LT1683 Просмотр технического описания (PDF) - Linear Technology

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LT1683

Slew Rate Controlled Ultralow Noise Push-Pull DC/DC Controller

Linear Technology 

LT1683

PIN FUNCTIONS

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 exter-

nal 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 sens-

ing 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 regula-

tion 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.

SS (Pin 13): The SS pin allows for ramping of the switch

current threshold at startup. Normally a capacitor is placed

on this pin to ground. An internal 9µA current source will

charge this capacitor up. The voltage on the VC pin cannot

exceed the voltage on SS. Thus peak current will ramp

up as the SS pin ramps up. During a short circuit fault

the SS pin will be discharged to ground thus reinitializing

soft-start.

When SS is below the VC clamp voltage the VC pin will

closely track the SS pin.

This pin can be left open if not used.

TEST CIRCUITS

5pF

CAP A/CAP B

20mA

IN5819

GATE A/GATE B

ZVN3306A –+ 10

2

1683 F01a

Figure 1a. Typical Test Circuitry

8

5pF

CAP A/CAP B

0.9A

IN5819

GATE A/GATE B

CS

Si4450DY –+ 10

0.1

1683 F01b

Figure 1b. Test Circuit for Slew

1683fd

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