Datasheet

LT3592
11
3592fc
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
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 fi xed
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.
Figure 1.
Figure 2. Figure 3.
1µs/DIV
V
SW
20V/DIV
3592 F01
V
OUT
50mV/DIV
I
L
500mA/DIV
1µs/DIV
V
SW
20V/DIV
3592 F02
V
OUT
50mV/DIV
I
L
500mA/DIV
1µs/DIV
V
SW
20V/DIV
3592 F03
V
OUT
50mV/DIV
I
L
500mA/DIV