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LTC3633A-2/LTC3633A-3

3633a23f

APPLICATIONS INFORMATION

Conversely, the minimum on-time is the smallest dura-

tion of time in which the top power MOSFET can be in

its “on” state. This time is typically 20ns. In continuous

mode operation, the minimum on-time limit imposes a

minimum duty cycle of:

DC(MIN) = f• tON(MIN)

()

where tON(MIN) is the minimum on-time. As the equation

shows, reducing the operating frequency will alleviate the

minimum duty cycle constraint.

In the rare cases where the minimum duty cycle is

surpassed, the output voltage will still remain in regula-

tion, but the switching frequency will decrease from its

programmed value. This constraint may not be of critical

importance in most cases, so high switching frequencies

may be used in the design without any fear of severe

consequences. As the sections on Inductor and Capacitor

selection show, high switching frequencies allow the use

of smaller board components, thus reducing the footprint

of the application circuit.

Internal/External Loop Compensation

The LTC3633A-2 provides the option to use a ﬁxed internal

loop compensation network to reduce both the required

external component count and design time. The internal

loop compensation network can be selected by connect-

ing the ITH pin to the INTVCC pin. To ensure stability it is

recommended that internal compensation only be used with

applications with fSW > 1MHz. Alternatively, the user may

choose speciﬁc external loop compensation components

to optimize the main control loop transient response as

desired. External loop compensation is chosen by simply

connecting the desired network to the ITH pin.

Suggested compensation component values are shown in

Figure 4. 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 3. Capacitor CF

provides phase lead by creating a high frequency zero

with R2 which improves the phase margin.

Figure 4. Compensation Component

ITH

RCOMP

13k

CCOMP

220pF

3633a23 F04

SGND

LTC3633A-2

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 4 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

(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

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