LTC3830ES datasheet(15/24 Pages) LINER

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LTC3830/LTC3830-1

sn3830 3830fs

where LO is the inductor value in µH. With proper fre-

quency compensation, the combination of the inductor

and output capacitor values determine the transient recov-

ery time. In general, a smaller value inductor improves

transient response at the expense of ripple and inductor

core saturation rating. A 2

µH inductor has a 0.81A/µs rise

time in this application, resulting in a 6.2

µs delay in

responding to a 5A load current step. During this 6.2

µs,

the difference between the inductor current and the output

current is made up by the output capacitor. This action

causes 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 5V input LTC3830

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 = LTC3830 oscillator frequency = 200kHz

LO = Inductor value

Solving this equation with our typical 5V to 3.3V applica-

tion with a 2

µH inductor, we get:

(– .

) • .

••

53 3

3 3

200

kHz

-P

Peak inductor current at 10A load:

10A + (2.8A/2) = 11.4A

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 LTC3830 design places significant demands on

both the input and the output capacitors. During normal

steady load operation, a buck converter like the LTC3830

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 the peak-to-peak ripple cur-

rent. Most of this current is supplied by the input bypass

capacitor. The resulting RMS current flow in the input

capacitor heats it and causes 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. Note that capacitor manufacturers’ ripple cur-

rent ratings are often based on only 2000 hours (3 months)

lifetime at rated temperature. Further derating of the input

capacitor ripple current beyond the manufacturer’s speci-

fication is recommended to extend the useful life of the

circuit. Lower operating temperature has the largest effect

on capacitor longevity.

APPLICATIO S I FOR ATIO

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