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LTC3630AIMSE#PBF

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LTC3630A
10
3630afc
For more information www.linear.com/LTC3630A
supply droop is desired, a capacitor can be placed from
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 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 1.2A. 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.
For applications requiring higher output current, the
LTC3630A provides a feedback comparator output pin
(FBO) for combining the output current of multiple
LTC3630As. By connecting the FBO pin of a “master”
LTC3630A to the V
FB
pin of one or more “slave” LTC3630As,
the output currents can be combined to source much
more than 500mA.
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 LTC3630A allows
the duty cycle to increase all the way to 100%. At 100%
duty cycle, the P-channel MOSFET stays on continu
-
ously, providing
output current equal to the peak current,
operaTion
which can be greater than 1A. The power dissipation of
the LTC3630A can increase dramatically during dropout
operation 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.
Input Voltage and Overtemperature Protection
When using the LTC3630A, 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 LTC3630A incorporates an overtemperature shutdown
feature. If the junction temperature reaches approximately
180°C, the LTC3630A 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 LTC3630A can provide a programmable undervoltage
lockout which can also serve as a precise input voltage
monitor by using a resistive divider from V
IN
to GND with
the tap connected to the RUN pin. Switching is enabled
when the RUN pin voltage exceeds 1.21V and is disabled
when dropping below 1.1V. Pulling the RUN pin below
700mV forces a low quiescent current shutdown (5µA).
Furthermore, if the input voltage falls below 3.5V typi
-
cal (3.7V maximum), an internal undervoltage detector
disables switching.
When switching is disabled, the
LTC3630A can safely
sustain
input voltages up to the absolute maximum rating
of 80V. Input supply undervoltage 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
differential of more than 65V, a minimum output load
current of 5mA is required to maintain a well-regulated
output voltage under all operating conditions, including
shutdown mode. If this 5mA minimum load is not available,
then the minimum output voltage that can be maintained
by the LTC3630A is limited to V
IN
– 65V.
(Refer to Block Diagram)
LTC3630A
11
3630afc
For more information www.linear.com/LTC3630A
applicaTions inForMaTion
The basic LTC3630A 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 program
-
ming 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 maximum
current limit of 1A (1.2A typical), which guarantees a
maximum average current of 500mA. For applications
that demand less current, the peak current threshold can
be reduced to as little as 100mA (120mA typical). This
lower peak current allows the use of lower value, smaller
components (input capacitor, output capacitor, and induc
-
tor), resulting in lower input supply ripple and a smaller
overall DC/DC converter.
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
= I
PEAK
• 0.2 • 10
6
where 100mA < I
PEAK
< 1A.
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 C
ISET
Selection
section of the Applications Information. For maxi-
mum 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 120mA to 1.2A. Shorting the I
SET
pin to
ground programs the current limit to 120mA, and leav-
ing it
float sets the current limit to the maximum value
of
1.2A. 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 condi
-
tions. 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
current
determine the switching frequency during a burst
cycle of the LTC3630A. 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 1.2A/I
PEAK
.
An additional constraint on the inductor value is the
LTC3630A’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)
50
R
ISET
(kΩ)
60
180
200
220
150
250
300 350
3630a F02
20
140
100
40
160
0
120
80
100
200
400 450
500
LTC3630A
12
3630afc
For more information www.linear.com/LTC3630A
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. 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. The inductor value chosen should also be large
enough to keep the inductor current from going very nega
-
tive which is more of a concern at higher V
OUT
(>~12V). For
a given inductor type, however, as inductance is increased,
DC resistance (DCR) also increases. Higher DCR trans
-
lates 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 surface
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
l
arger 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 converters 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
depends
on the price versus size requirements and any
radiated field/EMI requirements. New designs for surface
mount inductors are available from Coiltronics, Coilcraft,
TDK, Toko, and Sumida.
Figure 4. Recommended Inductor Values for Maximum Efficiency
Figure 3. Switching Frequency for V
OUT
= 3.3V
V
IN
INPUT VOLTAGE (V)
0
SWITCHING FREQUENCY (kHz)
400
500
600
7060
3630a F03
300
200
0
10
20 30 40 50
100
V
OUT
= 3.3V
I
SET
OPEN
L = 4.2µH
L = 10µH
L = 22µH
L = 47µH
L = 100µH
PEAK INDUCTOR CURRENT (mA)
100
10
INDUCTOR VALUE (µH)
100
1000
1000
3630a F04
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LTC3630AIMSE#PBF

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Buy LTC3630AIMSE#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators High Efficiency, 76V 500mA Synchronous Step-Down Converter
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