LT3477IFE Ver la hoja de datos (PDF) - Linear Technology

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LT3477IFE

3A, DC/DC Converter with Dual Rail-to-Rail Current Sense

Linear Technology 

LT3477

APPLICATIO S I FOR ATIO

is a triangular waveform (not a DC waveform like the

output current) some tweaking of the compensation val-

ues (RC and CC on the VC pin) may be required to ensure

a clean inductor ripple current while the constant-current

loop is in effect. For these applications, the constant-

current loop response can usually be improved by reduc-

ing the RC value or by adding a capacitor (with a value of

approximately CC/10) in parallel with the RC and CC com-

pensation network.

Frequency Compensation

The LT3477 has an external compensation pin (VC), which

allows the loop response to be optimized for each applica-

tion. An external resistor and capacitor (or sometimes just

a capacitor) are placed at the VC pin to provide a pole and

a zero (or just a pole) to ensure proper loop compensation.

Several other poles and zeroes are present in the closed-

loop transfer function of a switching regulator, so the VC

pin pole and zero are positioned to provide the best loop

response. A thorough analysis of the switching regulator

control loop is not within the scope of this data sheet, and

will not be presented here, but values of 1k and 4.7nF will

be a good choice for many designs. For those wishing to

optimize the compensation, use the 1k and 4.7nF as a

starting point.

Board Layout

As with all switching regulators, careful attention must be

paid to the PCB board layout and component placement.

To maximize efficiency, switch rise and fall times are made

as short as possible. To prevent radiation and high fre-

quency resonance problems, proper layout of the high

frequency switching path is essential. Minimize the length

and area of all traces connected to the SW pin and always

use a ground plane under the switching regulator to

minimize interplane coupling. The signal path including

the switch, output diode D1 and output capacitor COUT,

contains nanosecond rise and fall times and should be

kept as short as possible.

Soft-Start

For many applications, it is necessary to minimize the

inrush current at start-up. The built-in soft-start circuit

significantly reduces the start-up current spike and output

voltage overshoot. A typical value for the soft-start capaci-

tor is 10nF.

Switching Frequency

The switching frequency of the LT3477 is set by an

external resistor attached to the RT pin. Do not leave this

pin open. A resistor must always be connected for proper

operation. See Table 4 and Figure 4 for resistor values and

corresponding frequencies.

Increasing switching frequency reduces output voltage

ripple but also reduces efficiency. The user should set the

frequency for the maximum tolerable output voltage ripple.

Table 4. Switching Frequency

SWITCHING FREQUENCY (MHz)

3.5

3

2.5

2

1.5

1

0.5

0.2

RT (kΩ)

2.43

3.65

4.87

6.81

10.2

17.4

43.2

107

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

0.1

10

RT (kΩ)

100

3477 F04

Figure 4. Switch Frequency

3477fb

10

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