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LTC1702A

1702afa

tors need type 3 compensation to obtain acceptable phase

margin with a high bandwidth feedback loop.

Feedback Component Selection

Selecting the R and C values for a typical type 2 or type 3

loop is a nontrivial task. The applications shown in this data

sheet show typical values, optimized for the power com-

ponents shown. They should give acceptable performance

with similar power components, but can be way off if even

one major power component is changed significantly.

Applications that require optimized transient response will

need to recalculate the compensation values specifically

for the circuit in question. The underlying mathematics are

complex, but the component values can be calculated in a

straightforward manner if we know the gain and phase of

the modulator at the crossover frequency.

Modulator gain and phase can be measured directly from

a breadboard, or can be simulated if the appropriate

parasitic values are known. Measurement will give more

accurate results, but simulation can often get close enough

to give a working system. To measure the modulator gain

and phase directly, wire up a breadboard with an LTC1702A

and the actual MOSFETs, inductor, and input and output

capacitors that the final design will use. This breadboard

should use appropriate construction techniques for high

speed analog circuitry: bypass capacitors located close to

the LTC1702A, no long wires connecting components,

appropriately sized ground returns, etc. Wire the feedback

amplifier as a simple type 1 loop, with a 10k resistor from

VOUT to FB and a 0.1µF feedback capacitor from COMP to

FB. Choose the bias resistor (RB) as required to set the

desired output voltage. Disconnect RB from ground and

connect it to a signal generator or to the source output of

a network analyzer (Figure 12) to inject a test signal into

the loop. Measure the gain and phase from the COMP pin

to the output node at the positive terminal of the output

capacitor. Make sure the analyzer’s input is AC coupled so

that the DC voltages present at both the COMP and VOUT

nodes don’t corrupt the measurements or damage the

analyzer.

Type 2 loops work well in systems where the ESR zero in

the LC roll-off happens close to the LC pole, limiting the

total phase shift due to the LC. The additional phase

compensation in the feedback amplifier allows the 0dB

point to be at or above the LC pole frequency, improving

loop bandwidth substantially over a simple type 1 loop. It

has limited ability to compensate for LC combinations

where low capacitor ESR keeps the phase shift near 180

for an extended frequency range. LTC1702A circuits using

conventional switching grade electrolytic output capaci-

tors can often get acceptable phase margin with type 2

compensation.

“Type 3” loops (Figure 11) use two poles and two zeros to

obtain a 180

° phase boost in the middle of the frequency

band. A properly designed type 3 circuit can maintain

acceptable loop stability even when low output capacitor

ESR causes the LC section to approach 180

° phase shift

well above the initial LC roll-off. As with a type 2 circuit, the

loop should cross through 0dB in the middle of the phase

bump to maximize phase margin. Many LTC1702A cir-

cuits using low ESR tantalum or OS-CON output capaci-

APPLICATIONS INFORMATION

OUT

1702A F11a

VREF

–

GAIN

(dB)

PHASE

(DEG)

1702A F11b

–90

–180

–270

+6dB/OCT

–6dB/OCT

PHASE

GAIN

–6dB/OCT

Figure 11a. Type 3 Amplifier Schematic Diagram

Figure 11B. Type 3 Amplifier Transfer Function

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