOST pin; if V
IN
> 25V, external open circuit protection
circuitry (as shown in Figure 1) may be necessary.The
output voltage during an open LED condition is shown in
the Typical Performance Characteristics section.
Undervoltage Lockout
Undervoltage lockout (UVLO) is typically used in situations
where the input supply is current limited, or has high source
resistance. A switching regulator draws constant power
from the source, so the source current increases as the
source voltage drops. This looks like a negative resistance
load to the source and can cause the source to current limit
or latch low under low source voltage conditions. UVLO
prevents the regulator from operating at source voltages
where these problems might occur.
An internal comparator will force the part into shut-
down when V
IN
falls below 3.7V. If an adjustable UVLO
threshold is required, the SHDN pin can be used. The
threshold voltage of the SHDN pin comparator is 2.6V. An
internal resistor pulls 9μA to ground from the SHDN pin
at the UVLO threshold.
Choose resistors according to the following formula:
R2 =
2.6V
V
TH
–2.6V
R1
–9μA
V
TH
= UVLO Threshold
Example: Switching should not start until the input is
above 8V.
V
TH
= 8V
R1=100k
R2 =
2.6V
8V – 2.6V
100k
–9μA
= 57.6k
Figure 2. Undervoltage Lockout
GND
9μA
2.6V
V
IN
V
C
LT3475
R1
C1
R2
V
IN
SHDN
3475 F02
Table 1. Switching Frequencies
SWITCHING FREQUENCY (MHz) R
T
(kΩ)
2 4.32
1.5 6.81
1.2 9.09
1 11.8
0.8 16.9
0.6 24.3
0.4 40.2
0.3 57.6
0.2 100
Keep the connections from the resistors to the SHDN pin
short and make sure the coupling to the SW and BOOST
pins is minimized. If high resistance values are used, the
SHDN pin should be bypassed with a 1nF capacitor to
prevent coupling problems from switching nodes.
Setting the Switching Frequency
The LT3475 uses a constant frequency architecture that
can be programmed over a 200kHz to 2MHz range with a
single external timing resistor from the R
T
pin to ground.
A graph for selecting the value of R
T
for a given operating
frequency is shown in the Typical Applications section.
V
C
100k
10k
22V
OUT
3475 F01
Figure 1. External Overvoltage Protection
Circuitry for the LT3475-1
APPLICATIONS INFORMATION
LT3475/LT3475-1
9
3475fb
Table 1 shows suggested R
T
selections for a variety of
switching frequencies.
Operating Frequency Selection
The choice of operating frequency is determined by
several factors. There is a tradeoff between effi ciency and
component size. A higher switching frequency allows the
use of smaller inductors at the cost of increased switching
losses and decreased effi ciency.
Another consideration is the maximum duty cycle. In certain
applications, the converter needs to operate at a high duty
cycle in order to work at the lowest input voltage possible.
The LT3475 has a fi xed oscillator off time and a variable
on time. As a result, the maximum duty cycle increases
as the switching frequency is decreased.
Input Voltage Range
The minimum operating voltage is determined either by the
LT3475’s undervoltage lockout of 4V, or by its maximum
duty cycle. The duty cycle is the fraction of time that the
internal switch is on and is determined by the input and
output voltages:
DC =
V
OUT
+ V
F
()
V
IN
–V
SW
+ V
F
()
where V
F
is the forward voltage drop of the catch diode
(~0.4V) and V
SW
is the voltage drop of the internal switch
(~0.4V at maximum load). This leads to a minimum input
voltage of:
V
IN MIN
()
=
V
OUT
+ V
F
DC
MAX
–V
F
+ V
SW
with DC
MAX
= 1–t
OFF(MIN)
• f
where t
0FF(MIN)
is equal to 167ns and f is the switching
frequency.
Example: f = 600kHz, V
OUT
= 4V
DC
MAX
= 1− 167ns • 600kHz = 0.90
V
IN MIN
()
=
4V + 0.4V
0.9
– 0.4V + 0.4V = 4.9V
The maximum operating voltage is determined by the
absolute maximum ratings of the V
IN
and BOOST pins,
and by the minimum duty cycle.
V
IN MAX
()
=
V
OUT
+ V
F
DC
MIN
–V
F
+ V
SW
with DC
MIN
= t
ON(MIN)
• f
where t
ON(MIN)
is equal to 140ns and f is the switching
frequency.
Example: f = 750kHz, V
OUT
= 3.4V
DC
MIN
= 140ns • 750kHz = 0.105
V
IN MAX
()
=
3.4V + 0.4V
0.105
– 0.4V + 0.4V = 36V
The minimum duty cycle depends on the switching fre-
quency. Running at a lower switching frequency might
allow a higher maximum operating voltage. Note that
this is a restriction on the operating input voltage; the
circuit will tolerate transient inputs up to the Absolute
Maximum Ratings of the V
IN
and BOOST pins. The input
voltage should be limited to the V
IN
operating range (36V)
during overload conditions (short circuit or start up).
Minimum On Time
The LT3475 will regulate the output current at input volt-
ages greater than V
IN(MAX)
. For example, an application
with an output voltage of 3V and switching frequency of
1.2MHz has a V
IN(MAX)
of 20V, as shown in Figure 3. Figure
4 shows operation at 35V. Output ripple and peak inductor
V
SW
20V/DIV
I
L
1A/DIV
V
OUT
500mV/DIV
(AC COUPLED)
3475 F03
Figure 3. Operation at V
IN(MAX)
= 20V.
V
OUT
= 3V and f
SW
= 1.2MHHz
APPLICATIONS INFORMATION
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20

LT3475IFE#TRPBF

Mfr. #:
Buy LT3475IFE#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
LED Lighting Drivers Dual Step-Down 1.5A LED Driver
Lifecycle:
New from this manufacturer.
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