LTC3613 datasheet(22/44 Pages) AD

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LTC7131-1

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Minimum Off-Time and Minimum On-Time

Considerations

The minimum off-time, tOFF(MIN), is the smallest amount of

time that the LTC7131-1 is capable of turning on the bottom

power MOSFET, tripping the current comparator and turning

the power MOSFET back off. This time is generally about

100ns. The minimum off-time limit imposes a maximum

duty cycle of tON/(tON + tOFF(MIN)). If the maximum duty

cycle is reached, due to a dropping input voltage for exam-

ple, then the output will drop out of regulation. The mini-

mum input voltage to avoid dropout is given by Equation 4.

VIN(MIN) = VOUT •

tON + tOFF(MIN)

tON

(4)

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 75ns. In continuous

mode operation, the minimum on-time limit imposes a

minimum duty cycle of Equation 5.

DCMIN = f • tON(MIN)

(5)

where tON(MIN) is the minimum on-time. As the Equation 5

shows, reducing the operating frequency will alleviate the

minimum duty cycle constraint.

In the cases where the minimum duty cycle is surpassed,

the output voltage will still remain in regulation, 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. 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 com-

ponents, thus reducing the size of the application circuit.

Inductor Selection

Given the desired input and output voltages, the inductor

value and operating frequency determine the ripple cur-

rent given by Equation 6.

ΔIL =

VOUT

f •L

• 1−

VOUT

VIN

⎛

⎝

⎜

⎞

⎠

⎟

(6)

Lower ripple current reduces core losses in the inductor,

ESR losses in the output capacitors and output voltage

ripple. Highest efficiency operation is obtained at low fre-

quency with small ripple current. However, achieving this

requires a large inductor. There is a trade-off between

component size, efficiency and operating frequency.

A reasonable starting point is to choose a ripple current

that is about 30-40% of IOUT(MAX). This is especially

important at low VOUT operation where VOUT is 1.8V

or below. Care must be given to choose an inductance

value that will generate a big enough current ripple so

that the chip’s valley current comparator has enough sig-

nal-to-noise ratio to force constant switching frequency.

Meanwhile, also note that the largest ripple current occurs

at the highest VIN. To guarantee that ripple current does

not exceed a specified maximum, the inductance should

be chosen according to Equation 7.

L =

VOUT

f • ΔIL(MAX)

• 1−

VOUT

VIN(MAX)

⎛

⎝

⎜

⎞

⎠

⎟

(7)

Once the value for L is known, the type of inductor must

be selected. Actual core loss is independent of core size

for a fixed inductor value, but is very dependent on the

inductance selected. As the inductance increases, core

losses decrease. Unfortunately, increased inductance

requires more turns of wire and therefore copper losses

will increase.

Ferrite designs have very low core losses and are pre-

ferred at high switching frequencies, so design goals can

concentrate on copper loss and preventing saturation.

Ferrite core material saturates “hard”, which means that

LTC7131-1 inductance collapses abruptly when the peak

design current is exceeded. This results in an abrupt

increase in inductor ripple current and consequent output

voltage ripple. Do not allow the core to saturate!

Different core materials and shapes will change the size/

current and price/current relationship of an inductor.

Toroid or shielded pot cores in ferrite or permalloy materi-

als are small and don’t radiate much energy, but generally

cost more than powdered iron core inductors with similar

characteristics. The choice of which style inductor to use

mainly depends on the price versus size requirements

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