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LTC3637HMSE#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P26
LTC3637
10
3637fa
For more information www.linear.com/LTC3637
the SS pin to ground. The 5µA current that is sourced
out of this pin will create a smooth voltage ramp on the
capacitor. If this ramp rate is slower than the internal
0.8ms soft-start, then the output voltage will be limited
by the ramp rate on the SS pin instead. The internal and
external soft-start functions are reset on start-up and after
an undervoltage or overvoltage event on the input supply.
The peak inductor current is not limited by the internal or
external soft-start functions; however, placing a capacitor
from the I
SET
pin to ground does provide this capability.
Peak Inductor Current Programming
The peak current comparator nominally limits the peak
inductor current to 2.4A. This peak inductor current can
be adjusted by placing a resistor from the I
SET
pin to
ground. The 5µA current sourced out of this pin through
the resistor generates a voltage that adjusts the peak cur-
rent comparator threshold.
During sleep mode, the current sourced out of the I
SET
pin
is reduced to 1µA. The I
SET
current is increased back to 5µA
on the first switching cycle after exiting sleep mode. The
I
SET
current reduction in sleep mode, along with adding
a filtering capacitor, C
ISET
, from the I
SET
pin to ground,
provides a method of reducing light load output voltage
ripple at the expense of lower efficiency and slightly de-
graded load step transient response.
Dropout Operation
When the input supply decreases toward the output sup-
ply, the duty cycle increases to maintain regulation. The
P-channel MOSFET top switch in the LTC3637 allows
the duty cycle to increase all the way to 100%. At 100%
duty cycle, the P-channel MOSFET stays on continuously,
providing output current equal to the peak current, which
can be greater than 2A. The power dissipation of the
LTC3637 can increase dramatically during dropout opera-
tion especially at input voltages less than 10V. The increased
power dissipation is due to higher potential output current
and increased P-channel MOSFET on-resistance. See
the Thermal Considerations section of the Applications
Information for a detailed example.
OPERATION
Input Voltage and Overtemperature Protection
When using the LTC3637, care must be taken not to
exceed any of the ratings specified in the Absolute Maxi-
mum Ratings section. As an added safeguard, however,
the LTC3637 incorporates an overtemperature shutdown
feature. If the junction temperature reaches approximately
180°C, the LTC3637 will enter thermal shutdown mode.
Both power switches will be turned off and the SW node
will become high impedance. After the part has cooled
below 160°C, it will restart. The overtemperature level is
not production tested.
The LTC3637 additionally implements protection features
which inhibit switching when the input voltage is not within
a programmed operating range. By using a resistive di-
vider from the input supply to ground, the RUN and OVLO
pins can serve as a precise input supply voltage monitor.
Switching is disabled when either the RUN pin falls below
1.1V or the OVLO pin rises above 1.21V, which can be
configured to limit switching to a specific range of input
supply voltage. Pulling the RUN pin below 700mV forces
a low quiescent current shutdown (3µA). Furthermore, if
the input voltage falls below 3.5V typical (3.7V maximum),
an internal undervoltage detector disables switching.
When switching is disabled, the LTC3637 can safely sustain
input voltages up to the absolute maximum rating of 80V.
Input supply undervoltage or overvoltage events trigger a
soft-start reset, which results in a graceful recovery from
an input supply transient.
High Input Voltage Considerations
When operating with an input voltage to output voltage dif-
ferential of more than 65V, a minimum output load current
of 10mA is required to maintain a well-regulated output
voltage under all operating conditions, including shutdown
mode. If this 10mA minimum load is not available, then
the minimum output voltage that can be maintained by
the LTC3637 is limited to V
IN
– 65V.
(Refer to Block Diagram)
LTC3637
11
3637fa
For more information www.linear.com/LTC3637
APPLICATIONS INFORMATION
The basic LTC3637 application circuit is shown on the front
page of the data sheet. External component selection is
determined by the maximum load current requirement and
begins with the selection of the peak current programming
resistor, R
ISET
. The inductor value L can then be determined,
followed by capacitors C
IN
and C
OUT
.
Peak Current Resistor Selection
The peak current comparator has a guaranteed peak current
limit of 2A (2.4A typical), which guarantees a maximum
average load current of 1A. For applications that demand
less current, the peak current threshold can be reduced to
as little as 200mA (240mA typical). This lower peak current
allows the use of lower value, smaller components (input
capacitor, output capacitor, and inductor), resulting in
lower supply ripple and a smaller overall DC/DC regulator.
The threshold can be easily programmed using a resis-
tor (R
ISET
) between the I
SET
pin and ground. The voltage
generated on the I
SET
pin by R
ISET
and the internal 5µA
current source sets the peak current. The voltage on the
I
SET
pin is internally limited within the range of 0.1V to
1.0V. The value of resistor for a particular peak current can
be selected by using Figure 2 or the following equation:
R
ISET
= 140k • I
PEAK
– 24k
where 200mA < I
PEAK
< 2A.
The internal 5μA current source is reduced to 1μA in sleep
mode to maximize efficiency and to facilitate a trade-off
between efficiency and light load output voltage ripple, as
described in the Optimizing Output Voltage Ripple section
of the Applications Information. For maximum efficiency,
minimize the capacitance on the I
SET
pin and place the
R
ISET
resistor as close to the pin as possible.
The typical peak current is internally limited to be within the
range of 240mA to 2.4A. Shorting the I
SET
pin to ground
programs the current limit to 240mA, and leaving it float
sets the current limit to the maximum value of 2.4A. When
selecting this resistor value, be aware that the maximum
average output current for this architecture is limited to
half of the peak current. Therefore, be sure to select a value
that sets the peak current with enough margin to provide
adequate load current under all conditions. Selecting the
peak current to be 2.2 times greater than the maximum
load current is a good starting point for most applications.
Inductor Selection
The inductor, input voltage, output voltage, and peak cur-
rent determine the switching frequency during a burst
cycle of the LTC3637. For a given input voltage, output
voltage, and peak current, the inductor value sets the
switching frequency during a burst cycle when the output
is in regulation. Generally, switching between 50kHz and
250kHz yields high efficiency, and 200kHz is a good first
choice for many applications. The inductor value can be
determined by the following equation:
L =
V
OUT
f •I
PEAK
• 1–
V
OUT
V
IN
The variation in switching frequency during a burst cycle
with input voltage and inductance is shown in Figure 3. For
lower values of I
PEAK
, multiply the frequency in Figure3
by 2.4A/I
PEAK
.
An additional constraint on the inductor value is the
LTC3637’s 150ns minimum on-time of the high side switch.
Therefore, in order to keep the current in the inductor
well-controlled, the inductor value must be chosen so that
Figure 2. R
ISET
Selection
MAXIMUM LOAD CURRENT (mA)
R
ISET
(kΩ)
60
180
200
260
220
240
600
3637 F02
20
140
100
40
160
0
120
80
0 200
400
800
1000
LTC3637
12
3637fa
For more information www.linear.com/LTC3637
APPLICATIONS INFORMATION
it is larger than a minimum value which can be computed
as follows:
L >
V
IN(MAX)
• t
ON(MIN)
I
PEAK
• 1.2
where V
IN(MAX)
is the maximum input supply voltage when
switching is enabled, t
ON(MIN)
is 150ns, I
PEAK
is the peak
current, and the factor of 1.2 accounts for typical inductor
tolerance and variation over temperature. For applications
that have large input supply transients, the OVLO pin can
be used to disable switching above the maximum operat-
ing voltage, V
IN(MAX)
, so that the minimum inductor value
is not artificially limited by a transient condition. Inductor
values that violate the above equation will cause the peak
current to overshoot and permanent damage to the part
may occur.
Although the above equation provides the minimum in-
ductor value, higher efficiency is generally achieved with
a larger inductor value, which produces a lower switching
frequency. For a given inductor type, however, as inductance
is increased, DC resistance (DCR) also increases. Higher
DCR translates into higher copper losses and lower current
rating, both of which place an upper limit on the inductance.
The recommended range of inductor values for small sur-
face mount inductors as a function of peak current is shown
in Figure 4. The values in this range are a good compromise
between the trade-offs discussed above. For applications
where board area is not a limiting factor, inductors with
larger cores can be used, which extends the recommended
range of Figure4 to larger values.
Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. High efficiency regulators generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of the more expensive ferrite cores. Actual
core loss is independent of core size for a fixed inductor
value but is very dependent of the inductance selected.
As the inductance increases, core losses decrease. Un-
fortunately, increased inductance requires more turns of
wire and therefore copper losses will increase.
Ferrite designs have very low core losses and are pre-
ferred at high switching frequencies, so design goals
can concentrate on copper loss and preventing satura-
tion. Ferrite core material saturates “hard,” which means
that inductance collapses abruptly when the peak design
current is exceeded. This results in an abrupt increase in
inductor ripple current and consequently output voltage
ripple. Do not allow the core to saturate!
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials are
small and do not radiate energy but generally cost more
than powdered iron core inductors with similar charac-
teristics. The choice of which style inductor to use mainly
Figure 4. Recommended Inductor Values for Maximum Efficiency
Figure 3. Switching Frequency for V
OUT
= 5.0V
V
IN
INPUT VOLTAGE (V)
0
SWITCHING FREQUENCY (kHz)
200
300
7060
3637 F03
100
0
10
20 30 40 50
V
OUT
= 5.0V
I
SET
OPEN
L = 5.6µH
L = 10µH
L = 22µH
L = 47µH
PEAK INDUCTOR CURRENT (mA)
100
1
INDUCTOR VALUE (µH)
10
100
1000
1000
3637 F04
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LTC3637HMSE#TRPBF

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Analog Devices / Linear Technology
Description:
Switching Voltage Regulators High Efficiency, 76V 1A Step-Down Regulator
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