LT1936
12
1936fd
APPLICATIONS INFORMATION
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 0.22μF capacitor and fast switching diode (such as the
1N4148 or 1N914) will work well. Figure 3 shows two
ways to arrange the boost circuit. The BOOST pin must
be at least 2.3V above the SW pin for best effi ciency. For
outputs of 3V and above, the standard circuit (Figure 3a)
is best. For outputs between 2.8V and 3V, use a 0.47μF
capacitor and a Schottky diode. For lower output voltages
the boost diode can be tied to the input (Figure 3b), or to
another supply greater than 2.8V. The circuit in Figure 3a is
more effi cient because the BOOST pin current comes from
a lower voltage. You must also be sure that the maximum
voltage rating of the BOOST pin is not exceeded.
A 2.5V output presents a special case. This is a popular
output voltage, and the advantage of connecting the
boost circuit to the output is that the circuit will accept a
36V maximum input voltage rather than 20V (due to the
BOOST pin rating). However, 2.5V is marginally adequate
to support the boosted drive stage at low ambient tem-
peratures. Therefore, special care and some restrictions
on operation are necessary when powering the BOOST pin
from a 2.5V output. Minimize the voltage loss in the boost
circuit by using a 1μF boost capacitor and a good, low drop
Schottky diode (such as the ON Semi MBR0540). Because
the required boost voltage increases at low temperatures,
the circuit will supply only 1A of output current when the
ambient temperature is –45°C, increasing to 1.2A at 0°C.
Also, the minimum input voltage to start the boost circuit
is higher at low temperature. See the Typical Applications
section for a 2.5V schematic and performance curves.
The minimum operating voltage of an LT1936 application
is limited by the undervoltage lockout (~3.45V) 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
LT1936 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 4 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 discontinu-
ous 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 LT1936, requiring a higher
input voltage to maintain regulation.
Soft-Start
The SHDN pin can be used to soft-start the LT1936, 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 5 shows the start-up waveforms
with and without the soft-start circuit. By choosing a large
Figure 3. Two Circuits for Generating the Boost Voltage
V
IN
BOOST
GND
SW
V
IN
LT1936
(3a)
D2
V
OUT
C3
V
BOOST
– V
SW
≅ V
OUT
MAX V
BOOST
≅ V
IN
+ V
OUT
V
IN
BOOST
GND
SW
V
IN
LT1936
(3b)
D2
1933 F03
V
OUT
C3
V
BOOST
– V
SW
≅ V
IN
MAX V
BOOST
≅ 2V
IN
