LTC1430AI datasheet(17/24 Pages) LINER

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LTC1430A

APPLICATI

I FOR ATIO

Figure 14. Typical Schematic Showing Layout Considerations

Figure 15. Using External Resistors to Set Output Voltages

LTC1430A

VOUT

SENSE+

SENSE–

1430 F15

be floated and an external resistor string should be con-

nected to FB (Figure 15). As before, connect the top

resistor (R1) to the output as close to the load as practical

and connect the bottom resistor (R2) to the common

GND/PGND point. In both cases, connecting the top of the

resistor divider (either SENSE + or R1) close to the load can

significantly improve load regulation by compensating for

any drops in PC traces or hookup wires between the

LTC1430A and the load.

Power Component Hook-Up/Heat Sinking

As current levels rise much above 1A, the power compo-

nents supporting the LTC1430A start to become physi-

cally large (relative to the LTC1430A, at least) and can

require special mounting considerations. Input and output

capacitors need to carry high peak currents and must have

low ESR; this mandates that the leads be clipped as short

as possible and PC traces be kept wide and short. The

power inductor will generally be the most massive single

component on the board; it can require a mechanical hold-

down in addition to the solder on its leads, especially if it

is a surface mount type.

The power MOSFETs used require some care to ensure

proper operation and reliability. Depending on the current

levels and required efficiency, the MOSFETs chosen may

be as large as TO-220s or as small as SO-8s. High

efficiency circuits may be able to avoid heat sinking the

power devices, especially with TO-220 type MOSFETs. As

an example, a 90% efficient converter working at a steady

3.3V/10A output will dissipate only (33W/90%)10% =

3.7W. The power MOSFETs generally account for the

majority of the power lost in the converter; even assuming

that they consume 100% of the power used by the

converter, that’s only 3.7W spread over two or three

devices. A typical SO-8 MOSFET with a RON suitable to

provide 90% efficiency in this design can commonly

dissipate 2W when soldered to an appropriately sized

piece of copper trace on a PC board. Slightly less efficient

or higher output current designs can often get by with

standing a TO-220 MOSFET straight up in an area with

some airflow; such an arrangement can dissipate as much

as 3W without a heat sink. Designs which must work in

high ambient temperatures or which will be routinely

overloaded will generally fare best with a heat sink.

1430 F14

3.3V

2.7

µH/15A

TOTAL

1980

µF

(330

µF

6.3V

×6)

Q1A*

Q2*

PVCC1

IMAX

FREQSET

GND

PGND

SHDN

COMP

PVCC2

VCC

PGND

GND

7.5k

0.1

µF

0.1

µF

4700pF

220pF

CSS

0.01

µF

4.7

µF

35V

100

Ω

IFB

SENSE+

LTC1430A

SENSE–

GND

PGND

* MOTOROLA MTD20N03HL

MBR0530T1

Q1B*

TOTAL

880

µF

(220

µF

10V

×4)

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