Datasheet LT3508 (Analog Devices) - 6

LT3508
PIN FUNCTIONS BOOST1, 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 inter- programmed level. Do not drive SHDN more than 6V nal 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, vides both electrical contact to ground and good thermal catch 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
GND:
Tie the GND pins directly to the Exposed Pad and to this pin. An internal 1.2µA soft-start current charges ground plane. the capacitor to create a voltage ramp at the pin. In the 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 the FB pin is within 10% of the final regulation voltage.
VC1, VC2:
The VC pins are the outputs of the internal error As well as indicating output regulation, the PG pins can amps. The voltages on these pins control the peak switch be used to sequence the two switching regulators. These currents. These pins are normally used to compensate the pins can be left unconnected. The PG outputs are valid control loops, but can also be used to override the loops. when V Pull these pins to ground with an open drain to shut down IN1 is greater than 3.7V and SHDN is high. The PG comparators are disabled in shutdown. each switching regulator separately.
RT/SYNC:
The RT/SYNC pin is used to set the internal
VIN1:
The VIN1 pin supplies current to the LT3508 internal oscillator frequency. Tie a 33.2k resistor from RT/SYNC circuitry and to the internal power switch connected to to GND for a 1MHz switching frequency. To synchronize SW1 and must be locally bypassed. VIN1 must be greater the part to an external frequency, drive the RT/SYNC pin than 3.7V for channel 1 or channel 2 to operate. If VIN1 with a logic-level signal with positive and negative pulse is greater than 3.7V, channel 2 can operate with VIN2 as widths of at least 80ns. low as 3V.
SHDN:
The shutdown pin is used to put the LT3508 in
VIN2:
The VIN2 pin supplies current to the internal power shutdown mode. Pull the pin below 0.3V to shut down the switch connected to SW2 and must be locally bypassed. LT3508. The 2.63V threshold can function as an accurate Connect this pin directly to VIN1 unless power for chan- undervoltage lockout (UVLO), preventing the regulator nel 2 is coming from a different source. VIN2 must be greater than 3V and VIN1 must be greater than 3.7V for channel 2 to operate. Rev. E 6 For more information www.analog.com Document Outline Features Applications Typical Application Description 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 Related Parts