Datasheet LT1683 (Analog Devices) - 8

LT1683
PIN FUNCTIONS
The switch maximum operating current will be equal to
VC (Pin 12):
The compensation pin is used for frequency 0.1V/RS. At CS = 0.1V, the gate drivers will be immediately compensation and current limiting. It is the output of the turned off (no slew control). error amplifier and the input of the current comparator. If CS = 0.22V in addition to the drivers being turned off, V Loop frequency compensation can be performed with an C and SS will be discharged to ground (short-circuit protec- RC network connected from the VC pin to ground. The tion). This will hasten turn off on subsequent cycles. voltage on VC is proportional to the switch peak current. The normal range of voltage on this pin is 0.25V to 1.27V.
FB (Pin 9):
The feedback pin is used for positive voltage However, during slope compensation the upper clamp sensing. It is the inverting input to the error amplifier. The voltage is allowed to increase with the compensation. noninverting input of this amplifier connects internally to a 1.25V reference. During a short-circuit fault the VC pin will be discharged to ground. If the voltage on this pin exceeds the reference by 220mV, then the output drivers will immediately turn off the exter-
SS (Pin 13):
The SS pin allows for ramping of the switch nal MOSFETs (no slew control). This provides for output current threshold at startup. Normally a capacitor is placed overvoltage protection on this pin to ground. An internal 9µA current source will charge this capacitor up. The voltage on the VC pin cannot When this input is below 0.9V then the current sense exceed the voltage on SS. Thus peak current will ramp blanking will be disabled. This will assist start up. up as the SS pin ramps up. During a short circuit fault
NFB (Pin 10):
The negative feedback pin is used for sens- the SS pin will be discharged to ground thus reinitializing ing a negative output voltage. The pin is connected to the soft-start. inverting input of the negative feedback amplifier through When SS is below the VC clamp voltage the VC pin will a 100k source resistor. The negative feedback amplifier closely track the SS pin. 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 This pin can be left open if not used. 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.
TEST CIRCUITS
20mA 0.9A 5pF IN5819 5pF IN5819 CAP A/CAP B CAP A/CAP B GATE A/GATE B ZVN3306A + GATE A/GATE B Si4450DY – 10 +– 10 CS 2 0.1 1683 F01a 1683 F01b
Figure 1a. Typical Test Circuitry Figure 1b. Test Circuit for Slew
1683fd 8 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Test Circuits Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts