LT1912 Linear Technology Corporation, LT1912 Datasheet - Page 14

• Manufacturer Part Number: LT1912
• Description: Step-Down Switching Regulator
• Manufacturer: Linear Technology Corporation

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APPLICATIONS INFORMATION
LT1912
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 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
For lower start-up voltage, the boost diode can be tied to
V
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
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 fi lter to
create a voltage ramp at this pin. Figure 6 shows the start-
up and shut-down 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
14
IN
; however, this restricts the input range to one-half of
T
resistor should be chosen to set the LT1912
OUT
. At higher load currents, the inductor
IN
is ramping very slowly.
switching frequency 20% below the lowest synchronization
input. For example, if the synchronization signal will be
250kHz and higher, the R
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
is also important to note that slope compensation is set
by the R
than the one set by R
signifi cantly reduced which may require a larger inductor
value to prevent subharmonic oscillation.
Figure 4. Three Circuits For Generating The Boost Voltage
4.7μF
4.7μF
4.7μF
V
V
V
IN
IN
IN
T
value: When the sync frequency is much higher
(4c) For V
T
resistor. See Inductor Selection section. It
(4b) For 2.5V < V
V
V
V
IN
IN
IN
(4a) For V
LT1912
LT1912
LT1912
GND
GND
GND
OUT
BD
BD
BD
T
BOOST
BOOST
BOOST
, the slope compensation will be
< 2.5V; V
T
SW
SW
SW
should be chosen for 200kHz.
OUT
OUT
> 2.8V
IN(MAX)
C3
C3
C3
< 2.8V
D2
= 30V
1912 FO4
V
OUT
V
V
OUT
OUT
1912f