LT3509 LINER [Linear Technology], LT3509 Datasheet - Page 12

• Manufacturer Part Number: LT3509
• Description: Dual 36V, 700mA Step-Down Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LT3509
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
fi xed frequency operation is given by:
It follows that:
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 97-98%, 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
12
DC
V
IN MIN
MAX
(
)
= − 1
=
3.5
2.5
5.5
4.5
V
0.001
5
4
3
2
Figure 5. Minimum V
DC
OUT
t
OFF MIN
MAX
TO START
+
TO RUN
(
V
F
)
−
0.01
LOAD CURRENT (A)
•
V
f
SW
F
+
V
SW
IN
0.1
for 3.3V V
3509 F05
OUT
1
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 5.0V.
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:
where f
V
IN MIN
(
SW
5.5
4.5
6.5
)
0.001
7
6
5
4
Figure 6. Minimum V
is in MHz and C
=
V
DC
TO START
OUT
TO RUN
MAX
+
0.01
LOAD CURRENT (A)
V
F
−
V
BOOST
F
+
IN
V
0.1
for 5.0V V
SW
is in μF
3509 F06
OUT
1
3509f