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LTC1753

1753fa

in the RDS(ON) calculations will generally decrease MOSFET

cost and circuit efficiency while increasing MOSFET heat

sink requirements.

Inductor Selection

The inductor is often the largest component in the LTC1753

design and should be chosen carefully. Inductor value and

type should be chosen based on output slew rate require-

ments, output ripple requirements and expected peak

current. Inductor value is primarily controlled by the

required current slew rate. The maximum rate of rise of

current in the inductor is set by its value, the input-to-

output voltage differential and the maximum duty cycle of

the LTC1753. In a typical 5V input, 2.8V output applica-

tion, the maximum current slew rate will be:

MAX

OUT

−

()

183

where L is the inductor value in

µH. With proper frequency

compensation, the combination of the inductor and output

capacitor will determine the transient recovery time. In

general, a smaller value inductor will improve transient

response at the expense of increased output ripple voltage

and inductor core saturation rating. A 2

µH inductor would

have a 0.9A/

µs rise time in this application, resulting in a

5.5

µsdelayinrespondingtoa5Aloadcurrentstep.During

this 5.5

µs,thedifferencebetweentheinductorcurrentand

the output current must be made up by the output capaci-

tor, causing a temporary voltage droop at the output. To

minimize this effect, the inductor value should usually be

in the 1

µH to 5µH range for most typical 5V input LTC1753

circuits. To optimize performance, different combinations

of input and output voltages and expected loads may

require different inductor values.

Once the required value is known, the inductor core type

can be chosen based on peak current and efficiency

requirements. Peak current in the inductor will be equal to

the maximum output load current plus half of the peak-to-

peak inductor ripple current. Ripple current is set by the

inductor value, the input and output voltage and the

operating frequency. The ripple current is approximately

equal to:

RIPPLE

OUT

OSC

−

()( )

()( )( )

fOSC = LTC1753 oscillator frequency = 300kHz

LO = Inductor value

Solving this equation with our typical 5V to 2.8V applica-

tion with a 2

µH inductor, we get:

22 056

300

()( )

kHz

P-P

Peak inductor current at 11.2A load:

11 2

12 2

The ripple current should generally be between 10% and

40% of the output current. The inductor must be able to

withstand this peak current without saturating, and the

copper resistance in the winding should be kept as low as

possible to minimize resistive power loss. Note that in

circuits not employing the current limit function, the

current in the inductor may rise above this maximum

under short circuit or fault conditions; the inductor should

be sized accordingly to withstand this additional current.

Inductors with gradual saturation characteristics are often

the best choice.

Input and Output Capacitors

A typical LTC1753 design puts significant demands on

both the input and the output capacitors. During constant

load operation, a buck converter like the LTC1753 draws

square waves of current from the input supply at the

switching frequency. The peak current value is equal to the

output load current plus 1/2 peak-to-peak ripple current,

and the minimum value is zero. Most of this current is

supplied by the input bypass capacitor. The resulting RMS

current flow in the input capacitor will heat it up, causing

premature capacitor failure in extreme cases. Maximum

RMS current occurs with 50% PWM duty cycle, giving an

RMS current value equal to IOUT/2. A low ESR input

capacitor with an adequate ripple current rating must be

used to ensure reliable operation.

APPLICATIO S I FOR ATIO

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