–12–

ADDC02812DA/ADDC02815DA

REV. A

Incremental Negative Resistance: A POL converter is designed

to hold its output voltage constant no matter how its input volt-

age varies. Given a constant load current, the power drawn from

the input bus is therefore also a constant. If the input voltage

increases by some factor, the input current must decrease by the

same factor to keep the power level constant. In incremental

terms, a positive incremental change in the input voltage results

in a negative incremental change in the input current. The POL

converter therefore looks, incrementally, as a negative resistor.

The value of this negative resistor at a particular operating

R

–V

IN

I

IN

Note that this resistance is a function of the operating point. At

full load and low input line, the resistance is its smallest, while

at light load and high input line, it is its largest.

Potential System Instability: The preceding analysis assumes

dc voltages and currents. For ac waveforms the incremental input

model for the POL converter must also include the effects of its

input filter and control loop dynamics. When the POL con-

verter is connected to a power source, modeled as a voltage

ADI DC/DC CONVERTER

INPUT

TERMINALS

Figure 25. Model of Power Source and POL Converter

Connection

The network shown in Figure 25 is second order and has the

following characteristic equation:

+ R









+1 = 0

ship must hold:

too small, the system might be unstable. This condition would

first be observed at low input line and full load since the abso-

If an instability results and it cannot be corrected by changing

LISN requirement, one possible solution is to place a capacitor

across the input of the POL converter. Another possibility is to

place a small resistor in series with this extra capacitor.

The analysis so far has assumed the source of power was a volt-

age source (e.g., a battery) with some source impedance. In

some cases, this source may be the output of a front-end (FE)

converter. Although each FE converter is different, a model for

a typical one would have an LC output filter driven by a voltage

source whose value was determined by the feedback loop. The

LC filter usually has a high Q, so the compensation of the feed-

back loop is chosen to help dampen any oscillations that result

from load transients. In effect, the feedback loop adds “positive

resistance” to the LC network.

When the POL converter is connected to the output of this FE

converter, the POL’s “negative resistance” counteracts the

effects of the FE’s “positive resistance” offered by the feedback

loop. Depending on the specific details, this might simply mean

that the FE converter’s transient response is slightly more oscil-

latory, or it may cause the entire system to be unstable.

mately 1

µH and C

µF. Figure 12 shows a

more accurate depiction of the input impedance of the converter

as a function of frequency. The negative resistance is, itself, a

very good incremental model for the power state of the con-

verter for frequencies into the several kHz range (see Figure 12).