LTC3114-1_15 Datasheet, PDF(17/34 Page) Linear Technology – 40V, 1A Synchronous Buck-Boost DC/DC Converter with Programmable Output Current

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LTC3114-1_15 Datasheet, PDF (17/34 Pages) Linear Technology – 40V, 1A Synchronous Buck-Boost DC/DC Converter with Programmable Output Current

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LTC3114-1

OPERATION

THERMAL CONSIDERATIONS

The power switches of the LTC3114-1 are designed to

operate 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. In addition, the LDO regulator can generate

a significant amount of heat when VIN is very high. This

adds to the total power dissipation of the IC. As described

elsewhere in this data sheet, bootstrapping of the LDO

for 5V output applications can essentially eliminate the

LDO power dissipation term and significantly improve

efficiency. As a result, 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

LTC3114-1 is able to provide its full rated output current.

Specifically, the exposed die attach pad of both the DHC

and FE packages must be soldered to a copper layer on

the PCB to maximize the conduction of heat out of the IC

package. This can be accomplished by utilizing multiple

vias from the die attach pad connection underneath the IC

package to other PCB layer(s) containing a large copper

plane. A typical board layout incorporating these concepts

is shown in Figure 4.

If the IC die temperature exceeds approximately 165Â°C,

overtemperature shutdown will be invoked and all switching

will be inhibited. The part will remain disabled until the die

temperature cools by approximately 10Â°C. The soft-start

circuit is re-initialized in overtemperature shutdown to

provide a smooth recovery when the IC die temperature

cools enough to resume operation.

Start-Up Into a Pre-Biased VOUT

Some applications require the LTC3114-1 to start up into

an output voltage (VOUT), that is pre-biased by an external

source to some level. It is desirable at LTC3114-1 start-up

to minimize current taken from the pre-bias voltage source

and VOUT storage capacitor to prevent VOUT glitches and

currents fed backwards into the VIN power source of the

LTC3114-1.

If the LTC3114-1 VIN voltage is higher than the pre-biased

VOUT, indicating buck mode operation, then there will

be minimal reverse current at start-up. However, if the

LTC3114-1 VIN voltage is lower than the pre-biased VOUT,

indicating boost mode operation, then it is possible for a

brief, but substantial reverse current to be taken by the

LTC3114-1 from VOUT. The duration of this reverse cur-

rent is approximately 100Âµs to 200Âµs. The magnitude is

inversely proportional to the VIN voltage and dependent

upon external component values.

Prevention of pre-biased VOUT reverse current in boost

mode can be achieved in two ways. The preferred method

is to ensure that the pre-biased VOUT voltage level is set

higher than the nominal VOUT regulation level. For example,

if VOUT is pre-biased to 13V, then setting the VOUT regu-

lation voltage of the LTC3114-1 to less than 13V, taking

into account error margins, will result in negligible or

zero reverse current at start-up. If this is not possible,

then a Schottky diode can be connected in series between

VOUT of the LTC3114-1 and the converter output to block

reverse current .

For more information www.linear.com/LTC3114-1

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