LT3972 datasheet(13/24 Pages) LINER

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LT3972

3972f

Frequency Compensation

The LT3972 uses current mode control to regulate the

output. This simpliﬁes loop compensation. In particular,

the LT3972 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 com-

pensation 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 stabil-

ity using a transient load. Figure 2 shows an equivalent

circuit for the LT3972 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 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 cur-

rent is stepped from 1A to 3A and back to 1A.

0.8V

gm =

500μmho

GND

LT3972

3680 F02

OUTPUT

ESR

ERROR

AMPLIFIER

CURRENT MODE

POWER STAGE

gm = 5.3mho

POLYMER

TANTALUM

CERAMIC

CPL

Figure 3. Transient Load Response of the LT3972 Front Page

Application as the Load Current is Stepped from 1A to 3A.

VOUT = 5V

Figure 2. Model for Loop Response

3680 F03

1A/DIV

VOUT

100mV/DIV

10μs/DIV

VIN = 12V

VOUT = 3.3V

APPLICATIONS INFORMATION

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