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LTC1149

LTC1149-3.3/LTC1149-5

circuitry. Turning on the N-channel MOSFET when this

fault is detected will then force the system fuse to blow.

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

handle this overcurrent condition. The typical delay from

pulling the CT Pin 6 high to when the NGATE Pin 13 goes

high is 250ns.

Under shutdown conditions, the N-channel

is held off and pulling Pin 6 high will not cause the output

to be crowbarred.

A small N-channel FET can be used as an interface between

the overvoltage detect circuitry and the LTC1149 as shown

in Figure 7.

APPLICATIO S I FOR ATIO

Figure 7. Output Crowbar Interface

Board Layout Checklist

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of the

LTC1149 series. These items are also illustrated graphi-

cally in the layout diagram of Figure 8. Check the following

in your layout:

1. Are the signal and power grounds segregated? The

LTC1149 signal ground Pin 11 must connect separately

to the (–) plate of COUT. The other ground Pins 12 and

14 should return to the source of the N-channel MOSFET,

anode of the Schottky diode and (–) plate of CIN, which

should have as short lead lengths as possible.

2. Does the LTC1149 SENSE– Pin 8 connect to a point

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

applications, 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 decou-

pling capacitor between Pins 8 and 9 should be as close

as possible to the LTC1149. Up to 100

Ω may be placed

Figure 6. Suppressing Burst Mode Operation

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 6. Two 100

Ω resistors are

inserted in series with the leads from the sense resistor.

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

causing an offset of:

VOFFSET = VOUT

)

) R1

R1 + R3

If VOFFSET > 25mV, the minimum threshold 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:

RSENSE ≈

75mV

IMAX

To prevent noise spikes from erroneously tripping the

current comparator, a 1000pF capacitor is needed across

Pins 8 and 9.

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 timing capacitor Pin 6

above 1.5V when the output voltage is greater than the

desired regulated value, will turn on the N-channel MOSFET.

A fault condition which causes the output voltage to go

above a maximum value can be detected by external

LTC1149

SENSE+

SENSE –

1000pF

100

Ω

100

Ω

RSENSE

COUT

1149 F06

LTC1149

VN2222LL

CROWBAR

ACTIVE WHEN CROWBAR = VIN

OFF WHEN CROWBAR = GROUND

1149 F07

VCC

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