LT3477 datasheet(10/20 Pages) LINER

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LT3477

3477fc

some tweaking of the compensation values (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 reducing the RC value or by

adding a capacitor (with a value of approximately CC/10)

in parallel with the RC and CC compensation 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 place-

ment. To maximize efﬁciency, switch rise and fall times

are made as short as possible. To prevent radiation and

high frequency 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 out-

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

capacitor is 10nF.

Switching Frequency

The switching frequency of the LT3477 is set by an ex-

ternal 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.

Figure 4. Switch Frequency

RT (kΩ)

1.5

1.0

0.5

3.5

3.0

2.5

2.0

3477 F04

0.1

100

APPLICATIONS INFORMATION

Table 4. Switching Frequency

SWITCHING FREQUENCY (MHz)

RT (kΩ)

3.5

2.43

3.65

2.5

4.87

6.81

1.5

10.2

17.4

0.5

43.2

0.2

107

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