LTC3129 datasheet(20/34 Pages) AD

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LTC3114-1

Rev. D

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APPLICATIONS INFORMATION

A standard application circuit for the LTC3114-1 is shown

on the front page of this data sheet. The appropriate

selection of external components is dependent upon the

required performance of the IC in each particular appli-

cation given considerations and trade-offs such as PCB

area, input and output voltage range, output voltage rip-

ple, required efficiency, thermal considerations and cost.

This section of the data sheet provides some basic guide-

lines and considerations to aid in the selection of external

components and the design of the applications circuit.

LDO Capacitor Selection

The LDO output of the LTC3114-1 is generated from VIN

by a low dropout linear regulator. The LDO regulator has

been designed for stable operation with a wide range

of output capacitors. For most applications, a low ESR

capacitor of at least 4.7µF should be used. The capacitor

should be located as close to the PLDO pin as possible

and connected to the LDO pin and ground through the

shortest traces possible. PLDO is the regulator output

and is also the internal supply pin for the gate drivers

and boost rail charging diodes. The LDO pin is the supply

connection for the remainder of the control circuitry. The

LDO and PLDO pins must be connected together on the

PCB. If the connecting trace cannot be made short, an

additional 0.1µF bypass capacitor should be connected

between the LDO pin and ground as close to the package

pins as possible.

Inductor Selection

The choice of inductor used in LTC3114-1 application circuits

influences the maximum deliverable output current, the con-

verter bandwidth, the magnitude of the inductor current ripple

and the overall converter efficiency. The inductor must have a

low DC series resistance or output current capability and effi-

ciency will be compromised. Larger inductor values reduce

inductor current ripple but will not increase output current

capability as is the case with peak current mode control as

described in the Maximum Output Current section of this data

sheet. Larger value inductors also tend to have a higher DC

series resistance for a given case size, which will have a nega-

tive impact on efficiency. Larger values of inductance will also

lower the right half plane (RHP) zero frequency when operat-

ing in boost mode, which requires the converter bandwidth to

be set lower in frequency, slowing the converter’s response

to load transients. Nearly all LTC3114-1 application circuits

deliver the best performance with an inductor value between

4.7µH and 15µH. Buck mode-only applications can use the

larger inductor values as they are unaffected by the RHP zero,

while mostly boost applications generally require inductance

on the low end of this range depending on how deep they will

operate in boost mode.

Regardless of inductor value, the saturation current rat-

ing 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

operational mode can be calculated from the following

formula, where f is the switching frequency (1.2MHz), L

is the inductance in µH and tLOW is the switch pin mini-

mum low time in µs. The switch pin minimum low time

is typically 0.05µs.

ΔIL(P-P)(BUCK) =

VOUT

VIN – VOUT

VIN

⎛

⎝⎜

⎞

⎠⎟

– tLOW

⎛

⎝⎜

⎞

⎠⎟

Amps

ΔIL(P-P)(BOOST) =

VIN

VOUT – VIN

VOUT

⎛

⎝⎜

⎞

⎠⎟

– tLOW

⎛

⎝⎜

⎞

⎠⎟

Amps

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

peak inductor ripple current occurs when the duty cycle

in buck mode is maximum (highest VIN) and in boost

mode when the duty cycle is 50% (VOUT = 2 • VIN). As an

example, if VIN (minimum) = 3.6V and VIN (maximum) =

40V, VOUT = 5V and L = 6.8µH, the peak-to-peak inductor

ripples at the voltage extremes (40V VIN for buck and 3.6V

VIN for boost) are:

Buck = 504mA peak-to-peak

Boost = 116mA peak-to-peak

One-half of this inductor ripple current must be added to

the highest expected average inductor current in order to

select the proper saturation current rating for the inductor.

In addition to its influence on power conversion efficiency,

the inductor DC resistance can also impact the maximum

output current capability of the buck-boost converter par-

ticularly at low input voltages. In buck mode, the output

current of the buck-boost converter is primarily limited

by the inductor current reaching the average current limit

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