LT3592IMSE datasheet(11/24 Pages) LINER

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LT3592

3592fa

APPLICATIONS INFORMATION

(~0.4V at maximum load). This leads to a minimum input

voltage of:

IN(MIN) =

OUT + VD

MAX

–V

D + VSW

with DCmax = 0.90.

The maximum input voltage is determined by the absolute

maximum ratings of the VIN 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:

DCMIN = FOSC • 70nsec

IN(MAX) =

OUT + VD

MIN

–V

D + VSW

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 VIN

and BOOST pins. The input voltage should be limited to

the VIN 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 VIN(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 VIN(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 VIN = 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 VIN(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

VSW

20V/DIV

3592 F01

VOUT

50mV/DIV

500mA/DIV

1μs/DIV

VSW

20V/DIV

3592 F02

VOUT

50mV/DIV

500mA/DIV

1μs/DIV

VSW

20V/DIV

3592 F03

VOUT

50mV/DIV

500mA/DIV

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