LT3501 pin FuncTionsVIN1 (Pin 1): The VIN1 pin powers the internal control SS1/SS2 (Pins 19, 12): The SS1/2 pins control the soft- circuitry for both channels and is monitored by the under- start and sequence of their respective outputs. A single voltage lockout comparator. The VIN1 pin is also connected capacitor from the SS pin to ground determines the output to the collector of channel 1’s on-chip power NPN switch. ramp rate. For soft-start and output tracking/sequencing The VIN1 pin has high dI/dt edges and must be decoupled details, see the Applications Information section. to ground close to the pin of the device. VC1/VC2 (Pins 18, 13): The VC pin is the output of the SW1/SW2 (Pins 2, 9): The SW pin is the emitter of the on- error amplifier and the input to the peak switch current chip power NPN. At switch-off, the inductor will drive this comparator. It is normally used for frequency compensa- pin below ground with a high dV/dt. An external Schottky tion, but can also be used as a current clamp or control catch diode to ground, close to the SW pin and respective loop override. If the error amplifier drives VC above the VIN decoupling capacitor’s ground, must be used to prevent maximum switch current level, a voltage clamp activates. this pin from excessive negative voltages. This indicates that the output is overloaded and current is IND1/IND2 (Pins 3, 8): The IND pin is the input to the pulled from the SS pin, reducing the regulation point. on-chip sense resistor that measures current flowing in FB1/FB2 (Pins 17, 14): The FB pin is the negative input the inductor. When the current in the resistor exceeds to the error amplifier. The output switches regulate this the current dictated by the VC pin, the SW latch is held in pin to 0.8V, with respect to the exposed ground pad. Bias reset, disabling the output switch. Bias current flows out current flows out of the FB pin. of the IND pin when IND is less than 1.6V. SHDN (Pin 15): The shutdown pin is used to turn off both VOUT1/VOUT2 (Pins 4, 7): The VOUT pin is the output to channels and control circuitry to reduce quiescent current the on-chip sense resistor that measures current flowing to a typical value of 9µA. The accurate 1.28V threshold and in the inductor. When the current in the resistor exceeds input current hysteresis can be used as an undervoltage the current dictated by the VC pin, the SW latch is held in lockout, preventing the regulator from operating until the reset, disabling the output switch. Bias current flows out input voltage has reached a predetermined level. Force of the VOUT pin when VOUT is less than 1.6V. the SHDN pin above its threshold or let it float for normal PG1/PG2 (Pins 5, 6): The power good pin is an open-col- operation. lector output that sinks current when the feedback falls RT/SYNC (Pin 16): This RT/SYNC pin provides two modes below 90% of its nominal regulating voltage. For VIN1 of setting the constant switch frequency. above 1V, its output state remains true, although during Connecting a resistor from the R shutdown, V T/SYNC pin to ground IN1 undervoltage lockout or thermal shutdown, will set the R its current sink capability is reduced. The PG pins can be T/SYNC pin to a typical value of 0.975V. The resultant switching frequency will be set by the resistor left open circuit or tied together to form a single power value. The minimum value of 15.4k and maximum value of good signal. 133k sets the switching frequency to 1.5MHz and 250kHz, VIN2 (Pin 10): The VIN2 pin is the collector of channel 2’s respectively. on-chip power NPN switch. This pin is independent of VIN1 Driving the R and may be connected to the same or a separate supply. In T/SYNC pin with an external clock signal will synchronize the switch to the applied frequency. Synchro- either case, high dI/dt edges are present and decoupling nization occurs on the rising edge of the clock signal after to ground must be used close to this pin. 3501fd 8 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Related Parts
