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LTC1142

Dual High Efficiency Synchronous Step-Down Switching Regulators

Linear Technology 

LTC1142/LTC1142L/LTC1142HV

APPLICATIO S I FOR ATIO

ing with Kelvin connections. Be sure to use a PCB

pattern similar to that shown in Figure 7 for the current

sense resistors.

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

P-channel MOSFET as closely as possible? This capaci-

tor provides the AC current to the P-channel MOSFET.

5. Is the input decoupling capacitor (1µF/0.22µF) con-

nected closely between Pin 24 (10) and power ground

[Pin 4 (18) for the LTC1142, Pin 5 (19) for the LTC1142-

ADJ]? This capacitor carries the MOSFET driver peak

currents.

6. Are the shutdown Pins 2 and 16 for the LTC1142 (Pins

3 and 17 for the LTC1142-ADJ) actively pulled to

ground during normal operation? Both Shutdown pins

are high impedance and must not be allowed to float.

Both pins can be driven by the same external signal if

needed.

7. For the LTC1142-ADJ adjustable applications, the re-

sistive divider R1, R2 must be connected between the

(+) plate of COUT and signal ground.

Output Crowbar

An added feature to using an N-channel MOSFET as the

synchronous switch is the ability to crowbar the output

with the same MOSFET. Pulling the CT , Pin 25 (11) above

1.5V when the output voltage is greater than the desired

regulated value will turn “on” the N-channel MOSFET for

that regulator section.

A fault condition which causes the output voltage to go

above a maximum allowable value can be detected by

external circuitry. Turning on the N-channel MOSFET

when this fault is detected will cause large currents to flow

and blow the system fuse.

The N-channel MOSFET needs to be sized so it will safely

handle this overcurrent condition. The typical delay from

pulling the CT pin high and the NDrive Pin 6 (20) going high

is 250ns. Note: Under shutdown conditions, the N-chan-

nel is held OFF and pulling the CT pin high will not cause

the N-channel MOSFET to crowbar the output.

A simple N-channel FET can be used as an interface

between the overvoltage detect circuitry and the LTC1142

as shown in Figure 8.

FROM CROWBAR

DETECT CIRCUIT

(ACTIVE WHEN VGATE = VIN

OFF WHEN VGATE = GND)

PIN 26(12)

INT VCC

VN2222LL LTC1142

PIN 25(11)

CT

1142 F08

Figure 8. Output Crowbar Interface

Troubleshooting Hints

Since efficiency is critical to LTC1142 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, Pins 25

and 11.

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.

3.3V

0V

(a) CONTINUOUS MODE OPERATION

3.3V

(b) Burst Mode OPERATION

0V

1142 F09

Figure 9. CT Waveforms

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.

If Pin 25 or Pin 11 is observed falling to ground at high

output currents, it indicates poor decoupling or improper

grounding. Refer to the Board Layout Checklist.

Auxiliary Windings––Suppressing Burst Mode

Operation

The LTC1142 synchronous switch removes the normal

limitation that power must be drawn from the inductor

primary winding in order to extract power from auxiliary

windings. With synchronous switching, auxiliary outputs

16

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