LT3475 LINER [Linear Technology], LT3475 Datasheet - Page 7

• Manufacturer Part Number: LT3475
• Description: Dual Step-Down l 1.5A LED Driver
• Manufacturer: LINER [Linear Technology]

OPERATION
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
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
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
through the inductor to the output. The internal error
amplifi er regulates the output current by continually
adjusting the V
on the V
the output current.
The voltage on the V
LED pin. The NPN, Q3, pulls a current proportional to the
voltage on the V
amplifi er servos the V
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
to 1.5A. Tying a resistor divider to the REF pin allows the
C
, current comparator C1 resets the fl ip-fl op, turning
IN
C
. If the SHDN pin exceeds 1V, the internal bias
pin is 0.8V, and an active clamp of 1.8V limits
C
ADJ
pin voltage. The threshold for switching
pin through the 100Ω resistor. The gm
ADJ
C
pin to set the current through the
pin sets the current through the
ADJ
pin sets the LED pin current
C
pin controls the current
programming of LED pin currents of less than 1.5A. LED
pin current can also be programmed by tying the V
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
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
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
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
Q3, can only operate correctly with suffi cient voltage
of ≈1.7V at V
the V
erating correctly.
The switching regulator performs frequency foldback
during overload conditions. An amplifi er senses when
V
frequency down from full frequency to 15% of the nominal
frequency when V
during startup, short circuit, and overload conditions.
Frequency foldback helps limit switch current under these
conditions.
The switch driver operates either from V
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.
OUT
C
T
C
is less than 2V and begins decreasing the oscillator
pin allows programming of the switching frequency.
pin high until V
pin and the output capacitor store the state of
OUT
, A comparator senses V
OUT
OUT
will be at a low voltage. The NPN,
= 0V. The OUT pin is less than 2V
OUT
LT3475/LT3475-1
rises above 2V, and Q3 is op-
OUT
IN
or from the
C
and forces
pin is dis-
ADJ
3475fb
7
pin