LT3668 datasheet(13/28 Pages) LINER

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LT3668

3668fa

For more information www.linear.com/LT3668

APPLICATIONS INFORMATION

SWITCHING REGULATOR

FB1 Resistor Network

The switching regulator output voltage of the LT3668 is

programmed with a resistor divider between the output

of the switching regulator and the FB1 pin. Choose the

resistor values according to:

R1=R2

VOUT1

1.2V

–1









Reference designators refer to the Block Diagram of the

LT3668.1%resistorsarerecommendedtomaintainoutput

voltage accuracy. Note that choosing larger resistors will

decrease the quiescent current of the application circuit.

Setting the Switching Frequency

The LT3668 regulators use a constant frequency PWM

architecture that can be programmed to switch from

250kHz to 2.2MHz by using a resistor tied from the RT

pin to ground. Table 1 shows the necessary RT value for

a desired switching frequency.

Table 1: Switching Frequency vs RT Value

SWITCHING FREQUENCY (MHz)

RT VALUE (kΩ)

0.25

475

0.3

383

0.4

274

0.5

215

0.6

174

0.8

124

95.3

1.2

1.4

61.9

1.6

51.1

1.8

43.2

37.4

2.2

32.4

Operating Frequency Trade-Offs

Selection of the operating frequency is a trade-off between

efficiency,componentsize,minimumdropoutvoltage,and

maximum input voltage. The advantage of high frequency

operationisthatsmallerinductorandcapacitorvaluesmay

be used. The disadvantages are lower efficiency, lower

maximum input voltage, and higher dropout voltage. The

highest acceptable switching frequency (fSW(MAX)) for a

given application can be calculated as follows:

fSW(MAX)=

VOUT1+ VD

tON(MIN) VIN1– VSW + VD

(

)

where VIN1 is the typical input voltage, VOUT1 is the output

voltage, VD is the catch diode drop (~0.5V) and VSW is the

internal switch drop (~0.5V at max load). This equation

shows that slower switching frequency is necessary to

accommodate high VIN1/VOUT1 ratio.

Lowerfrequencyalsoallowsalowerdropoutvoltage.Input

voltagerangedependsontheswitchingfrequencybecause

the LT3668 switch has finite minimum on and off times.

The switch can turn on for a minimum of ~150ns and turn

off for a minimum of ~190ns (note that the minimum on-

time is a strong function of temperature). The minimum

and maximum duty cycles that can be achieved taking

minimum on and off times into account are:

DCMIN = fSW • tON(MIN)

DCMAX = 1 − fSW • tOFF(MIN)

where fSW is the switching frequency, tON(MIN) is the

minimum switch on-time (~150ns), and tOFF(MIN) is the

minimumswitchoff-time(~190ns).Theseequationsshow

that the duty cycle range increases when the switching

frequency is decreased.

A good choice of switching frequency should allow an

adequate input voltage range (see Input Voltage Range

section) and keep the inductor and capacitor values small.

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