Datasheet

LT3991/LT3991-3.3/LT3991-5
11
3991fa
applicaTions inForMaTion
Operating Frequency Tradeoffs
Selection of the operating frequency is a tradeoff between
efficiency, component size, minimum dropout voltage, and
maximum input voltage. The advantage of high frequency
operation is that smaller inductor and capacitor values may
be used. The disadvantages are lower efficiency, lower
maximum input voltage, and higher dropout voltage. The
highest acceptable switching frequency (f
SW(MAX)
) for a
given application can be calculated as follows:
f
SW(MAX)
=
V
OUT
+
V
D
t
ON(MIN)
(V
IN
− V
SW
+ V
D
)
where V
IN
is the typical input voltage, V
OUT
is the output
voltage, V
D
is the catch diode drop (~0.5V), and V
SW
is
the internal switch drop (~0.5V at max load). This equation
shows that slower switching frequency is necessary to
safely accommodate high V
IN
/V
OUT
ratio. Also, as shown
in the Input Voltage Range section, lower frequency allows
a lower dropout voltage. The input voltage range depends
on the switching frequency because the LT3991 switch has
finite minimum on and off times. The minimum switch on
and off times are strong functions of temperature. Use
the typical minimum on and off curves to design for an
application’s maximum temperature, while adding about
30% for part-to-part variation. The minimum and maximum
duty cycles that can be achieved taking minimum on and
off times into account are:
DC
MIN
=
f
SW
t
ON(MIN)
DC
MAX
= 1− f
SW
t
OFF(MIN)
where f
SW
is the switching frequency, the t
ON(MIN)
is the
minimum switch on-time, and the t
OFF(MIN)
is the minimum
switch off-time. These equations show that duty cycle
range increases when switching frequency is decreased.
See the Electrical Characteristics section for t
ON(MIN)
and
t
OFF(MIN)
values.
A good choice of switching frequency should allow ad-
equate input voltage range (see Input Voltage Range sec-
tion) and keep the inductor and capacitor values small.
Input V
oltage Range
The minimum input voltage is determined by either the
LT3991’s minimum operating voltage of 4.3V or by its
maximum duty cycle (see equation in Operating Frequency
Tradeoffs section). The minimum input voltage due to
duty cycle is:
V
IN(MIN)
=
V
OUT
+
V
D
1− f
SW
t
OFF(MIN)
− V
D
+ V
SW
where V
IN(MIN)
is the minimum input voltage, V
OUT
is
the output voltage, V
D
is the catch diode drop (~0.5V),
V
SW
is the internal switch drop (~0.5V at max load), f
SW
is the switching frequency (set by R
T
), and t
OFF(MIN)
is
the minimum switch off-time. Note that higher switch-
ing frequency will increase the minimum input voltage.
If a lower dropout voltage is desired, a lower switching
frequency should be used.
The maximum input voltage for LT3991 applications
depends on switching frequency, the Absolute Maximum
Ratings of the V
IN
and BOOST pins, and the operating
mode. For a given application where the switching fre-
quency and the output voltage are already selected, the
maximum input voltage (V
IN(OP-MAX)
) that guarantees
optimum output voltage ripple for that application can be
found by applying the following equation:
V
IN(OP-MAX)
=
V
OUT
+
V
D
f
SW
• t
ON(MIN)
– V
D
+ V
SW
where t
ON(MIN)
is the minimum switch on-time. Note that
a higher switching frequency will decrease the maximum
operating input voltage. Conversely, a lower switching
frequency will be necessary to achieve normal operation
at higher input voltages.
The circuit will tolerate inputs above the maximum op
-
erating input voltage and up to the Absolute Maximum
Ratings of the V
IN
and BOOST pins, regardless of chosen
switching frequency. However, during such transients
where V
IN
is higher than V
IN(OP-MAX)
, the LT3991 will enter
pulse-skipping operation where some switching pulses are
skipped to maintain output regulation. The output voltage
ripple and inductor current ripple will be higher than in
typical operation. Do not overload when V
IN
is greater
than V
IN(OP-MAX)
.