lt1913 Linear Technology Corporation, lt1913 Datasheet - Page 16

• Manufacturer Part Number: lt1913
• Description: 25v, 3.5a, 2.4mhz Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT1913
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 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
is ramping very slowly. For lower start-up voltage, the
boost diode can be tied to V
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 LT1913, requiring a higher
input voltage to maintain regulation.
Soft-Start
The RUN/SS pin can be used to soft-start the LT1913,
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 LT1913 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 LT1913 may be synchronized over a 250kHz to 2MHz
range. The R
switching frequency 20% below the lowest synchronization
16
T
resistor should be chosen to set the LT1913
OUT
. At higher load currents, the inductor
IN
.
IN
input. For example, if the synchronization signal will be
250kHz and higher, the R
To assure reliable and safe operation the LT1913 will only
synchronize when the output voltage is near regulation
as indicated by the PG fl ag. It is therefore necessary to
choose a large enough inductor value to supply the required
output current at the frequency set by the R
Inductor Selection section. It is also important to note that
slope compensation is set by the R
frequency is much higher than the one set by R
compensation will be signifi cantly reduced which may
require a larger inductor value to prevent subharmonic
oscillation.
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, an LT1913 buck regulator will tolerate a shorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
LT1913 is absent. This may occur in battery charging ap-
plications or in battery backup systems where a battery
or some other supply is diode OR-ed with the LT1913’s
output. If the V
pin is held high (either by a logic signal or because it is
tied to V
its quiescent current through its SW pin. This is fi ne if
your system can tolerate a few mA in this state. If you
ground the RUN/SS pin, the SW pin current will drop to
essentially zero. However, if the V
0.22μF
RUN
15k
Figure 6. To Soft-Start the LT1913, Add a Resisitor
and Capacitor to the RUN/SS Pin
IN
), then the LT1913’s internal circuitry will pull
RUN/SS
GND
IN
pin is allowed to fl oat and the RUN/SS
T
should be chosen for 200kHz.
2ms/DIV
IN
T
pin is grounded while
value: When the sync
T
1913 F06
resistor. See
T
, the slope
I
1A/DIV
V
2V/DIV
V
2V/DIV
L
RUN/SS
OUT
1913f