Datasheet

LT1912
14
1912fa
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 volt-
ages, and on the arrangement of the boost circuit. The
minimum load generally goes to zero once the circuit has
started. Figure 5 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 LT1912, requiring a higher
input voltage to maintain regulation.
Soft-Start
The RUN/SS pin can be used to soft-start the LT1912,
reducing the maximum input current during start-up.
The RUN/SS pin is driven through an external RC filter to
create a voltage ramp at this pin. Figure 6 shows the start-
up and shutdown waveforms with 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 RUN/
SS pin reaches 2.5V.
Synchronization
Synchronizing the LT1912 oscillator to an external fre-
quency can be done by connecting a square wave (with
20% to 80% duty cycle) to the SYNC pin. The square
wave amplitude should have valleys that are below 0.3V
and peaks that are above 0.8V (up to 6V).
The LT1912 may be synchronized over a 250kHz to 500kHz
range. The R
T
resistor should be chosen to set the LT1912
switching frequency 20% below the lowest synchronization
V
IN
BOOST
SW
BD
V
IN
V
OUT
4.7µF
C3
GND
LT1912
V
IN
BOOST
SW
BD
V
IN
V
OUT
4.7µF
C3
D2
GND
LT1912
V
IN
BOOST
SW
BD
V
IN
V
OUT
4.7µF
C3
GND
LT1912
1912 FO4
(4a) For V
OUT
> 2.8V
(4b) For 2.5V < V
OUT
< 2.8V
(4c) For V
OUT
< 2.5V; V
IN(MAX)
= 30V
Figure 4. Three Circuits For Generating The Boost Voltage
input. For example, if the synchronization signal will be
250kHz and higher, the R
T
should be chosen for 200kHz.
To assure reliable and safe operation the LT1912 will only
synchronize when the output voltage is near regulation. It
is therefore necessary to choose a large enough inductor
value to supply the required output current at the frequency
set by the R
T
resistor. See Inductor Selection section. It
is also important to note that slope compensation is set
by the R
T
value: When the sync frequency is much higher
than the one set by R
T
, the slope compensation will be
significantly reduced which may require a larger inductor
value to prevent subharmonic oscillation.
APPLICATIONS INFORMATION