LT3991EMSE-5#PBF Linear Technology, LT3991EMSE-5#PBF Datasheet - Page 11

• Manufacturer Part Number: LT3991EMSE-5#PBF
• Description: IC, DC-DC CONV, 2MHz, MSOP10
• Manufacturer: Linear Technology

Specifications of LT3991EMSE-5#PBF

• Primary Input Voltage: 55V
• No. Of Outputs: 1
• Output Voltage: 5V
• Output Current: 1.2A
• No. Of Pins: 10
• Operating Temperature Range: -40°C To +125°C
• Peak Reflow Compatible (260 C): Yes
• Msl: MSL 1 - Unlimited
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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
given application can be calculated as follows:
where V
voltage, V
the internal switch drop (~0.5V at max load). This equation
shows that slower switching frequency is necessary to
safely accommodate high V
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:
where f
minimum switch on-time, and the t
switch off-time. These equations show that duty cycle
range increases when switching frequency is decreased.
See the Electrical Characteristics section for t
t
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.
OFF(MIN)
DC
DC
f
SW(MAX)
MIN
MAX
SW
IN
values.
= f
D
is the typical input voltage, V
= 1− f
is the switching frequency, the t
is the catch diode drop (~0.5V), and V
=
SW
t
ON(MIN)
t
SW
ON(MIN)
t
OFF(MIN)
V
(V
OUT
IN
− V
IN
+ V
/V
D
SW
OUT
OFF(MIN)
+ V
ratio. Also, as shown
D
)
OUT
is the minimum
SW(MAX)
ON(MIN)
is the output
ON(MIN)
) for a
is the
SW
and
is
Input Voltage 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:
where V
the output voltage, V
V
is the switching frequency (set by R
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
mode. For a given application where the switching fre-
quency and the output voltage are already selected, the
maximum input voltage (V
optimum output voltage ripple for that application can be
found by applying the following equation:
where t
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
switching frequency. However, during such transients
where V
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
than V
LT3991/LT3991-3.3/LT3991-5
SW
V
V
IN(OP-MAX)
IN(MIN)
is the internal switch drop (~0.5V at max load), f
IN(OP-MAX)
ON(MIN)
IN
IN(MIN)
is higher than V
=
1− f
=
is the minimum switch on-time. Note that
IN
is the minimum input voltage, V
V
.
IN
SW
f
OUT
SW
and BOOST pins, regardless of chosen
V
and BOOST pins, and the operating
OUT
t
OFF(MIN)
D
• t
+ V
is the catch diode drop (~0.5V),
ON(MIN)
+ V
D
IN(OP-MAX)
D
IN(OP-MAX)
− V
– V
D
+ V
D
, the LT3991 will enter
+ V
T
SW
), and t
) that guarantees
SW
IN
OFF(MIN)
is greater
11
OUT
3991fa
SW
is
is