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LT3668

3668fa

For more information www.linear.com/LT3668

APPLICATIONS INFORMATION

Figure 1. Burst Mode Operation

wherefSWisinMHz,andCOUT1istherecommendedoutput

capacitance in μF. Use X5R or X7R types. This choice will

provide low output ripple and good transient response.

Transientperformancecanbeimprovedwithahighervalue

capacitorifcombinedwithaphaseleadcapacitor(typically

22pF) between the output and pin FB1. Note that a larger

phase lead capacitor should be used with a large output

capacitor. A lower value of output capacitor can be used to

save space and cost but transient performance will suffer.

When choosing a capacitor, look carefully through the

data sheet to find out what the actual capacitance is under

operating conditions (applied voltage and temperature).

A physically larger capacitor, or one with a higher voltage

rating, may be required. Table 3 lists several capacitor

vendors.

Table 3: Capacitor Vendors

VENDOR

URL

Panasonic

www.panasonic.com

Kemet

www.kemet.com

Sanyo

www.sanyovideo.com

Murata

www.murata.com

AVX

www.avxcorp.com

Taiyo Yuden

www.taiyo-yuden.com

Audible Noise

Ceramic capacitors are small, robust and have very

low ESR. However, ceramic capacitors can sometimes

cause problems when used with the LT3668 due to their

piezoelectric nature. When in Burst Mode operation, the

LT3668’sswitchingfrequencydependsontheloadcurrent,

and at very light loads the LT3668 can excite the ceramic

capacitor at audio frequencies, generating audible noise.

Since the LT3668 operates at a lower current limit during

Burst Mode operation, the noise is typically very quiet. If

this is unacceptable, use a high performance tantalum or

electrolytic capacitor at the output.

Low Ripple Burst Mode Operation

To enhance efficiency at light loads, the LT3668 oper-

ates in low ripple Burst Mode operation which keeps

the output capacitor charged to the proper voltage while

minimizingtheinputquiescentcurrent.DuringBurstMode

operation, the LT3668 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 LT3668 delivers power

to the output with single, low current pulses, the output

ripple is kept below 5mV for a typical application. As the

load current decreases towards a no load condition, the

percentage of time that the LT3668 operates in sleep mode

increases and the average input current is greatly reduced

resulting in high efficiency even at very low loads. Note

thatduringBurstModeoperation,theswitchingfrequency

will be lower than the programmed switching frequency.

At higher output loads (above ~50mA for the front page

application) the LT3668 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 operation is seamless, and will not

disturb the output voltage.

3668 F01

1µs/DIV

FRONT PAGE APPLICATION, VOUT1 = 5V

VSW

5V/DIV

VOUT1

5mV/DIV

100mA/DIV

ILOAD = 10mA

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