Datasheet

LT3980
15
3980fa
For more information www.linear.com/LT3980
on input and output voltages, and on the arrangement of
the boost circuit. The minimum load generally goes to
zero once the circuit has started. Figure 6 shows a plot
of minimum load to start and to run as a function of input
voltage. In many cases the discharged output capacitor
will present a load to the switcher, which will allow it to
start. The plots show the worst-case situation where V
IN
is ramping very slowly. For lower start-up voltage, the
boost diode can be tied to V
IN
; however, this restricts the
input range to one-half of the absolute maximum rating
of the BOOST pin.
At light loads, the inductor current becomes discontinuous
and the effective duty cycle can be very high. This reduces
the minimum input voltage to approximately 300mV above
V
OUT
. At higher load currents, the inductor current is
continuous and the duty cycle is limited by the maximum
duty cycle of the LT3980, requiring a higher input voltage
to maintain regulation.
Soft-Start
The RUN/SS pin can be used to soft-start the LT3980,
reducing the maximum input current during start-up.
The RUN/SS pin is driven through an external RC filter to
While operating with high boost voltages (>10V), it is
important to ensure that the power dissipation from the
boost circuit is not too high. See the Typical Performance
Characteristics section for the plot, BOOST Pin Current
vs Switch Current. Boost circuit power dissipation is
calculated as follows:
P
BOOST
= I
BOOST
V
BOOST
– SW
DC
Where DC is the switch duty cycle, I
BOOST
is the boost pin
current, and V
BOOST
– V
SW
is the voltage between the boost
pin and switch pin. If the P
BOOST
> 0.5W, a Zener can be put
between the boost pin and the boost capacitor such that
the power is dissipated in the Zener instead of the LT3980.
The minimum operating voltage of an LT3980 application
is limited by the minimum input voltage (3.6V) and by the
maximum duty cycle as outlined in a previous section. For
proper startup, the minimum input voltage is also limited
by the boost circuit. If the input voltage is ramped slowly,
or the LT3980 is turned on with its RUN/SS pin when the
output is already in regulation, then the boost capacitor
may not be fully charged. Because the boost capacitor is
charged with the energy stored in the inductor, the circuit
will rely on some minimum load current to get the boost
circuit running properly. This minimum load will depend
Figure 6. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit
applicaTions inFormaTion
1 10 100 1000 2000
f = 400kHz
I
LOAD
(mA)
INPUT VOLTAGE (V)
4
5
3980 F06a
3
2
6
TO START
TO RUN
I
LOAD
(mA)
INPUT VOLTAGE (V)
6
7
3980 F06b
5
4
8
1 10 100 1000 2000
TO START
TO RUN
f = 400kHz
Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.