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LTC3604

3604f

APPLICATIONS INFORMATION

Minimum Off-Time/On-Time Considerations

The minimum off-time is the smallest amount of time that

the LTC3604 can turn on the bottom power MOSFET, trip

the current comparator and turn the power MOSFET back

off. This time is typically 40ns. For the controlled on-time

current mode control architecture, the minimum off-time

limit imposes a maximum duty cycle of:

f t

MAX

OFF MIN

()

(

)

–•

()

where f is the switching frequency and tOFF(MIN) is the

minimum off-time. If the maximum duty cycle is surpassed,

due to a dropping input voltage for example, the output

will drop out of regulation. The minimum input voltage to

avoid this dropout condition is:

IN MIN

OUT

OFF MIN

()

•

−

()

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:

f t

MIN

ON 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 is an acceptable result in many

applications, so this constraint may not be of critical

importance in most cases, and high switching frequen-

cies 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 LTC3604 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 connecting

the ITH pin to the INTVCCpin.Toensurestability,itisrecom-

mended that the output capacitance be at least 47μF when

using internal compensation. 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 3. For a 2MHz application, an R-C network of 150pF

and 14kΩ 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 R1 which improves the phase margin.

Figure 3. Compensation Components

ITH

RCOMP

14k

CCOMP

150pF

CBYP

3604 F03

SGND

LTC3604

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