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LT3509EMSE#PBF

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LT3509
10
3509fd
For more information www.linear.com/LT3509
applicaTions inForMaTion
External Synchronization
The external synchronization provides a trigger to the
internal oscillator. As such, it can only raise the frequency
above the free-run value. To allow for device and
component tolerances, the free run frequency should be
set to at least 12% lower than the lowest supplied external
synchronization reference. The oscillator and hence the
switching frequency can then pushed up from 12% above
the free-run frequency, set by the selected R
T
. For example,
if the minimum external clock is 300kHz, the R
T
should
be chosen for 264kHz.
The SYNC input has a threshold of 1.0V nominal so it is
compatible with most logic levels. The duty cycle is not
critical provided the high or low pulse width is at least
80ns. If not used, the SYNC input should be tied low with
10kΩ less to avoid noise pickup.
Design Procedure
Before starting detailed design a number of key design
parameters should be established as these may affect
design decisions and component choices along the way.
One of the main things to determine apart from the desired
output voltages is the input voltage range. Both the normal
operating range and the extreme conditions of surges
and/or dips or brown-outs need to be known. Then the
operating frequency should be considered and if there
are particular requirements to avoid interference. If there
are very specific frequencies that need to be avoided then
external synchronization may be needed. This could also be
desirable if multiple switchers are used as low frequency
beating between similar devices can be undesirable. For
efficient operation this converter requires a boost supply so
that the base of the output transistor can be pumped above
the input voltage during the switch on time. Depending
on the input and output voltages the boost supply can be
provided by the input voltage, one of the regulated outputs
or an independent supply such as an LDO.
Input Voltage Range
Firstly, the LT3509 imposes some hard limits due to the
undervoltage lock-out and the overvoltage protection. A
given application will also have a reduced, normal operating
range over which maximum efficiency and lowest ripple
are obtained. This usually requires that the device is
operating at a fixed frequency without skipping pulses.
There may also be zones above and below the normal
range where regulation is maintained but efficiency and
ripple may be compromised. At the low end, insufficient
input voltage will cause loss of regulation and increased
ripple—this is the dropout range. At the high end if the
duty cycle becomes too low this will cause pulse skipping
and excessive ripple. This is the pulse-skip region. Both
situations also lead to higher noise at frequencies other than
the chosen switching frequency. Occasional excursions
into pulse-skip mode, during surges for example, may be
tolerable. Pulse skipping will also occur at light loads even
within the normal operating range but ripple is usually not
degraded because at light load the output capacitor can
hold the voltage steady between pulses.
For input voltages greater than 30V, there are restrictions
on the inductor value. See the Inductor Selection section
for details.
To ensure the regulator is operating in continuous mode
it is necessary to calculate the duty cycle for the required
output voltage over the full input voltage range. This must
then be compared with minimum and maximum practical
duty cycles.
LT3509
11
3509fd
For more information www.linear.com/LT3509
applicaTions inForMaTion
In any step-down switcher the duty cycle when operating
in continuous, or fixed frequency, mode is dependent
on the step-down ratio. This is because for a constant
average load current the decay of the inductor current
when the switch is off must match the increase in inductor
current when the switch is on. The can be estimated by
the following formula:
DC
VV
VV V
OUT F
IN SW F
=
+
−+
where:
DC =
Duty Cycle (Fraction of Cycle when Switch is On)
V
OUT
= Output Voltage
V
IN
= Input Voltage
V
F
= Catch Diode Forward Voltage
V
SW
= Switch Voltage Drop
Note: This formula neglects switching and inductor losses
so in practice the duty cycle may be slightly higher.
It is clear from this equation that the duty cycle will approach
100% as the input voltage is reduced and become smaller
as the input voltage increases. There are practical limits to
the minimum and maximum duty cycles for continuous
operation due to the switch minimum off and on times.
These are independent of operating frequency so it is clear
that range of usable duty cycle is inverserly proportional to
frequency. Therefore at higher frequency the input voltage
range (for constant frequency operation) will narrow.
The minimum duty cycle is given by:
DC ft
MINSWONMIN
=
•
()
where:
f
SW
= Switching Frequency
t
ON(MIN)
= Switch Minimum On-Time
The minimum on time increases with increasing tempera-
ture so the value for the maximum operating temperature
should be used. See the Minimum On-T
ime vs T
emperature
graph in the Typical Performance Characteristics.
The maximum input voltage for this duty cycle is given by:
V
VV
DC
VV
IN MAX
OUT F
MIN
FS
W()
=
+
−+
Above this voltage the only way the LT3509 can maintain
regulation is to skip cycles so the effective frequency will
reduce. This will cause an increase in ripple and the switch
-
ing noise will shift to a lower frequency. This calculation
will in practice drive the maximum switching frequency
for a desired step-down ratio.
Figure 3. Continuous Mode
I
L
0.5A/DIV
V
OUT
100mV/DIV
(AC COUPLED)
TIME 1µs/DIV
3509 F03
Figure 4. Pulse Skipping
I
L
0.5A/DIV
V
OUT
100mV/DIV
(AC COUPLED)
TIME 1µs/DIV
3509 F04
LT3509
12
3509fd
For more information www.linear.com/LT3509
applicaTions inForMaTion
Minimum Input Voltage and Boost Architecture
The minimum operating voltage is determined either by
the LT3509’s internal undervoltage lockout of ~3.6V or
by its maximum duty cycle. The maximum duty cycle for
fixed frequency operation is given by:
DC tf
MAXOFF MI
NS
W
=−
1
()
•
It follows that:
V
VV
DC
VV
IN MIN
OUT F
MAX
FS
W()
=
+
−+
If a reduction in switching frequency can be tolerated the
minimum input voltage can drop to just above output
voltage. Not only is the output transistor base pumped
above the input voltage by the boost capacitor, the
switch can remain on through multiple switching cycles
resulting in a high effective duty cycle. Thus, this is a
true low dropout regulator. As it is necessary to recharge
the boost capacitor from time to time, a minimum width
off-cycle will be forced occasionally to maintain the charge.
Depending on the operating frequency
, the duty cycle can
reach
97% to 98%, although at this point the output pulses
will be at a sub-multiple of the programmed frequency.
One other consideration is that at very light loads or no
load the part will go into pulse skipping mode. The part
will then have trouble getting enough voltage on to the
boost capacitors to fully saturate the switch. This is most
problematic when the BD pin is supplied from the regulated
output. The net result is that a higher input voltage will be
required to start up the boost system. The typical minimum
input voltage over a range of loads is shown in Figure 5
for 3.3V and Figure 6 for 5V.
When operating at such high duty cycles the peak currents
in the boost diodes are greater and this will require a the
BD supply to be somewhat higher than would be required
at less extreme duty cycles. If operation at low input/output
ratios and low BD supply voltages is required it may be
desirable to augment the internal boost diodes with external
discrete diodes in parallel.
Boost Pin Considerations
The boost capacitor, in conjunction with the internal boost
diode, provides a bootstrapped supply for the power switch
that is above the input voltage. For operation at 1MHz and
above and at reasonable duty cycles a 0.1µF capacitor
will work well. For operation at lower frequencies and/or
higher duty cycles something larger may be needed. A
good rule of thumb is:
C
f
BOOST
SW
=
1
10 •
where f
SW
is in MHz and C
BOOST
is in µF
Figure 5. Minimum V
IN
for 3.3V V
OUT
LOAD CURRENT (A)
0.001
V
IN
TO START (V)
4
4.5
5
1
3509 F05
3
3.5
2.5
2
0.01
0.1
5.5
TO START
TO RUN
Figure 6. Minimum V
IN
for 5V V
OUT
LOAD CURRENT (A)
0.001
V
IN
TO START (V)
6
6.5
1
3509 F06
5
5.5
4.5
4
0.01
0.1
7
TO START
TO RUN
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LT3509EMSE#PBF

Mfr. #:
Buy LT3509EMSE#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Dual Integrated 700mA Wide Input Rane Step-Down Regulator
Lifecycle:
New from this manufacturer.
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