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LTC3633

3633f

APPLICATIONS INFORMATION

Suggested compensation component values are shown in

Figure 3. For a 2MHz application, an R-C network of 220pF

and 13kΩ provides a good starting point. The bandwidth

of the loop increases with decreasing C. If R is increased

by the same factor that C 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.

A 10pF bypass capacitor on the ITH pin is recommended

for the purposes of ﬁltering out high frequency coupling

from stray board capacitance. In addition, a feedforward

capacitor CF can be added to improve the high frequency

response, as previously shown in Figure 2. Capacitor CF

provides phase lead by creating a high frequency zero

with R2 which improves the phase margin.

(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 their various types and values determine the

loop gain and phase. An output current pulse of 20% to

100% of full load current having a rise time of ~1μs will

produce output voltage and ITH pin waveforms that will

give a sense of the overall loop stability without breaking

the feedback loop.

Switching regulators take several cycles to respond to a

step in load current. When a load step occurs, VOUT im-

mediately shifts by an amount equal to ΔILOAD • ESR, where

ESR is the effective series resistance of COUT. ΔILOAD also

begins to charge or discharge COUT generating a feedback

error signal used by the regulator to return VOUT to its

steady-state value. During this recovery time, VOUT can

be monitored for overshoot or ringing that would indicate

a stability problem.

When observing the response of VOUT to a load step, the

initial output voltage step may not be within the bandwidth

of the feedback loop, so the standard second order over-

shoot/DC ratio cannot be used to determine 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 Linear Technology

Application Note 76.

In some applications, a more severe transient can be caused

by switching in loads with large (>10μF) input capacitors.

The discharged input capacitors are effectively put in paral-

lel with COUT, causing a rapid drop in VOUT. No regulator

can deliver enough current to prevent this problem, if the

switch connecting the load has low resistance and is driven

quickly. The solution is to limit the turn-on speed of the load

switch driver. A hot swap controller is designed speciﬁcally

for this purpose and usually incorporates current limiting,

short-circuit protection, and soft starting.

Figure 3. Compensation Component

ITH

RCOMP

13k

CCOMP

220pF

3633 F03

SGND

LTC3633

Checking Transient Response

The regulator loop response can be checked by observing

the response of the system to a load step. When conﬁgured

for external compensation, the availability of the ITH pin

not only allows 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

behavior at this test point reﬂect the closed 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 components shown in Figure 3 circuit

will provide an adequate starting point for most applica-

tions. The series R-C ﬁlter sets the dominant pole-zero

loop compensation. The values can be modiﬁed slightly

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