LT1934-1 Linear Technology, LT1934-1 Datasheet - Page 10

• Manufacturer Part Number: LT1934-1
• Description: Micropower Step-Down Switching Regulators in ThinSOT
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LT1934/LT1934-1
when used with ceramic capacitors will be lower and may
be acceptable.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT1934. A
ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank
circuit. If the LT1934 circuit is plugged into a live supply,
the input voltage can ring to twice its nominal value,
possibly exceeding the LT1934’s rating. This situation is
easily avoided; see the Hot Plugging Safely section.
Catch Diode
A 0.5A Schottky diode is recommended for the catch
diode, D1. The diode must have a reverse voltage rating
equal to or greater than the maximum input voltage. The
ON Semiconductor MBR0540 is a good choice; it is rated
for 0.5A forward current and a maximum reverse voltage
of 40V.
Schottky diodes with lower reverse voltage ratings usually
have a lower forward drop and may result in higher
efficiency with moderate to high load currents. However,
these diodes also have higher leakage currents. This
leakage current mimics a load current at the output and
can raise the quiescent current of the LT1934 circuit,
especially at elevated temperatures.
BOOST Pin Considerations
Capacitor C3 and diode D2 are used to generate a boost
voltage that is higher than the input voltage. In most cases
a 0.1 F capacitor and fast switching diode (such as the
1N4148 or 1N914) will work well. Figure 2 shows two
ways to arrange the boost circuit. The BOOST pin must be
more than 2.5V above the SW pin for best efficiency. For
outputs of 3.3V and above, the standard circuit (Figure 2a)
is best. For outputs between 2.8V and 3V, use a 0.22 F
capacitor and a small Schottky diode (such as the
BAT-54). For lower output voltages the boost diode can be
tied to the input (Figure 2b). The circuit in Figure 2a is more
efficient 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 LT1934 application
is limited by the undervoltage lockout (~3V) and by the
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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 LT1934
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 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 3 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
Use a Schottky diode (such as the BAT-54) for the lowest
start-up voltage.
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
300mV above V
current is continuous and the duty cycle is limited by the
Figure 2. Two Circuits for Generating the Boost Voltage
V
V
IN
IN
V
MAX V
V
MAX V
BOOST
BOOST
OUT
V
BOOST
V
BOOST
– V
– V
IN
D2
IN
SW
SW
. At higher load currents, the inductor
LT1934
LT1934
BOOST
BOOST
GND
GND
V
2V
V
IN
V
IN
OUT
IN
+ V
SW
SW
OUT
(2a)
(2b)
IN
www.DataSheet4U.com
is ramping very slowly.
C3
C3
D2
1934 F02
V
OUT
V
OUT
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