LT3480 datasheet(13/24 Pages) LINER

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LT3480

3480fb

APPLICATIONS INFORMATION

operation, the noise is typically very quiet to a casual ear.

If this is unacceptable, use a high performance tantalum

or electrolytic capacitor at the output.

A ﬁnal precaution regarding ceramic capacitors concerns

the maximum input voltage rating of the LT3480. A ceramic

input capacitor combined with trace or cable inductance

forms a high quality (under damped) tank circuit. If the

LT3480 circuit is plugged into a live supply, the input volt-

age can ring to twice its nominal value, possibly exceeding

the LT3480’s rating. This situation is easily avoided (see

the Hot Plugging Safely section).

Frequency Compensation

The LT3480 uses current mode control to regulate the

output. This simpliﬁes loop compensation. In particular, the

LT3480 does not require the ESR of the output capacitor

for stability, so you are free to use ceramic capacitors to

achieve low output ripple and small circuit size. Frequency

compensation is provided by the components tied to the

VC pin, as shown in Figure 2. Generally a capacitor (CC)

and a resistor (RC) in series to ground are used. In addi-

tion, there may be lower value capacitor in parallel. This

capacitor (CF) is not part of the loop compensation but

is used to ﬁlter noise at the switching frequency, and is

required only if a phase-lead capacitor is used or if the

output capacitor has high ESR.

Loop compensation determines the stability and transient

performance. Designing the compensation network is

a bit complicated and the best values depend on the

application and in particular the type of output capacitor.

A practical approach is to start with one of the circuits in

this data sheet that is similar to your application and tune

the compensation network to optimize the performance.

Stability should then be checked across all operating

conditions, including load current, input voltage and

temperature. The LT1375 data sheet contains a more

thorough discussion of loop compensation and describes

how to test the stability using a transient load. Figure 2

shows an equivalent circuit for the LT3480 control loop.

The error ampliﬁer is a transconductance ampliﬁer with

ﬁnite output impedance. The power section, consisting

of the modulator, power switch and inductor, is modeled

as a transconductance ampliﬁer generating an output

0.8V

gm =

420μmho

GND

LT3480

3480 F02

OUTPUT

ESR

ERROR

AMPLIFIER

CURRENT MODE

POWER STAGE

gm = 3.5mho

POLYMER

TANTALUM

CERAMIC

CPL

Figure 2. Model for Loop Response

current proportional to the voltage at the VC pin. Note that

the output capacitor integrates this current, and that the

capacitor on the VC pin (CC) integrates the error ampliﬁer

output current, resulting in two poles in the loop. In most

cases a zero is required and comes from either the output

capacitor ESR or from a resistor RC in series with CC.

This simple model works well as long as the value of the

inductor is not too high and the loop crossover frequency

is much lower than the switching frequency. A phase lead

capacitor (CPL) across the feedback divider may improve

the transient response. Figure 3 shows the transient

response when the load current is stepped from 500mA

to 1500mA and back to 500mA.

Figure 3. Transient Load Response of the LT3480 Front Page

Application as the Load Current is Stepped from 500mA to

1500mA. VOUT = 3.3V

3480 F03

0.5A/DIV

VOUT

100mV/DIV

10μs/DIV

VIN = 12V; FRONT PAGE APPLICATION

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