Datasheet

LT3505
16
3505fc
APPLICATIONS INFORMATION
Figure 7
(7a) Typical Minimum Input Voltage, V
OUT
= 5V, f
SW
= 750kHz (7b) Typical Minimum Input Voltage, V
OUT
= 3.3V, f
SW
= 750kHz
capacitor and a small Schottky diode (such as the BAT-54).
For lower output voltages tie a Schottky diode to the input
(Figure 6b). The circuit in Figure 6a is more effi cient because
the BOOST pin current comes from a lower voltage source.
You must also be sure that the maximum voltage rating
of the BOOST pin is not exceeded.
The minimum operating voltage of an LT3505 applica-
tion is limited by the undervoltage lockout (3.6V) and by
the maximum duty cycle as outlined above. For proper
start-up, the minimum input voltage is also limited by
the boost circuit. If the input voltage is ramped slowly,
or the LT3505 is turned on with its SHDN 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
on the 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 7 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 verly 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 discontinu-
ous and the effective duty cycle can be very high. This
reduces the minimum input voltage to approximately
400mV 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 LT3505, requiring a higher
input voltage to maintain regulation.
Soft-Start
The SHDN pin can be used to soft-start the LT3505, reducing
the maximum input current during start-up. The SHDN pin
is driven through an external RC fi lter to create a voltage
ramp at this pin. Figure 8 shows the start-up waveforms
with and without the soft-start circuit. By choosing a large
RC time constant, the peak start up current can be reduced
to the current that is required to regulate the output, with
no overshoot. Choose the value of the resistor so that it
can supply 20µA when the SHDN pin reaches 2.3V.
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, an LT3505 buck regulator will tolerate a shorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
LT3505 is absent. This may occur in battery charging ap-
LOAD CURRENT (mA)
1
6.0
INPUT VOLTAGE (V)
6.2
6.4
6.6
6.8
10 100 1000
3505 G15
5.8
5.6
5.4
5.2
7.0
7.2
TO START
TO RUN
T
A
= 25°C
LOAD CURRENT (mA)
1
4.3
INPUT VOLTAGE (V)
4.5
4.7
4.9
5.1
10 100 1000
4.1
3.9
3.7
3.5
5.3
5.5
TO START
T
A
= 25°C
TO RUN