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LT3972

3972f

Low-Ripple Burst Mode and Pulse-Skip Mode

The LT3972 is capable of operating in either Low-Ripple

Burst Mode or Pulse-Skip Mode which are selected us-

ing the SYNC pin. See the Synchronization section for

details.

To enhance efﬁciency at light loads, the LT3972 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 LT3972 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 LT3972 delivers power to the output

with single, low current pulses, the output ripple is kept

below 15mV for a typical application. In addition, VIN and

BD quiescent currents are reduced to typically 30μA and

80μA respectively during the sleep time. As the load cur-

rent decreases towards a no load condition, the percentage

of time that the LT3972 operates in sleep mode increases

and the average input current 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 LT3972 will be running at the frequency

programmed by the RT 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 LT3972 can

operate in Pulse-Skip mode. The beneﬁt of this mode is

that the LT3972 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.

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 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 VIN 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 LT3972 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 LT3972 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

Figure 4. Burst Mode Operation

3680 F04

0.2A/DIV

VSW

5V/DIV

VOUT

10mV/DIV

5μs/DIV

VIN = 12V

VOUT = 3.3V

ILOAD = 10mA

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