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LT1683

1683f

CS (Pin 4): This is the input to the current sense amplifier.

It is used for both current mode control and current

slewing of the external MOSFETs. Current sense is accom-

plished via a sense resistor (RS) connected from the

sources of the external MOSFETs to ground. CS is con-

nected to the top of RS. Current sense is referenced to the

GND pin.

The switch maximum operating current will be equal to

0.1V/RS. At CS = 0.1V, the gate drivers will be immediately

turned off (no slew control).

If CS = 0.22V in addition to the drivers being turned off, VC

and SS will be discharged to ground (short-circuit protec-

tion). This will hasten turn off on subsequent cycles.

FB (Pin 9): The feedback pin is used for positive voltage

sensing. It is the inverting input to the error amplifier. The

noninverting input of this amplifier connects internally to

a 1.25V reference.

If the voltage on this pin exceeds the reference by 220mV,

then the output drivers will immediately turn off the

external MOSFETs (no slew control). This provides for

output overvoltage protection

When this input is below 0.9V then the current sense

blanking will be disabled. This will assist start up.

NFB (Pin 10): The negative feedback pin is used for

sensing a negative output voltage. The pin is connected to

the inverting input of the negative feedback amplifier

through a 100k source resistor. The negative feedback

amplifier provides a gain of –0.5 to the FB pin. The nominal

regulation point would be –2.5V on NFB. This pin should

be left open if not used.

If NFB is being used then overvoltage protection will occur

at 0.44V below the NFB regulation point.

At NFB < –1.8 current sense blanking will be disabled.

VC (Pin 12): The compensation pin is used for frequency

compensation and current limiting. It is the output of the

error amplifier and the input of the current comparator.

Loop frequency compensation can be performed with an

RC network connected from the VC pin to ground. The

voltage on VC is proportional to the switch peak current.

The normal range of voltage on this pin is 0.25V to 1.27V.

However, during slope compensation the upper clamp

voltage is allowed to increase with the compensation.

During a short-circuit fault the VC pin will be discharged to

ground.

PI FU CTIO S

–

5pF

IN5819

20mA

1683 F01a

ZVN3306A

GATE A/GATE B

CAP A/CAP B

Figure 1a. Typical Test Circuitry

Figure 1b. Test Circuit for Slew

–

5pF

IN5819

0.9A

0.1

1683 F01b

Si4450DY

GATE A/GATE B

CAP A/CAP B

TEST CIRCUITS

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