LT3474/LT3474-1
8
3474fd
APPLICATIONS INFORMATION
Operation
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.
If the SHDN pin is tied to ground, the LT3474 is shut
down and draws minimal current from the input source
tied to V
IN
. If the SHDN pin exceeds 1.5V, the internal bias
circuits turn on, including the internal regulator, reference,
and oscillator. The switching regulator will only begin to
operate when the SHDN pin exceeds 2.65V.
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 1.9V limits
the output current.
The voltage on the V
ADJ
pin sets the current through the
LED pin. The NPN Q2 pulls a current proportional to the
voltage on the V
ADJ
pin through the 100Ω resistor. The
g
m
amplifi er servos the V
C
pin to set the current through
the 0.1Ω resistor and the LED pin. When the voltage drop
across the 0.1Ω 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 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
ADJ
pin
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.
If the PWM pin is pulled low, the V
C
pin is disconnected
from the internal circuitry and draws minimal current from
the compensation capacitor. Circuitry drawing 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 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 very
low 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 Q2
can only operate correctly with suffi cient voltage at V
OUT
,
around 1.7V. A comparator senses V
OUT
and forces the V
C
pin high until V
OUT
rises above 2V, and Q2 is operating
correctly.
The switching regulator performs frequency foldback dur-
ing overload conditions. An amplifi er senses when V
OUT
is
less than 2V and begins decreasing the oscillator frequency
down from full frequency to 20% 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.
