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LT3970/LT3970-3.3/LT3970-5

3970fa

operates at a lower current limit during Burst Mode

operation, the noise is typically very quiet to a casual ear.

If this is unacceptable, use a high performance tantalum

or electrolytic capacitor at the output.

A ﬁnal precaution regarding ceramic capacitors concerns

the maximum input voltage rating of the LT3970. As pre-

viously mentioned, a ceramic input capacitor combined

with trace or cable inductance forms a high quality (under

damped) tank circuit. If the LT3970 circuit is plugged into a

live supply, the input voltage can ring to twice its nominal

value, possibly exceeding the LT3970’s rating. This situation

is easily avoided (see the Hot Plugging Safely section).

Low Ripple Burst Mode Operation

To enhance efﬁciency at light loads, the LT3970 operates

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 opera-

tion, the LT3970 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 capaci-

tor. Because the LT3970 delivers power to the output with

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

5mV for a typical application. See Figure 2.

As the load current decreases towards a no load condition,

the percentage of time that the LT3970 operates in sleep

mode increases and the average input current is greatly

reduced resulting in high efﬁciency even at very low loads.

Note that during Burst Mode operation, the switching

frequency will be lower than the programmed switching

frequency. See Figure 3.

At higher output loads (above ~45mA for the front page

application) the LT3970 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.

BOOST and BD 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 4 shows two ways to ar-

range the boost circuit. The BOOST pin must be more than

1.9V above the SW pin for best efﬁciency. For outputs of

2.2V and above, the standard circuit (Figure 4a) is best.

For outputs between 2.2V and 2.5V, use a 0.47μF boost

capacitor. For output voltages below 2.2V, the boost diode

can be tied to the input (Figure 4b), or to another external

supply greater than 2.2V. However, the circuit in Figure 4a

is more efﬁcient because the BOOST pin current and BD

pin quiescent current come from a lower voltage source.

Also, be sure that the maximum voltage ratings of the

BOOST and BD pins are not exceeded.

The minimum operating voltage of an LT3970 application

is limited by the minimum input voltage (4.2V) and by the

maximum duty cycle as outlined in a previous section. For

output voltages greater than 3.4V, the minimum input volt-

age is also limited by the boost circuit for proper start-up.

If the input voltage is ramped slowly, the boost capacitor

APPLICATIONS INFORMATION

Figure 2. Burst Mode Operation

Figure 3. Switching Frequency in Burst Mode Operation

VOUT

5mV/DIV

VSW

5V/DIV

100mA/DIV

2μs/DIV

FRONT PAGE APPLICATION

VIN = 12V

VOUT = 5V

ILOAD = 10mA

3970 F02

LOAD CURRENT (mA)

400

500

700

150

250

3070 F03

300

200

100

200

300

350

100

600

FRONT PAGE APPLICATION

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