Datasheet

LT3680
14
Rev C
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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 efficiency at light loads, the LT3680 can be
operated in Low-Ripple Burst Mode operation which
keeps the output capacitor charged to the proper volt-
age 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 out-
put 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 oper-
ates in sleep mode increases and the average input cur-
rent is greatly reduced resulting in high efficiency 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 benefit 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 efficiency. 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 exter-
nal Schottky diode (such as the ON Semi MBR0540), and a
1µF boost capacitor (see Figure 5b). For lower output volt-
ages 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 efficient 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
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