LTC4449 datasheet(46/108 Pages) LINER

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LTC3882-1

Rev A

For more information www.analog.com

APPLICATIONS INFORMATION

Figure 22. Type 3 Compensation Circuit

handling requirements of the application. Aluminum elec-

trolytic and dry tantalum capacitors are both available in

surfacemountconfigurations.Newspecialpolymersurface

mount capacitors offer very low ESR also but have much

lower capacitive density per unit volume. In the case of

tantalum, it is critical that the capacitors are surge tested

for use in switching power supplies. Several excellent out-

put capacitor choices include the Sanyo POSCAP TPD/E/F

series, the Kemet T520, T530 and A700 series, NEC/Tokin

NeoCapacitors and Panasonic SP series. Other suitable

capacitor types include Nichicon PL series and Sprague

595D series. Consult the manufacturer for other specific

recommendations.

Feedback Loop Compensation

The LTC3882-1 is a voltage mode controller with a second,

dedicated current sharing loop to provide excellent phase-

to-phase current sharing in PolyPhase applications. The

current sharing loop is internally compensated.

While Type 2 compensation for the voltage control loop

may be adequate in some applications (such as with the

use of high ESR bulk capacitors), Type 3 compensation

and ceramic capacitors are recommended for optimum

transient response.

Figure 22 shows a simplified view of the error amplifier EA

for one LTC3882-1 channel. The positive input of the error

amplifier is connected to the output of an internal 12-bit

DAC fed by a 1.024V reference, while the negative input is

connected to the FB pin and other internal circuits (not all

shown). R1 is internal to the IC with a value range given

by the RVSFB parameter in the Electrical Characteristics

table. The output is connected to COMP, from which the

PWM controller derives the required output duty cycle. To

speedupovershootrecoverytime,themaximumpotential

at the COMP pin is internally clamped.

Unlike many regulators that use a transconductance (gm)

amplifier, the LTC3882-1 is designed to use an inverting

summing amplifier topology with the FB pin configured

as a virtual ground. This allows feedback gain to be tightly

controlled by external components, which is not possible

with a simple gm amplifier. The voltage feedback amplifier

also provides flexibility in choosing pole and zero loca-

tions. In particular, it allows the use of Type 3 compensa-

tion to provide phase boost at the LC pole frequency for

significantly improving the control loop phase margin,

as shown in Figure 23.

VOUT

VDAC

COMP

INTERNAL

3882 F22

In a typical LTC3882-1 circuit, the feedback loop closed

around this control amplifier and compensation network

consists of the line feedforward circuit, the modulator,

the external inductor and the output capacitor. All these

componentsaffectloopbehaviorandneedtobeaccounted

for in the frequency compensation.

The modulator consists of the PWM generator, the output

MOSFET drivers and the external MOSFETs themselves.

Step-down modulator gain varies linearly with the input

voltage. The line feedforward circuit compensates for this

change in gain, and provides a constant gain AMOD of 4V/V

from the error amplifier output COMP to the inductor input

(average DC voltage) regardless of VIN. The combination

of the line feedforward circuit and the modulator looks

like a linear voltage transfer function from COMP to the

inductor input with a fairly benign AC behavior at typical

loop compensation frequencies. Significant phase shift

will not begin to occur in this transfer function until half

the switching frequency.

Figure 23. Type 3 Compensation Frequency Response

FREQ

–1

GAIN

PHASE

BOOST

–90

–180

–270

–380

38821 F23

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