An interesting DC/DC
converter IC is available from Linear Technology. The LT1615 step-up
switching voltage regulator can generate an output voltage of up to +34V
from a +1.2 to +15V supply, using only a few external components. The
tiny 5-pin SOT23 package makes for very compact construction. This IC
can for example be used to generate the high voltage needed for an LCD
screen, the tuning voltage for a varicap diode and so on. The internal
circuit diagram of the LT1615 is shown in Figure 1. It contains a
monostable with a pulse time of 400 ns, which determines the off time of
the transistor switch.
If the voltage sampled at the feedback input drops below the
reference threshold level of 1.23 V, the transistor switches on and the
current in the coil starts to increase. This builds up energy in the
magnetic ﬁeld of the coil. When the current through the coil reaches 350
mA, the monostable is triggered and switches the transistor off for the
following 400 ns. Since the energy stored in the coil must go
somewhere, current continues to ﬂow through the coil, but it decreases
linearly. This current charges the output capacitor via the Schottky
diode (SS24, 40V/2A). As long as the voltage at FB remains higher than
1.23V, nothing else happens.
As soon as it drops below this level, however, the whole cycle is
repeated. The hysteresis at the FB input is 8mV. The output voltage can
be calculated using the formula Vout = 1.23V (R1+R2) / R2 The value of
R1 can be selected in the megohm range, since the current into the FB
input is only a few tens of nano-amperes. When the supply voltage is
switched on, or if the output is short-circuited, the IC enters the
power-up mode. As long as the voltage at FB is less than 0.6V, the
LT1615 output current is limited to 250mA instead of 350mA, and the
monostable time is increased to 1.5µs.
These measures reduce the power dissipation in the coil and diode
while the output voltage is rising. In order to minimize the noise
voltages produced when the coil is switched, the IC must be properly
decoupled by capacitors at the input and output. The series resistance
of these capacitors should be as low as possible, so that they can short
noise voltages to earth. They should be located as close to the IC as
possible, and connected directly to the earth plane. The area of the
track at the switch output (SW) should be as small as possible.
Connecting a 4.7-µF capacitor across the upper feedback capacitor helps
to reduce the level of the output ripple voltage.
The selection of the coil inductance is described in detail in the LT1615 data sheet at http://www.linear-tech.com.
Normally, a 4.7µH ﬁlter choke is satisfactory for output voltages less
than 7V. For higher voltages, a 10-µH choke should be used. In the data
sheet, the Coilcraft DO1608-472 (4.7 µF) and DO1608-100 (10 µF) are
recommended. The Schottky diode must naturally have a reverse blocking
voltage that is significantly greater than the value of the output
voltage. The types MBR0530 and SS24 are recommended. The shutdown input
(/SHDN) can be used to disable the step-up regulator by applying a
voltage that is less than +0.25V.
If the voltage at this pin is +0.9 V or higher, the LT1615 is
active. You must bear in mind that even when the IC is disabled, the
input voltage still can reach the output via the coil and the diode,
reduced only by the forward voltage drop of the diode. The second
circuit diagram for the LT1615 (Figure 2) shows how you can make a
symmetric power supply using this switching regulator. Here the switch
output of the IC is tapped off and rectified using a symmetrical
rectifier. The voltage divider at the positive output of the rectifier
determines the output voltage.