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LT3844

3844fb

APPLICATIONS INFORMATION

Minimum On-Time Considerations (Buck Mode)

Minimum on-time, tON(MIN), is the smallest amount of time

that the LT3844 is capable of turning the top MOSFET on

and off again. It is determined by internal timing delays

and the amount of gate charge required turning on the

top MOSFET. Low duty cycle applications may approach

this minimum on-time limit and care should be taken to

ensure that:

OUT

ON MIN

•

()

where tON(MIN) is typically 350ns worst case.

If the duty cycle falls below what can be accommodated by

the minimum on-time, the LT3844 will begin to skip cycles.

The output will be regulated, but the ripple current and

ripple voltage will increase. If lower frequency operation

is acceptable, the on-time can be increased above tON(MIN)

for the same step-down ratio.

Boost Converter Design

The LT3844 can be used to conﬁgure a boost converter

to step-up voltages to as high as hundreds of volts. An

example of a boost converter circuit schematic is shown

in the Typical Applications section. The following sections

are a guide to designing a boost converter:

The maximum duty cycle of the main switch is:

MAX

OUT

IN MIN

OUT

−

()

Boost Converter: Inductor Selection

The critical parameters for selection of an inductor are

minimum inductance value and saturation current. The

minimum inductance value is calculated as follows:

MIN

IN MIN

LSW

MAX

()

•

fSW is the switch frequency.

Similar to the buck converter, the typical range of values

for

ΔIL is (0.2 • IL(MAX)) to (0.5 • IL(MAX)), where IL(MAX)

is the maximum average inductor current.

L MAX

OUT MAX

OUT

IN MIN

()

•

Using

ΔIL = 0.3 • IL(MAX) yields a good design compromise

between inductor performance versus inductor size and

cost.

The inductor must not saturate at the peak operating

current, IL(MAX) + ΔIL/2. The inductor saturation current

speciﬁcation is the current at which the inductance, mea-

sured at zero current, decreases by a speciﬁed amount,

typically 30%.

One drawback of boost regulators is that they cannot be

current limited for output shorts because the current steer-

ing diode makes a direct connection between input and

output. Therefore, the inductor current during an output

short circuit is only limited by the available current of the

input supply.

After calculating the minimum inductance value and the

saturation current for your design, select an off-the-shelf

inductor. For more detailed information on selecting an

inductor, please see the “Inductor Selection” section of

Linear Technology Application Note 19.

Boost Converter: MOSFET Selection

The selection criteria of the external N-channel standard

level power MOSFET include on resistance (RDS(ON)), re-

verse transfer capacitance (CRSS), maximum drain source

voltage (VDSS), total gate charge (QG) and maximum

continuous drain current.

For maximum efﬁciency, minimize RDS(ON) and CRSS.

Low RDS(ON) minimizes conduction losses while low

CRSS minimizes transition losses. The problem is that

RDS(ON) is inversely related to CRSS. Balancing the tran-

sition losses with the conduction losses is a good idea

in sizing the MOSFET. Select the MOSFET to balance the

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