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componentes Descripción

Fabricante

LTC1267

Dual High Efficiency Synchronous Step-Down Switching Regulators

Linear Technology 

LTC1267

LTC1267-ADJ/LTC1267-ADJ5

UU W U

APPLICATIO S I FOR ATIO

VIN

1N4148

1

VCC

2

0.15µF 3

VIN

CAP3

EXT VCC 28

27

MSHDN

MSHDN

26 0.15µF

CAP5

1N4148

P-CH

4 PGATE 3

PGATE 5 25

P-CH

N-CH

D3

L3

0.1µF 5

PDRIVE 3

24 0.1µF

PDRIVE 5

6

NGATE 3

NGATE 5 23

CIN3

7

PGND3

1µF

+ 8 VCC3

22

PGND5

CIN5

1µF

VCC5 21

N-CH

D5

L5

RSENSE3

COUT3

VOUT3

CT3

9

1k 3300pF 10

CT3

ITHR3

LTC1267

SGND5

20

19

SHDN5

SHDN5

11 SGND3

12

SHDN3

SHDN3

SENSE+5 18

SENSE – 5 17

1000pF

13

1000pF 14

SENSE – 3

SENSE+3

16

CT5

3300pF

15

1k

ITHR5

CT5

COUT5

RSENSE5

VOUT5

BOLD LINES INDICATE HIGH CURRENT PATHS

LTC1267 • F08

Figure 8. LTC1267 Layout Diagram

6. In adjustable versions, the feedback pin is very sensi-

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

to isolate VFB1, 2 from possible capacitive coupling of

the inductor switch signal.

7. Are MSHDN and SHDN1, 3, 5 actively pulled to ground

during normal operation? These shutdown pins are

high impedance and must not be allowed to float.

Troubleshooting Hints

Since efficiency is critical to LTC1267 applications, it is

very important to verify that the circuit is functioning

correctly in both continuous and Burst Mode operation.

The waveform to monitor is the voltage on the CT pin.

In continuous mode (ILOAD > IBURST) the voltage on the CT

pin should be a sawtooth with a 0.9VP-P swing. This

voltage should never dip below 2V as shown in Figure 9a.

When load currents are low (ILOAD < IBURST) Burst Mode

operation occurs. The voltage on the CT pin now falls to

ground for periods of time as shown in Figure 9b.

If the CT is observed falling to ground at high output

currents, it indicates poor decoupling or improper ground-

ing. Refer to the Board Layout Checklist.

Inductor current should also be monitored. Look to verify

that the peak-to-peak ripple current in continuous mode

operation is approximately the same as in Burst Mode

operation.

3.3V

0V

(a) CONTINUOUS MODE OPERATION

3.3V

(b) Burst Mode OPERATION

Figure 9. CT Waveforms

0V

LTC1267 • F09

14

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