LTC1143CS datasheet(15/20 Pages) LINER

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LTC1143/LTC1143L

LTC1143L-ADJ

APPLICATIONS INFORMATION

With the addition of R3 a current is generated though R1,

causing an offset of:

OFFSET

OUT









If VOFFSET > 25mV, the built-in offset will be cancelled and

Burst Mode operation is prevented from occurring. Since

VOFFSET is constant, the maximum load current is also

decreased by the same offset. Thus, to get back to the

same IMAX, the value of the sense resistor must be lower:

SENSE

MAX

≈

To prevent noise spikes from erroneously tripping the

current comparator, a 1000pF capacitor is needed across

Pins 1 (16) and 9 (8).

Board Layout Checklist

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of the

LTC1143 series. These items are also illustrated graphi-

cally in the layout diagram of Figure 9. In general, each

block should be self-contained with little cross coupling

for best performance. Check the following in your layout:

1) Are the signal and power grounds segregated? The

LTC1143 series GND Pin 3 (11) must return separately

to: a) the power and b) the signal grounds. The

power ground returns to the anode of the Schottky

diode and (–) plate of CIN, which should have as short

lead lengths as possible.The signal ground (b) con-

nects to the (–) plate of COUT.

2) Does the LTC1143 series SENSE– Pin 16 (8) connect

to a point close to RSENSE and the (+) plate of COUT?

3) Are the SENSE– and SENSE+ leads routed together

with minimum PC trace spacing? The 1000pF

capacitor between Pins 1 (9) and 16 (8) should be as

close as possible to the LTC1143 series.

4) Does the (+) plate of CIN connect to the source of the

P-channel MOSFET as closely as possible? This

capacitor provides the AC current to the P-channel

MOSFET.

3.3V

(a) CONTINUOUS MODE OPERATION

3.3V

(b) Burst Mode OPERATION

LTC1143 • F07

Figure 7. CT Waveforms

Auxiliary Windings––Suppressing Burst Mode

Operation

The LTC1143 series operates nonsynchronously with the

normal limitation that the power drawn from the inductor

primary winding must not be less than twice the power

drawn from the auxiliary windings. (With synchronous

switching, using the LTC1142 series, auxiliary outputs

may be loaded without regard to the primary output load,

providing that the loop remains in continuous mode

operation.)

Burst Mode operation can be suppressed at low output

currents with a simple external network that cancels the

25mV minimum current comparator threshold. This tech-

nique is also useful for eliminating audible noise from

certain types of inductors in high current (IOUT > 5A)

applications when they are lightly loaded.

An external offset is put in series with the Sense– pin to

subtract from the built-in 25mV offset. An example of this

technique is shown in Figure 8. Two 100

Ω resistors are

inserted in series with the sense leads from the sense

resistor.

RSENSE

1000pF

100

Ω

100

Ω

COUT

VOUT

SENSE+

[PIN 16 (8)]

SENSE –

[PIN 1(9)]

1143 F08

Figure 8. Suppression of Burst Mode Operation

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