LT3480 查看數據表（PDF） - Linear Technology

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LT3480

36V, 2A, 2.4MHz Step-Down Switching Regulator with 70µA Quiescent Current

Linear Technology 

LT3480

OPERATION

The LT3480 contains a power good comparator which trips

when the FB pin is at 86% of its regulated value. The PG

output is an open-collector transistor that is off when the

output is in regulation, allowing an external resistor to pull

the PG pin high. Power good is valid when the LT3480 is

enabled and VIN is above 3.6V.

The LT3480 has an overvoltage protection feature which

disables switching action when the VIN goes above 38V

typical (36V minimum). When switching is disabled, the

LT3480 can safely sustain input voltages up to 60V.

APPLICATIONS INFORMATION

FB Resistor Network

Operating Frequency Tradeoffs

The output voltage is programmed with a resistor divider

between the output and the FB pin. Choose the 1% resis-

tors according to:

R1=

R2





VOUT

0.79V

−

1

Reference designators refer to the Block Diagram.

Selection of the operating frequency is a tradeoff 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:

Setting the Switching Frequency

The LT3480 uses a constant frequency PWM architecture

that can be programmed to switch from 200kHz to 2.4MHz

by using a resistor tied from the RT pin to ground. A table

showing the necessary RT value for a desired switching

frequency is in Figure 1.

SWITCHING FREQUENCY (MHz)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

RT VALUE (kΩ)

187

121

88.7

68.1

56.2

46.4

40.2

34

29.4

23.7

19.1

16.2

13.3

11.5

9.76

8.66

Figure 1. Switching Frequency vs. RT Value

( ) fSW(MAX)

=

VD + VOUT

tON(MIN) VD + VIN

–

VSW

where VIN is the typical input voltage, VOUT 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

safely accommodate high VIN/VOUT ratio. Also, as shown

in the next section, lower frequency allows a lower dropout

voltage. The reason input voltage range depends on the

switching frequency is because the LT3480 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 ~150ns.

Typical minimum on time at 25°C is 80ns. This means that

the minimum and maximum duty cycles are:

DCMIN = fSW tON(MIN)

DCMAX = 1– fSW tOFF(MIN)

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

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

the minimum switch off time (~150ns). These equations

show that duty cycle range increases when switching

frequency is decreased.

3480fe

For more information www.linear.com/LT3480

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