LT3595
OPERATION
The LT3595 uses a constant-frequency, current mode
control scheme to provide excellent line and load regula-
tion. Operation is best understood by referring to the Block
Diagram. The oscillator, V-I converter and internal regulator
are shared by the sixteen converters. The control circuitry,
power switches, PWM comparators and dimming control
(DFC) are identical for all converters.
The LT3595 enters shutdown mode when the
⎯
S
⎯
H
⎯
D
⎯
N pin
is lower than 400mV. If the
⎯
S
⎯
H
⎯
D
⎯
N pin is above 1.6V,
then the LT3595 turns on. At the start of each oscillator
cycle, the power switch is turned on. Current ramps up
through the output capacitor, the inductor, and the switch
to ground. When the voltage across the output capacitor
is larger than the LEDs’ forward voltage, current flows
through the LEDs.
When the switch is on, a voltage proportional to the switch
current is added to a stabilizing ramp and the resulting
sum is fed into the positive terminal of the PWM compara-
tor. When this voltage exceeds the level at the negative
input of the PWM comparator, the PWM logic turns off
the power switch. The level at the negative input of the
PWM comparator is set by the error amplifier output. This
voltage is set by the LED current and the bandgap refer-
ence. In this manner, the error amplifier sets the correct
peak current level in the inductor to keep the LED output
current in regulation. The external R
SET
resistor is used
to program the LED current from 10mA to 50mA.
Input Voltage Range
The minimum input voltage required to generate a specific
output voltage in an LT3595 application is limited by its
4.5V input voltage or by its maximum duty cycle. The duty
cycle is the fraction of time that the internal switch is on
divided by the total period. It is determined by the input
voltage and the voltage across the LEDs:
DC
=
V
LED
+
V
D
V
VIN
−
V
CESAT
+
V
D
where V
LED
is the voltage drop across the LEDs, V
D
is
the Schottky forward drop, and V
CESAT
is the saturation
voltage of the internal switch. This leads to a minimum
input voltage of:
V
IN(MIN)
=
V
LED
+
V
D
+
V
CESAT
−
V
D
DC
MAX
where DC
MAX
is the minimum rating of maximum duty
cycle.
The maximum input voltage is limited by the absolute
maximum rating of 45V.
Pulse-Skipping
At low duty cycles, the LT3595 may enter pulse-skipping
mode. Low duty cycle occurs at higher input voltages and
lower LED count. The LT3595 can drive currents without
pulse-skipping provided the voltage across the LED
string is greater than 15% of the input supply voltage. If
the current decreases to the point that the LED voltage is
less than 15% of the input supply, the device may begin
skipping pulses. This will result in some low frequency
ripple, although the LED current remains regulated on an
average basis down to 10mA.
3595f
8
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