3681 datasheet(14/24 Pages) LINER

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LT3681

3681f

boost diode can be tied to VIN; however, this restricts the

input range to one-half of the absolute maximum rating

of the BOOST pin.

At light loads, the inductor current becomes discontinuous

and the effective duty cycle at the BOOST pin (not the SW

pin) can be very high. This reduces the minimum input

voltage to approximately 300mV above VOUT.Athigherload

currents, the inductor current is continuous and the duty

cycle is limited by the maximum duty cycle of the LT3681,

requiring a higher input voltage to maintain regulation.

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 5 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 2.8V 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. The circuit in Figure 5a

is more efﬁcient because the BOOST pin current and BIAS

pin quiescent current comes from a lower voltage source.

You must also be sure that the maximum voltage ratings

of the BOOST and BIAS pins are not exceeded.

The minimum operating voltage of an LT3681 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 LT3681 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. If, however, the LT3681 is

started by the RUN/SS pin and the output is discharged, the

discharged output capacitance will often present enough

of a load to allow the circuit to start. Figure 6 gives plots

of the input voltage required for three different situations:

the worst case situation where RUN/SS is tied to VIN and

VIN is ramped up very slowly, the minimum input voltage

at which the circuit will regulate when start-up is controlled

by RUN/SS, and the minimum input voltage required to

maintain output regulation. For lower start-up voltage, the

VIN

BOOST

VIN

VOUT

4.7

GND

LT3681

VIN

BOOST

VIN

VOUT

4.7

GND

LT3681

VIN

BOOST

VIN

VOUT

4.7

GND

LT3681

3681 FO5

COUT

(5a) For VOUT > 2.8V

(5b) For 2.5V < VOUT < 2.8V

(5c) For VOUT < 2.5V

Figure 5. Three Circuits For Generating The Boost Voltage

APPLICATIONS INFORMATION

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