LT3154AVPBF datasheet(18/32 Pages) AD

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LT3154

Rev. 0

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OPERATION

Thermal Considerations

The power switches of the LT3154 are designed to oper‑

ate continuously with currents up to the internal current

limit thresholds. However, when operating at high current

levels, there may be significant heat generated within the

IC. Careful consideration must be given to the thermal

environment of the IC in order to provide a means to

remove heat from the IC and ensure that the LT3154 is

able to provide its full rated output current. Specifically,

the exposed die attach pad of both the LQFN package must

be soldered to a copper layer on the PCB to maximize the

conduction of heat out of the IC package. This can be ac‑

complished by utilizing multiple vias from the die attach

pad connection underneath the IC package to other PCB

layer(s) containing large copper planes. A recommended

board layout incorporating these concepts is show in Fig‑

ure 4. Typical temperature rise versus load current curves

using Figure 4 PCB are given in the Typical Performance

Characteristics section.

If the IC die temperature exceeds approximately 170°C,

thermal shutdown will be invoked and all switching will

be inhibited. The part will remain disabled until the die

temperature cools by approximately 10°C. The soft‑start

circuit is re‑initialized in thermal shutdown to provide a

smooth recovery when the die temperature cools enough

to resume operation.

Application circuits for the LT3154 are shown through‑

out the data sheet with varied use of pin‑strapped/default

or programmable operation as described in the Quick

Reference section. The selection of external components

is dependent upon the required performance of each par‑

ticular application given considerations and trade‑offs

such as PCB area, input and output voltage range, output

voltage ripple, required efficiency, thermal considerations

and cost. This section of the data sheet provides some

basic guidelines and considerations to aid in the selection

of external components and the design of the applications

circuit.

Inductor Selection

The choice of inductor used in LT3154 applications influ‑

ences 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 efficiency 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 “Inductor

Current Sense and Maximum Output Current” section of

APPLICATIONS INFORMATION

this data sheet. 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 (RHP) zero

frequency when operating in boost mode, which requires

the converter bandwidth to be set lower in frequency,

slowing the converter’s response to load transients.

f RHPZ=

VIN2 •RL

VOUT2 •2π •L

(Hz)

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 cur‑

rent. The peak‑to‑peak inductor current ripple for each

operational mode can be calculated from the following

formula, where fSW is the switching frequency in MHz

and L is the inductance in µH.

ΔIL(P−P)(BUCK) =

VOUT

fSW •L

VIN – VOUT

VIN

⎛

⎝

⎜

⎞

⎠

⎟Amps

ΔIL(P−P)(BOOST) =

VIN

fSW •L

VOUT – VIN

VOUT

⎛

⎝

⎜

⎞

⎠

⎟Amps

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

peak inductor ripple current occurs when the duty cycle in

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