LT3667EUDD View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT3667EUDD

Iq= 50uA at 12Vin to 5V, 3.3V and 2.5V with No Load / 40V 400mA Step-Down Switching Regulator with Dual Fault Protected LDOs

Linear Technology 

LT3667

APPLICATIONS INFORMATION

SWITCHING REGULATOR

FB1 Resistor Network

The switching regulator output voltage of the LT3667 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

LT3667. 1% resistors are recommended to maintain output

voltage accuracy. Note that choosing larger resistors will

decrease the quiescent current of the application circuit.

Setting the Switching Frequency

The LT3667 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 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

1

95.3

1.2

75

1.4

61.9

1.6

51.1

1.8

43.2

2

37.4

2.2

32.4

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)

=

VOUT1 +

tON(MIN) VIN1 –

VD

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.

Lower frequency also allows a lower dropout voltage. Input

voltage range depends on the switching frequency because

the LT3667 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 ~170ns (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

minimum switch off-time (~170ns). These equations show

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.

16

For more information www.linear.com/LT3667

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