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LT3975EMSE#PBF

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LT3975
13
3975f
APPLICATIONS INFORMATION
A good choice of switching frequency should allow ad-
equate input voltage range (see next two sections) and
keep the inductor and capacitor values small.
Maximum Input Voltage Range
The LT3975 can operate from input voltages of up to 42V.
Often the highest allowed V
IN
during normal operation
(V
IN(OP-MAX)
) is limited by the minimum duty cycle rather
than the absolute maximum ratings of the V
IN
pin. It can
be calculated using 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. A lower
switching frequency can be used to extend normal opera-
tion to 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 LT3975 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)
.
Minimum Input Voltage Range
The minimum input voltage is determined by either the
LT3975’s minimum operating voltage of 4.3V, its maximum
duty cycle, or the enforced minimum dropout voltage.
See the Typical Performance Characteristics section for
the minimum input voltage across load for outputs of
3.3V and 5V.
The duty cycle is the fraction of time that the internal
switch is on during a clock cycle. Unlike many fixed fre-
quency regulators, the LT3975 can extend its duty cycle
by remaining on for multiple clock cycles. The LT3975
will not switch off at the end of each clock cycle if there
is sufficient voltage across the boost capacitor (C3 in
the Block Diagram). Eventually, the voltage on the boost
capacitor falls and requires refreshing. When this occurs,
the switch will turn off, allowing the inductor current to
recharge the boost capacitor. This places a limitation on
the maximum duty cycle as follows:
DC
MAX
=
β
SW
β
SW
+1
where β
SW
is equal to the beta of the internal power switch.
The beta of the power switch is typically about 50, which
leads to a DC
MAX
of about 98%. This leads to a minimum
input voltage of approximately:
V
IN(MIN1)
=
V
OUT
+ V
D
DC
MAX
– V
D
+ V
SW
where V
OUT
is the output voltage, V
D
is the catch diode
drop, V
SW
is the internal switch drop and DC
MAX
is the
maximum duty cycle.
The final factor affecting the minimum input voltage is
the minimum dropout voltage. When the OUT pin is tied
to the output, the LT3975 regulates the output such that
it stays 500mV below V
IN
. This enforced minimum drop-
out voltage is due to reasons that are covered in the next
section. This places a limitation on the minimum input
voltage as follows:
V
IN(MIN2)
= V
OUT
+ V
DROPOUT(MIN)
where V
OUT
is the programmed output voltage and
V
DROPOUT(MIN)
is the minimum dropout voltage of 500mV.
Combining these factors leads to the overall minimum
input voltage:
V
IN(MIN)
= Max (V
IN(MIN1)
, V
IN(MIN2)
, 4.3V)
Minimum Dropout Voltage
To achieve a low dropout voltage, the internal power switch
must always be able to fully saturate. This means that the
boost capacitor, which provides a base drive higher than
V
IN
, must always be able to charge up when the part starts
up and then must also stay charged during all operating
conditions.
LT3975
14
3975f
APPLICATIONS INFORMATION
During start-up if there is insufficient inductor current, such
as during light load situations, the boost capacitor will be
unable to charge. When the LT3975 detects that the boost
capacitor is not charged, it activates a 100mA (typical)
pull-down on the OUT pin. If the OUT pin is connected to
the output, the extra load will increase the inductor current
enough to sufficiently charge the boost capacitor. When
the boost capacitor is charged, the current source turns
off, and the part may re-enter Burst Mode operation.
To keep the boost capacitor charged regardless of load
during dropout conditions, a minimum dropout voltage
is enforced. When the OUT pin is tied to the output, the
LT3975 regulates the output such that:
V
IN
– V
OUT
> V
DROPOUT(MIN)
where V
DROPOUT(MIN)
is 500mV. The 500mV dropout volt-
age limits the duty cycle and forces the switch to turn off
regularly to charge the boost capacitor. Since sufficient
voltage across the boost capacitor is maintained, the switch
is allowed to fully saturate and the internal switch drop
stays low for good dropout performance. Figure 3 shows
the overall V
IN
to V
OUT
performances during start-up and
dropout conditions.
measured dropout voltage, can be significantly reduced.
Additionally, when operating in dropout at high currents,
high ripple voltage on the input and output can generate
audible noise. This noise can also be significantly reduced
by adding bulk capacitance to the input and output to
reduce the voltage ripple.
Inductor Selection and Maximum Output Current
For a given input and output voltage, the inductor value
and switching frequency will determine the ripple current.
The ripple current increases with higher V
IN
or V
OUT
and
decreases with higher inductance and faster switching
frequency. A good first choice for the inductor value is:
L =
V
OUT
+ V
D
1.5• f
SW
where f
SW
is the switching frequency in MHz, V
OUT
is the
output voltage, V
D
is the catch diode drop (~0.5V) and L
is the inductor value is μH.
The inductor’s RMS current rating must be greater than
the maximum load current and its saturation current
should be about 30% higher. For robust operation in fault
conditions (start-up or overload) and high input voltage
(>30V), the saturation current should be above 8.5A.
To keep the efficiency high, the series resistance (DCR)
should be less than 0.1Ω, and the core material should
be intended for high frequency applications. Table 2 lists
several inductor vendors.
Table 2. Inductor Vendors
VENDOR URL
Coilcraft www.coilcraft.com
Sumida www.sumida.com
Toko www.tokoam.com
Würth Elektronik www.we-online.com
Coiltronics www.cooperet.com
Murata www.murata.com
The inductor value must be sufficient to supply the desired
maximum output current (I
OUT(MAX)
), which is a function
of the switch current limit (I
LIM
) and the ripple current.
I
OUT(MAX)
= I
LIM
–
∆I
L
2
Figure 3. V
IN
to V
OUT
Performance
V
IN
1V/DIV
V
OUT
1V/DIV
V
OUT
V
IN
100ms/DIV1kΩ LOAD
(5mA IN REGULATION)
3975 F03
It is important to note that the 500mV dropout voltage
specified is the minimum difference between V
IN
and
V
OUT
. When measuring V
IN
to V
OUT
with a multimeter,
the measured value will be higher than 500mV because
you have to add half the ripple voltage on the input and
half the ripple voltage on the output. With the normal
ceramic capacitors specified in the data sheet, this mea-
sured dropout voltage can be as high as 650mV at high
load. If some bulk electrolytic capacitance is added to the
input and output the voltage ripple, and subsequently the
LT3975
15
3975f
APPLICATIONS INFORMATION
The LT3975 limits its peak switch current in order to protect
itself and the system from overload and short-circuit faults.
The LT3975’s switch current limit (I
LIM
) is typically 5.4A at
low duty cycles and decreases linearly to 4.4A at DC = 0.8.
When the switch is off, the potential across the inductor
is the output voltage plus the catch diode drop. This gives
the peak-to-peak ripple current in the inductor:
∆I
L
=
1–DC
( )
• V
OUT
+ V
D
( )
L • f
SW
where f
SW
is the switching frequency of the LT3975, DC is
the duty cycle and L is the value of the inductor. Therefore,
the maximum output current that the LT3975 will deliver
depends on the switch current limit, the inductor value,
and the input and output voltages. The inductor value may
have to be increased if the inductor ripple current does
not allow sufficient maximum output current (I
OUT(MAX)
)
given the switching frequency, and maximum input voltage
used in the desired application.
The optimum inductor for a given application may differ
from the one indicated by this simple design guide. A larger
value inductor provides a higher maximum load current and
reduces the output voltage ripple. If your load is lower than
the maximum load current, than you can relax the value of
the inductor and operate with higher ripple current. This
allows you to use a physically smaller inductor, or one with
a lower DCR resulting in higher efficiency. Be aware that if
the inductance differs from the simple rule above, then the
maximum load current will depend on the input voltage. In
addition, low inductance may result in discontinuous mode
operation, which further reduces maximum load current.
For details of maximum output current and discontinuous
operation, see Linear Technology’s Application Note 44.
Finally, for duty cycles greater than 50% (V
OUT
/V
IN
> 0.5),
a minimum inductance is required to avoid sub-harmonic
oscillations, see Application Note 19.
One approach to choosing the inductor is to start with
the simple rule given above, look at the available induc-
tors, and choose one to meet cost or space goals. Then
use the equations above to check that the LT3975 will be
able to deliver the required output current. Note again
that these equations assume that the inductor current is
continuous. Discontinuous operation occurs when I
OUT
is less than ΔI
L
/2.
Current Limit Foldback and Thermal Protection
The LT3975 has a large peak current limit to ensure a 2.5A
max output current across duty cycle and current limit
distribution, as well as allowing a reasonable inductor
ripple current. During a short-circuit fault, having a large
current limit can lead to excessive power dissipation and
temperature rise in the LT3975, as well as the inductor and
catch diode. To limit this power dissipation, the LT3975
starts to fold back the current limit when the FB pin falls
below 0.8V. The LT3975 typically lowers the peak current
limit about 40% from 5.4A to 3.3A.
During start-up, when the output voltage and FB pin are low,
current limit foldback could hinder the LT3975’s ability to
start up into a large load. To avoid this potential problem,
the LT3975’s current limit foldback will be disabled until
the SS pin has charged above 2V. Therefore, the use of
a soft-start capacitor will keep the current limit foldback
feature out of the way while the LT3975 is starting up.
The LT3975 has thermal shutdown to further protect the
part during periods of high power dissipation, particularly
in high ambient temperature environments. The thermal
shutdown feature detects when the LT3975 is too hot
and shuts the part down, preventing switching. When the
thermal event passes and the LT3975 cools, the part will
restart and resume switching. A thermal shutdown event
actively discharges the soft-start capacitor.
Input Capacitor
Bypass the input of the LT3975 circuit with a ceramic capaci-
tor of X7R or X5R type. Y5V types have poor performance
over temperature and applied voltage, and should not be
used. A 4.7μF to 10μF ceramic capacitor is adequate to
bypass the LT3975 and will easily handle the ripple cur-
rent. Note that larger input capacitance is required when
a lower switching frequency is used (due to longer on
times). If the input power source has high impedance, or
there is significant inductance due to long wires or cables,
additional bulk capacitance may be necessary. This can
be provided with a low performance electrolytic capacitor.
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LT3975EMSE#PBF

Mfr. #:
Buy LT3975EMSE#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 42V, 2.5A, 2mHz Step-Down Switching Regulator with 3.4uA Quiescent Current
Lifecycle:
New from this manufacturer.
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