Data Sheet

LT7200S

analog.com

Rev 0

23 of 35

Figure 36. External Compensation Network

Select the proper ITH components for OPTI-LOOP® optimization. The compensation network is shown in Figure 36.

The RC filter sets the dominant pole-zero loop compensation. The gain of the loop increases with the RITH and the

bandwidth of the loop increases with decreasing CITH. If RITH is increased by the same factor that CITH is decreased, the

zero frequency is kept the same, thereby keeping the phase the same in most critical frequency ranges of the

feedback loop.

For a 1MHz application, an R-C network of 470pF and 10kΩ provides a good starting point. A 4.7pF bypass capacitor,

CITHP, on the ITH pin is recommended to filter out high frequency coupling from stray board capacitance. Table 6

provides a basic guideline for the compensation values to use, given the frequency of the part. Slight tweaks to those

values may be required depending on the application's required output capacitance.

Table 6. Compensation Values

Frequency

RITH

CITH

CITHP

500kHz

15k

470pF

4.7pF

700kHz

10k

470pF

4.7pF

1MHz

10k

470pF

4.7pF

1.5MHz

10k

470pF

4.7pF

2MHz

10k

220pF

4.7pF

Checking Transient Response

The OPTI-LOOP compensation allows the transient response to be optimized for a wide range of loads and output

capacitors. The availability of the ITH pin not only allows for optimization of the control loop behavior but also

provides a DC-coupled and AC-filtered closed loop response test point. The DC step, rise time, and settling at this test

point truly reflects these close loop response. Assuming a predominantly second order system, phase margin and/or

damping factor can be estimated using the percentage of overshoot seen at this pin.

The ITH external component shown in the Table 6 circuit provides an adequate starting point for most applications.

The RC filter sets the dominant pole-zero loop compensation. The values can be modified slightly (from 0.5 to 2 times

their suggested value) to optimize transient response once the final PC layout is done and the output capacitor type

and value are determined. Select the output capacitors because their various types and values determine the loop

feedback factor gain and phase. In addition, add a feedforward capacitor CFF to improve the high frequency response,

as shown in Figure 33. Capacitor CFF provides phase lead by creating a high frequency zero with R2, which improves

the phase margin.

An output current pulse of 20% to 100% of full load current having a rise time of ~1μs produces output voltage and

ITH pin waveforms that give a sense of the overall loop stability without breaking the feedback loop. Switching

regulators may take several cycles to respond to a step in load current. When a load step occurs, VOUT immediately

shifts by an amount equal to the ΔILOAD x ESR, where ESR is the effective series resistance of COUT. ΔILOAD also begins to

RITH

ITH

CITHP

CITH