LT3680
14
3680fb
Low-Ripple Burst Mode and Pulse-Skip Mode
The LT3680 is capable of operating in either Low-Ripple Burst
Mode or pulse-skipping mode which are selected using the
SYNC pin. See the Synchronization section for details.
To enhance effi ciency at light loads, the LT3680 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 LT3680 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 LT3680 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
90μA respectively during the sleep time. As the load current
decreases towards a no load condition, the percentage of
time that the LT3680 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 LT3680 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 LT3680 can
operate in Pulse-Skip mode. The benefi t of this mode is
that the LT3680 will enter full frequency standard PWM
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. Select
pulse-skipping mode by applying a clock signal or a DC
voltage higher than 0.8V to the SYNC pin.
BOOST and BIAS Pin Considerations
Capacitor C3 and the internal boost Schottky diode (see
the Block Diagram) are used to generate a boost volt-
age 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 28V. 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 LT3680 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 LT3680 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
Figure 4. Burst Mode Operation
3680 F04
I
L
0.2A/DIV
V
SW
5V/DIV
V
OUT
10mV/DIV
5μs/DIV
V
IN
= 12V
V
OUT
= 3.3V
I
LOAD
= 10mA
APPLICATIONS INFORMATION
