LTC3124 datasheet(20/36 Pages) AD

20 / 36 page

LT3120

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Input Capacitor Selection

PVIN pins carry the full inductor current and provides

power to internal control circuits in the IC. To minimize

input voltage ripple and ensure proper operation of the IC,

a low ESR bypass capacitor with a value of at least 10µF

should be located as close to this pin as possible. The

traces connecting this capacitor to PVIN and the ground

plane should be made as short as possible. The VIN pin

provides power to the VCC regulator and other internal

circuitry. If the PCB trace connecting VIN to PVIN is long, it

may be necessary to add an additional small value bypass

capacitor near the VIN pin.

When powered through long leads or from a high ESR

power source, a larger value bulk input capacitor may be

required. In such applications, a 47µF to 100µF electrolytic

capacitor in parallel with a 1µF ceramic capacitor generally

yields a high performance, low cost solution.

When powered through an inductive connection such as

a long cable, the inductance of the power source and the

input bypass capacitor form a high-Q resonant LC filter. In

suchapplications,hot-pluggingintoapoweredsourcecan

lead to a significant voltage overshoot, even up to twice

the nominal input source voltage. Care must be taken in

suchsituationstoensurethattheabsolutemaximuminput

voltage rating of the LT3120 is not violated. See Analog

DevicesApplicationNote88forsolutionstoincreasedamp-

ing in the input filter and minimize this voltage overshoot.

Inductor Selection

The choice of inductor used in LT3120 application circuits

influences the maximum deliverable output current, the

converterbandwidth,themagnitudeoftheinductorcurrent

ripple and the overall converter efficiency. The inductor

must have a low DC series resistance, when compared to

the internal switch resistance (25mΩ), or output current

capability and efficiency will be compromised. Larger in-

ductor values reduce inductor current ripple but may not

increase output current capability as is the case with peak

current mode control. Larger value inductors also tend to

have a higher DC series resistance for a given case size,

which will have a negative impact on efficiency. Larger

values of inductance will also lower the right half plane

zero (RHPZ) frequency when operating in boost mode,

which can compromise loop stability. Nearly all LT3120

application circuits deliver the best performance with an

inductor value between 1.5µH and 15µH. Buck mode only

applications can use the larger inductor values as they are

unaffected by the RHPZ, while mostly boost applications

generally require inductance on the lower end of this range

depending on how large the step-up ratio is. Regardless

of inductor value, the saturation current rating should be

selected such that it is greater than the worst-case average

inductor current plus half of the ripple current.

The peak-to-peak inductor current ripple for each opera-

tional mode can be calculated using Equation 10, where

fSW is the programmed switching frequency, L is the

inductance and tLOW is the switch pin minimum low time,

typically 70ns.

ΔIL P-P

( ) BUCK

(

) ≅

VOUT

VIN −VOUT

VIN

⎛

⎝

⎜

⎞

⎠

⎟

fSW

− tLOW

⎛

⎝

⎜

⎞

⎠

⎟

ΔIL P-P

( ) BOOST

(

) ≅

VIN

VOUT −VIN

VOUT

⎛

⎝

⎜

⎞

⎠

⎟

fSW

− tLOW

⎛

⎝

⎜

⎞

⎠

⎟

(10)

It should be noted that the worst-case peak-to-peak in-

ductor ripple current occurs when the duty cycle in buck

mode is minimum (highest VIN) and in boost mode when

the duty cycle is 50% (VOUT ≅ 2 • VIN).

As an example, if VIN (minimum) = 2.5V and VIN (maxi-

mum) = 15V, VOUT = 5V, fSW = 1MHz and L = 4.7µH, the

peak-to-peakinductorripplesatthevoltageextremes(15V

VIN for buck and 2.5V VIN for boost). See Equation 11.

ΔIL P-P

( ) BUCK

(

) ≅

4.7µH

15V−5V

15V

⎛

⎝

⎜

⎞

⎠

⎟•930ns = 659mA

ΔIL P-P

( ) BOOST

(

) ≅

2.5V

4.7µH

5V−2.5V

⎛

⎝

⎜

⎞

⎠

⎟•930ns = 247mA

(11)

LTC3124 Datasheet(PDF) 20 Page - Analog Devices

LTC3124 Datasheet(HTML) 20 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

Datasheet Download

Link URL