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LTC3815

3815fa

For more information www.linear.com/LTC3815

APPLICATIONS INFORMATION

Minimum Off-Time and Minimum On-Time

Considerations

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

of time that the LTC3815 is capable of turning on the bot-

tom 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 maxi-

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

duty cycle is reached, due to a dropping input voltage for

example, then the output will drop out of regulation. The

minimum input voltage to avoid dropout is:

VIN(MIN) = VOUT •

tON + tOFF(MIN)

tON

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:

DCMIN = 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 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

valueandoperatingfrequencydeterminetheripplecurrent:

∆IL =

VOUT

f •L

• 1−

VOUT

VIN













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

frequency with small ripple current. However, achieving

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

component size, efficiency and operating frequency.

Areasonablestartingpointistochoosearipplecurrentthat

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 gen-

erate a big enough current ripple so that the chip’s valley

current comparator has enough signal-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:

L =

VOUT

f • ∆IL(MAX)

• 1−

VOUT

VIN(MAX)













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 or frequency in-

creases, core losses decrease. Unfortunately, increased

inductance requires more turns of wire and therefore

copper losses will increase.

Ferritedesignshaveverylowcorelossesand arepreferred

at high switching frequencies, so design goals can con-

centrate on copper loss and preventing saturation. Ferrite

corematerialsaturates“hard”,whichmeansthatLTC3815

inductancecollapsesabruptlywhenthepeakdesigncurrent

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/

currentandprice/currentrelationshipofaninductor.Toroid

or shielded pot cores in ferrite or permalloy materials 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

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