DC1205A Datasheet, PDF (11/24 Pages) Linear Integrated Systems – 500mA Wide Input Voltage Range Step-Down LED Driver with 10:1 Dimming

LT3592
APPLICATIONS INFORMATION
(~0.4V at maximum load). This leads to a minimum input
voltage of:
V_IN(MIN)
=
V_OUT + V_D
DC_MAX
–
V_D
+
V_SW
with DC_MAX = 0.90.
The maximum input voltage is determined by the absolute
maximum ratings of the V_IN and BOOST pins. The con-
tinuous mode operation, the maximum input voltage is
determined by the minimum duty cycle, which is dependent
upon the oscillator frequency:
DC_MIN = f_OSC • 70nsec
V_{IN(MAX )}
=
V_OUT + V_D
DC_MIN
–
V_D
+
V_SW
Note that this is a restriction on the operating input voltage
for continuous mode operation. The circuit will tolerate
transient inputs up to the absolute maximum of the V_IN
and BOOST pins. The input voltage should be limited to
V_OUT
50mV/DIV
I_L
500mA/DIV
V_SW
20V/DIV
1μs/DIV
Figure 1.
3592 F01
the V_IN absolute maximum range (36V) during overload
conditions (short circuit or startup).
Minimum On Time
The LT3592 will still regulate the output properly at input
voltages that exceed V_IN(MAX) (up to 36V); however, the
output voltage ripple increases as the input voltage is
increased.
Figure 1 illustrates switching waveforms in a 2.2MHz single
red LED application near V_IN(MAX) = 24V.
As the input voltage is increased, the part is required to
switch for shorter periods of time. Delays associated with
turning off the power switch dictate the minimum on time
of the part. The minimum on time for the LT3592 is ~70ns.
Figure 2 illustrates the switching waveforms when the
input voltage is increased to V_IN = 26V.
Now the required on time has decreased below the mini-
mum on time of 70ns. Instead of the switch pulse width
becoming narrower to accommodate the lower duty
cycle requirement, the switch pulse width remains fixed
at 70ns. In Figure 2, the inductor current ramps up to a
value exceeding the load current and the output ripple
increases to about 70mV. The part then remains off until
the output voltage dips below the programmed value
before it switches again.
Provided that the load can tolerate the increases output
voltage ripple and the the components have been properly
selected, operation about V_IN(MAX) is safe and will not dam-
age the part. Figure 3 illustrates the switching waveforms
when the input voltage is increased to 36V.
V_OUT
50mV/DIV
I_L
500mA/DIV
V_SW
20V/DIV
1μs/DIV
Figure 2.
3592 F02
V_OUT
50mV/DIV
I_L
500mA/DIV
V_SW
20V/DIV
1μs/DIV
Figure 3.
3592 F03
3592fc
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