LT3508 PIN FUNCTIONSBOOST1, BOOST2: The BOOST pins are used to provide from operating until the input voltage has reached the drive voltages, higher than the input voltage, to the internal programmed level. Do not drive SHDN more than 6V NPN power switches. Tie through a diode to a 2.8V or above VIN1. higher supply, such as VOUT or VIN. SW1, SW2: The SW pins are the outputs of the internal Exposed Pad: The exposed pad metal of the package pro- power switches. Connect these pins to the inductors, catch vides both electrical contact to ground and good thermal diodes and boost capacitors. contact to the printed circuit board. The exposed pad must TRACK/SS1, TRACK/SS2: The TRACK/SS pins are used be soldered to the circuit board for proper operation. to soft-start the two channels, to allow one channel to FB1, FB2: The LT3508 regulates each feedback pin to track the other output, or to allow both channels to track 0.800V. Connect the feedback resistor divider taps to another output. For tracking, tie a resistor divider to this these pins. pin from the tracked output. For soft-start, tie a capacitor to this pin. An internal 1.2μA soft-start current charges GND: Tie the GND pins directly to the Exposed Pad and the capacitor to create a voltage ramp at the pin. In the ground plane. TSSOP package, if these pins are unused, tie a 47pF or PG1, PG2: The power good pins are the open-collector greater capacitor from each pin to GND. outputs of an internal comparator. PG remains low until V the FB pin is within 10% of the fi nal regulation voltage. C1, VC2: The VC pins are the outputs of the internal error amps. The voltages on these pins control the peak switch As well as indicating output regulation, the PG pins can currents. These pins are normally used to compensate the be used to sequence the two switching regulators. These control loops, but can also be used to override the loops. pins can be left unconnected. The PG outputs are valid Pull these pins to ground with an open drain to shut down when VIN1 is greater than 3.7V and SHDN is high. The PG each switching regulator separately. comparators are disabled in shutdown. VRT/SYNC: The RT/SYNC pin is used to set the internal IN1: The VIN1 pin supplies current to the LT3508 internal circuitry and to the internal power switch connected to oscillator frequency. Tie a 33.2k resistor from RT/SYNC SW1 and must be locally bypassed. V to GND for a 1MHz switching frequency. To synchronize IN1 must be greater than 3.7V for channel 1 or channel 2 to operate. If V the part to an external frequency, drive the RT/SYNC pin IN1 is greater than 3.7V, channel 2 can operate with V with a logic-level signal with positive and negative pulse IN2 as low as 3V. widths of at least 80ns. SHDNV: The shutdown pin is used to put the LT3508 in IN2: The VIN2 pin supplies current to the internal power switch connected to SW2 and must be locally bypassed. shutdown mode. Pull the pin below 0.3V to shut down the Connect this pin directly to V LT3508. The 2.63V threshold can function as an accurate IN1 unless power for chan- nel 2 is coming from a different source. V undervoltage lockout (UVLO), preventing the regulator IN2 must be greater than 3V and VIN1 must be greater than 3.7V for channel 2 to operate. 3508fd 6 Document Outline FEATURES DESCRIPTION APPLICATIONS TYPICAL APPLICATION ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION ORDER INFORMATION ELECTRICAL CHARACTERISTICS TYPICAL PERFORMANCE CHARACTERISTICS PIN FUNCTIONS BLOCK DIAGRAM OPERATION APPLICATIONS INFORMATION TYPICAL APPLICATIONS PACKAGE DESCRIPTION REVISION HISTORY TYPICAL APPLICATION RELATED PARTS
