Datasheet LT1738 (Analog Devices) - 10

LT1738
U OPERATIO
The current slew feedback loop consists of the voltage The voltage on GCL determines two features. The first is across the external sense resistor, which is internally the maximum gate drive voltage. This will protect the amplified and differentiated. The derivative is limited to a MOSFET gate from overvoltage. value set by RCSL. The current slew rate is thus inversely With GCL tied to a zener or an external voltage source then proportional to both the value of sense resistor and RCSL. the maximum gate driver voltage is approximately The two control loops are combined internally so that a VGCL␣ – 0.2V. If GCL is tied to VIN, then the maximum gate smooth transition from current slew control to voltage voltage is determined by VIN and is approximately slew control is obtained. When turning on, the driver VIN – 1.6V. There is an internal 19V zener on the GCL pin current will slew before voltage. When turning off, voltage that prevents the gate driver pin from exceeding approxi- will slew before current. In general it is desirable to have mately 19V. RVSL and RCSL of similar value. In addition, the GCL voltage determines undervoltage lockout of the gate drive. This feature disables the gate
Internal Regulator
driver if VIN is too low to provide adequate voltage to turn Most of the control circuitry operates from an internal 2.4V on the MOSFET. This is helpful during start up to insure the low dropout regulator that is powered from VIN. The MOSFET has sufficient gate drive to saturate. internal low dropout design allows VIN to vary from 2.7V If GCL is tied to a voltage source or zener less than 6.8V, to 20V with stable operation of the controller. When SHDN the gate driver will not turn on until V < 1.3V the internal regulator is completely disabled. IN exceeds GCL voltage by 0.8V. For VGCL above 6.5V, the gate drive is
5V Regulator
insured to be off for VIN < 7.3V and it will be turned on by VGCL + 0.8V. A 5V regulator is provided for powering external circuitry. This regulator draws current from V If GCL is tied to V IN and requires VIN to IN, the gate driver is always on be greater than 6.5V to be in regulation. It can sink or (undervoltage lockout is disabled). source 10mA. The output is current limited to prevent The gate drive has current limits for the drive currents. If against destruction from accidental short circuits. the sink or source current is greater than 300mA then the current will be limited.
Safety and Protection Features
The V5 regulator also has internal current limiting that will There are several safety and protection features on the only guarantee ±10mA output current. chip. The first is overcurrent limit. Normally the gate driver will go low when the output of the internal sense amplifier There is also an on chip thermal shutdown circuit that will exceeds the voltage on the V turn off the output in the event the chip temperature rises C pin. The VC pin is clamped such that maximum output current is attained when the CS to dangerous levels. Thermal shutdown has hysteresis pin voltage is 0.1V. At that level the outputs will be that will cause a low frequency (<1kHz) oscillation to occur immediately turned off (no slew). The effect of this control as the chip heats up and cools down. is that the output voltage will foldback with overcurrent. The chip has an undervoltage lockout feature that will In addition, if the CS voltage exceeds 0.22V, the V force the gate driver low in the event that V C and SS IN drops below pins will be discharged to ground, resetting the soft-start 2.5V. This insures predictable behavior during start up and function. Thus if a short is present this will allow for faster shut down. SHDN can be used in conjuction with an MOSFET turnoff and less MOSFET stress. external resistor divider to completely disable the part if the input voltage is too low. This can be used to insure If the voltage on the FB pin exceeds regulation by approxi- adequate voltage to reliably run the converter. See the mately 0.22V, the outputs will immediately go low. The section in Applications Information. implication is that there is an overvoltage fault. Table 1 summarizes these features. 1738fa 10