LT3493-3 LINER [Linear Technology], LT3493-3 Datasheet - Page 9

• Manufacturer Part Number: LT3493-3
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
Minimum On Time
The part will still regulate the output at input voltages that
exceed V
ripple increases as the input voltage is increased. Figure 1
illustrates switching waveforms in continuous mode for a
3V output application near V
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 LT3493-3 is
130ns. Figure 2 illustrates the switching waveforms when
the input voltage is increased to V
Now the required on time has decreased below the mini-
mum on time of 130ns. Instead of the switch pulse width
becoming narrower to accommodate the lower duty cycle
requirement, the switch pulse width remains fixed at
130ns. In Figure 2 the inductor current ramps up to a value
exceeding the load current and the output ripple increases
to ~200mV. The part then remains off until the output
voltage dips below 100% of the programmed value before
it begins switching again.
Provided that the output remains in regulation and that the
inductor does not saturate, operation above V
and will not damage the part. Figure 3 illustrates the
switching waveforms when the input voltage is increased
to its absolute maximum rating of 40V.
AC COUPLED
200mV/DIV
0.5A/DIV
20V/DIV
V
V
IN(PS)
OUT
SW
I
L
C
V
V
I
L = 10µH
LOAD
OUT
OUT
IN
= 35V
(up to 40V), however, the output voltage
= 10µF
= 3V
= 0.75A
U
Figure 2
U
2µs/DIV
IN(PS)
IN
= 33V.
W
= 35V.
3493-3 F02
IN(PS)
U
is safe
As the input voltage increases, the inductor current ramps
up quicker, the number of skipped pulses increases and
the output voltage ripple increases. For operation above
V
components be adequately rated for operation at the
intended voltage levels.
The part is robust enough to survive prolonged operation
under these conditions as long as the peak inductor
current does not exceed 2.2A. Inductor current saturation
may further limit performance in this operating regime.
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
where V
L is in µH. With this value there will be no subharmonic
oscillation for applications with 50% or greater duty cycle.
The inductor’s RMS current rating must be greater than
your maximum load current and its saturation current
should be about 30% higher. For robust operation in fault
conditions, the saturation current should be above 2.2A.
To keep efficiency high, the series resistance (DCR) should
be less than 0.1Ω. Table 1 lists several vendors and types
that are suitable.
Of course, such a simple design guide will not always
result in the optimum inductor for your application. A
IN(MAX)
L = 1.6 (V
AC COUPLED
200mV/DIV
0.5A/DIV
20V/DIV
D
V
V
OUT
is the voltage drop of the catch diode (~0.4V) and
SW
the only component requirement is that the
I
L
C
V
V
I
L = 10µH
LOAD
OUT
OUT
OUT
IN
= 40V
= 10µF
= 3V
= 0.75A
+ V
D
)
Figure 3
2µs/DIV
LT3493-3
3493-3 F03
3493-3f
9