LT3480
14
3480fb
APPLICATIONS INFORMATION
operation at a lower output load current than when in Burst
Mode. The front page application circuit will switch at full
frequency at output loads higher than about 60mA.
BOOST and BIAS Pin Considerations
Capacitor C3 and the internal boost Schottky diode (see the
Block Diagram) are used to generate a boost voltage that is
higher than the input voltage. In most cases a 0.22μF capacitor
will work well. Figure 2 shows three ways to arrange the boost
circuit. The BOOST pin must be more than 2.3V above the
SW pin for best effi ciency. For outputs of 3V and above, the
standard circuit (Figure 5a) is best. For outputs between 2.8V
and 3V, use a 1μF boost capacitor. A 2.5V output presents a
special case because it is marginally adequate to support the
boosted drive stage while using the internal boost diode. For
reliable BOOST pin operation with 2.5V outputs use a good
external Schottky diode (such as the ON Semi MBR0540),
and a 1μF boost capacitor (see Figure 5b). For lower output
voltages the boost diode can be tied to the input (Figure 5c), or
to another supply greater than 2.8V. Tying BD to V
IN
reduces
the maximum input voltage to 30V. The circuit in Figure 5a
is more effi cient because the BOOST pin current and BD pin
quiescent current comes from a lower voltage source. You
must also be sure that the maximum voltage ratings of the
BOOST and BD pins are not exceeded.
The minimum operating voltage of an LT3480 application
is limited by the minimum input voltage (3.6V) and by the
maximum duty cycle as outlined in a previous section. For
proper startup, the minimum input voltage is also limited
by the boost circuit. If the input voltage is ramped slowly,
or the LT3480 is turned on with its RUN/SS 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 6 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
Figure 4. Burst Mode Operation
3480 F04
I
L
0.2A/DIV
V
SW
5V/DIV
V
OUT
10mV/DIV
5μs/DIV
V
IN
= 12V; FRONT PAGE APPLICATION
I
LOAD
= 10mA
Low-Ripple Burst Mode and Pulse-Skip Mode
The LT3480 is capable of operating in either Low-Ripple
Burst Mode or Pulse-Skip Mode which are selected using the
SYNC pin. See the Synchronization section for details.
To enhance effi ciency at light loads, the LT3480 can be
operated in Low-Ripple Burst Mode operation which keeps
the output capacitor charged to the proper voltage while
minimizing the input quiescent current. During Burst Mode
operation, the LT3480 delivers single cycle bursts of current
to the output capacitor followed by sleep periods where the
output power is delivered to the load by the output capacitor.
Because the LT3480 delivers power to the output with single,
low current pulses, the output ripple is kept below 15mV
for a typical application. In addition, V
IN
and BD quiescent
currents are reduced to typically 30μA and 80μA respec-
tively during the sleep time. As the load current decreases
towards a no load condition, the percentage of time that the
LT3480 operates in sleep mode increases and the average
input current is greatly reduced resulting in high effi ciency
even at very low loads. See Figure 4. At higher output loads
(above 140mA for the front page application) the LT3480
will be running at the frequency programmed by the R
T
resistor, and will be operating in standard PWM mode. The
transition between PWM and Low-Ripple Burst Mode is
seamless, and will not disturb the output voltage.
If low quiescent current is not required the LT3480 can
operate in Pulse-Skip mode. The benefi t of this mode is
that the LT3480 will enter full frequency standard PWM
