7

LTC1701

Setting the Output Voltage

The LTC1701 develops a 1.25V reference voltage between

Figure 2. The output voltage is set by a resistive divider

according to the following formula:

VV

R

R









125

1

2

1

.

To prevent stray pickup, a capacitor of about 5pF can be

added across R1, located close to the LTC1701. Unfortu-

nately, the load step response is degraded by this capaci-

tor. Using a good printed circuit board layout eliminates

the need for this capacitor. Great care should be taken to

inductor or the SW line.

APPLICATIO S I FOR ATIO

SGND

LTC1701

R2

1%

R1

100k

1%

1701 F02

5pF

Transient Response

The OPTI-LOOP compensation allows the transient re-

sponse to be optimized for a wide range of loads and

allows optimization of the control loop behavior but also

provides a DC coupled and AC filtered closed-loop re-

sponse test point. The DC step, rise time and settling at this

test point truly reflects the closed-loop response. Assum-

ing a predominately second order system, phase margin

and/or damping factor can be estimated using the percent-

age of overshoot seen at this pin. The bandwidth can also

be estimated by examining the rise time at the pin.

will provide an adequate starting point for most applica-

tions. The series R3-C3 filter sets the dominant pole-zero

loop compensation. The values can be modified slightly

Figure 2. Setting the Output Voltage

(from 0.5 to 2 times their suggested values) to optimize

transient response once the final PC layout is done and the

particular output capacitor type and value have been

determined. The output capacitors need to be selected

because the various types and values determine the loop

feedback factor gain and phrase. An output current pulse

of 20% to 100% of full-load current having a rise time of

1

waveforms that will give a sense of the overall loop

stability without breaking the feedback loop.

The initial output voltage step may not be within the

bandwidth of the feedback loop, so the standard second-

order overshoot/DC ratio cannot be used to determine

phase margin. The gain of the loop increases with R3 and

the bandwidth of the loop increases with decreasing C3. If

R3 is increased by the same factor that C3 is decreased,

the zero frequency will be kept the same, thereby keeping

the phase the same in the most critical frequency range of

the feedback loop. In addition, a feed-forward capacitor,

creating a high frequency zero with R2 which improves the

phase margin.

The output voltage settling behavior is related to the

stability of the closed-loop system and will demonstrate

the actual overall supply performance. For a detailed

explanation of optimizing the compensation components,

including a review of control loop theory, refer to Applica-

tion Note 76.

RUN Function

loop compensation and a means to shut down the LTC1701.

Soft-start can also be implemented with this pin. Soft-start

the internal peak inductor current. Power supply sequenc-

ing can also be accomplished using this pin.

An external pull-up is required to charge the external

capacitor C3 in Figure 1. Typically, a 1M resistor between

reaches about 0.8V the LTC1701 begins operating. At this

normal operating range of 1.25V to 2.25V.