LT3474/LT3474-1
APPLICATIONS INFORMATION
Operation
acrossthe0.1Ωresistorisequaltothevoltagedropacross
the 100Ω resistor, the servo loop is balanced.
The LT3474 is a constant frequency, current mode regula-
tor with an internal power switch capable of generating
a constant 1A output. Operation can be best understood
by referring to the Block Diagram.
Tying the REF pin to the V pin sets the LED pin current
ADJ
to 1A. Tying a resistor divider to the REF pin allows the
programming of LED pin currents of less than 1A. LED
pin current can also be programmed by tying the V pin
ADJ
If the SHDN pin is tied to ground, the LT3474 is shut
down and draws minimal current from the input source
directly to a voltage source up to 1.25V.
An LED can be dimmed with pulse width modulation us-
ing the PWM pin and an external NFET. If the PWM pin is
unconnected or pulled high, the part operates nominally.
tied to V . If the SHDN pin exceeds 1.5V, the internal bias
IN
circuitsturnon, includingtheinternalregulator, reference,
and oscillator. The switching regulator will only begin to
operate when the SHDN pin exceeds 2.65V.
If the PWM pin is pulled low, the V pin is disconnected
C
from the internal circuitry and draws minimal current from
Theswitcherisacurrentmoderegulator.Insteadofdirectly
modulatingthedutycycleofthepowerswitch,thefeedback
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.
thecompensationcapacitor.Circuitrydrawingcurrentfrom
the OUT pin is also disabled. This way, the V pin and the
C
output capacitor store the state of the LED pin current
until 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.
A pulse from the oscillator sets the RS flip-flop 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
TheR pinallowsprogrammingoftheswitchingfrequency.
T
Forapplicationsrequiringthesmallestexternalcomponents
possible, a fast switching frequency can be used. If very
low or very high input voltages are required, a slower
switching frequency can be programmed.
V , current comparator C1 resets the flip-flop, turning
C
off the switch. The current in the inductor flows through
the external Schottky diode and begins to decrease. The
cycle begins again at the next pulse from the oscillator.
During startup V
will be at a low voltage. The NPN Q2
OUT
can only operate correctly with sufficient voltage at V
,
OUT
around 1.7V. A comparator senses V
and forces the V
In this way, the voltage on the V pin controls the current
OUT
C
C
pin high until V
correctly.
rises above 2V, and Q2 is operating
through the inductor to the output. The internal error
OUT
amplifier regulates the output current by continually
adjusting the V pin voltage. The threshold for switching
C
The switching regulator performs frequency foldback dur-
ing overload conditions. An amplifier senses when V is
on the V pin is 0.8V, and an active clamp of 1.9V limits
C
OUT
the output current.
lessthan2Vandbeginsdecreasingtheoscillatorfrequency
down from full frequency to 20% of the nominal frequency
The voltage on the V
pin sets the current through the
ADJ
whenV
=0V.TheOUTpinislessthan2Vduringstartup,
LED pin. The NPN Q2 pulls a current proportional to the
voltage on the V pin through the 100Ω resistor. The
OUT
shortcircuit, andoverloadconditions. Frequencyfoldback
ADJ
helps limit switch current under these conditions.
g amplifier servos the V pin to set the current through
m
C
the 0.1Ω resistor and the LED pin. When the voltage drop
3474fd
8
Linear [ Linear ]
