LT3669EUFD 查看數據表（PDF） - Linear Technology

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LT3669EUFD

IO-Link Transceiver with Integrated Step-Down Regulator and LDO

Linear Technology 

LT3669/LT3669-2

APPLICATIONS INFORMATION

SWITCHING REGULATOR

FBOUT Resistor Network

The switching regulator output voltage is programmed

with a resistor divider between its output and the FBOUT

pin. Choose the resistor values according to:

R1= R2





VOUT

0.794V

−

1

Reference designators refer to the Block Diagram. Use 1%

resistors to maintain output voltage accuracy.

Setting the Switching Frequency

The LT3669 switching regulator uses 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 2 lists the required RT values for various

switching frequencies.

Table 2. Switching Frequency vs RT Value

SWITCHING FREQUENCY (MHz)

0.25

RT VALUE (kΩ)

110

0.3

88.7

0.4

63.4

0.5

47.5

0.6

38.3

0.7

31.6

0.8

26.7

0.9

22.6

1.0

19.6

1.2

15.4

1.4

12.1

1.6

10.0

1.8

8.06

2.0

6.65

2.2

5.49

Operating Frequency Trade-Offs

Selection of the operating frequency is a trade-off between

efficiency, component size, minimum dropout voltage and

maximum input voltage. The advantage of high frequency

operation is that smaller inductor and capacitor values may

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 )

=

tON(MIN)

VOUT + VD

• (VL+ − VSW

+

VD )

where VL+ is the typical input voltage, VOUT is the output

voltage, VD is the catch diode drop (~0.72V in LT3669)

and VSW is the internal drop from L+ to SW pins (~1.0V

in LT3669 and ~1.4V in LT3669-2 at maximum load).

This equation shows that a slower switching frequency

is necessary to safely accommodate a high VL+/ VOUT

ratio. Lower frequency allows lower dropout voltage. The

input voltage range depends on the switching frequency

because the LT3669 switch has finite minimum on- and

off-times. The switch can turn off for a minimum of

~210ns, but the minimum on-time is a strong function

of temperature. Use the minimum switch on-time curve

(see Typical Performance Characteristics) to design for an

application’s maximum temperature, while adding about

30% for LT3669 part-to-part variation. 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, and tOFF(MIN) is the minimum

switch-off time. These equations show that the duty cycle

range increases when the switching frequency is decreased.

A good choice of switching frequency allows adequate

input voltage range (see the Input Voltage Range section)

and keeps the inductor and capacitor values small.

For more information www.linear.com/LT3669

3669fa

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