LTC1159CS datasheet(15/20 Pages) LINER

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LTC1159

LTC1159-3.3/LTC1159-5

1) Are the signal and power grounds segregated? The

LTC1159 signal ground must connect separately to the

(–) plate of COUT. The other ground pin(s) should return to

the source of the N-channel MOSFET, anode of the Schot-

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

lead lengths as possible.

2) Does the LTC1159 SENSE– pin connect to a point close

to RSENSE and the (+) plate of COUT? In adjustable applica-

tions, the resistive divider R1, R2 must be connected

between the (+) plate of COUT and signal ground.

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

with minimum PC trace spacing? The differential

decoupling capacitor between the two SENSE pins should

be as close as possible to the LTC1159. Up to 100

Ω may

be placed in series with each sense lead to help decouple

the SENSE pins. However, when these resistors are used,

the capacitor should be no larger than 1000pF.

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

P-channel MOSFET as closely as possible? An additional

0.1

µF ceramic capacitor between VIN and power ground

may be required in some applications.

5) Is the VCC decoupling capacitor connected closely be-

tween the VCC pins of the LTC1159 and power ground?

This capacitor carries the MOSFET driver peak currents.

6) In adjustable versions, the feedback pin is very sensitive

to pickup from the switch node. Care must be taken to

isolate VFB from possible capacitive coupling of the induc-

tor switch signal.

7) Is the SHDN1 pin actively pulled to ground during

normal operation? SHDN1 is a high impedance pin and

must not be allowed to float.

Troubleshooting Hints

Since efficiency is critical to LTC1159 applications it is very

important to verify that the circuit is functioning correctly

in both continuous and Burst Mode operation. The wave-

form to monitor is the voltage on the CT pin .

In continuous mode (ILOAD > IBURST) the voltage should be

a sawtooth with a 0.9VP-P swing. This voltage should never

dip below 2V as shown in Figure 9a. When the load current

is low (ILOAD < IBURST), Burst Mode operation should occur

with the CT waveform periodically falling to ground as

shown in Figure 9b.

If the CT pin is observed falling to ground at high output

currents, it indicates poor decoupling or improper ground-

ing. Refer to the Board Layout Checklist.

3.3V

(a) CONTINUOUS MODE OPERATION

3.3V

(b) Burst Mode OPERATION

LTC1159 • F09

Figure 9. CT Pin 6 Waveforms

APPLICATIO S I FOR ATIO

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