Datasheet LT1738 (Analog Devices) - 7

LT1738
U U U PI FU CTIO S
Approximately 50µA of current can be sourced from this exceed the voltage on SS. Thus peak current will ramp up pin if VGCL < VIN – 0.8V. as the SS pin ramps up. During a short circuit fault the SS pin will be discharged to ground thus reinitializing soft- This pin also controls undervoltage lockout of the gate start. drive. If the pin is tied to a zener or voltage source, the gate drive will not be enabled until VIN > VGCL + 0.8V. If this pin When SS is below the VC clamp voltage the VC pin will is tied to VIN, then undervoltage lockout is disabled. closely track the SS pin. There is an internal 19V zener tied from this pin to ground This pin can be left open if not used. to provide a fail-safe for maximum gate voltage.
CS (Pin 4):
This is the input to the current sense amplifier. It is used for both current mode control and current
Slew Control
slewing of the external MOSFET. Current sense is accom-
CAP (Pin 2):
This pin is the feedback node for the external plished via a sense resistor (RS) connected from the voltage slewing capacitor. Normally a small 1pf to 5pf source of the external MOSFET to ground. CS is connected capacitor is connected from this pin to the drain of the to the top of RS. Current sense is referenced to the GND MOSFET. pin. The voltage slew rate is inversely proportional to this The switch maximum operating current will be equal to capacitance and proportional to the current that the part 0.1V/RS. At CS = 0.1V, the gate driver will be immediately will sink and source on this pin. That current is inversely turned off (no slew control). proportional to RVSL. If CS = 0.22V in addition to the drivers being turned off, VC
RCSL (Pin 15):
A resistor to ground sets the current slew and SS will be discharged to ground (short-circuit protec- rate for the external drive MOSFET during switching. The tion). This will hasten turn off on subsequent cycles. minimum resistor value is 3.3k and the maximum value is
FB (Pin 9):
The feedback pin is used for positive voltage 68k. The time to slew between on and off states of the sensing. It is the inverting input to the error amplifier. The MOSFET current will determine how the di/dt related noninverting input of this amplifier connects internally to harmonics are reduced. This time is proportional to RCSL a 1.25V reference. and RS (the current sense resistor) and maximum current. Longer times produce a greater reduction of higher fre- If the voltage on this pin exceeds the reference by 220mV, quency harmonics. then the output driver will immediately turn off the external MOSFET (no slew control). This provides for output over-
RVSL (Pin 16):
A resistor to ground sets the voltage slew voltage protection rate for the drain of the external drive MOSFET. The minimum resistor value is 3.3k and the maximum value is When this input is below 0.9V then the current sense 68k. The time to slew between on and off states on the blanking will be disabled. This will assist start up. MOSFET drain voltage will determine how dv/dt related
NFB (Pin 10):
The negative feedback pin is used for harmonics are reduced. This time is proportional to RVSL, sensing a negative output voltage. The pin is connected to CV and the input voltage. Longer times produce more the inverting input of the negative feedback amplifier rolloff of harmonics. CV is the equivalent capacitance from through a 100k source resistor. The negative feedback CAP to the drain of the MOSFET. amplifier provides a gain of –0.5 to the FB pin. The nominal regulation point would be –2.5V on NFB. This pin should
Switch Mode Control
be left open if not used.
SS (Pin 3):
The SS pin allows for ramping of the switch If NFB is being used then overvoltage protection will occur current threshold at startup. Normally a capacitor is placed at 0.44V below the NFB regulation point. on this pin to ground. An internal 9µA current source will charge this capacitor up. The voltage on the VC pin cannot At NFB < –1.8 current sense blanking will be disabled. 1738fa 7