Datasheet LT3988 (Analog Devices) - 6

LT3988
Typical perForMance characTerisTics VIN1 Overvoltage Lockout TRACK/SS Pin Current vs Temperature vs Temperature
65 1.42 1.38 64 VIN1 RISING 1.34 63 OVLO (V) 1.30 V IN1 62 TRACK/SS PIN CURRENT (µA) 1.26 VIN1 FALLING 61 1.22 –50 –25 0 25 50 75 100 125 150 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) TEMPERATURE (°C) 3988 G18 3988 G19
pin FuncTions BD:
Internal boost diodes are connected between the BD and negative pulse widths of at least 100ns. If the SYNC pin and the BOOST pins. Connect BD to a 3V or higher function is not used, connect the SYNC pin to ground. If supply, such as VOUT. using SYNC, minimize coupling to RT and FB2, and add
BOOST1, BOOST2:
The BOOST pins are used to provide decoupling capacitors as needed up to 22pF. drive voltages, higher than the input voltage, to the internal
SW1, SW2:
The SW pins are the outputs of the internal NPN power switches. power switches. Connect these pins to the inductors, catch
DA1, DA2:
Tie the DA pin to the anode of the external diodes and boost capacitors. Schottky catch diode. If the DA pin current exceeds 1.32A,
TRACK/SS1, TRACK/SS2:
The TRACK/SS pins are used which could occur in an overload or short-circuit condi- to soft-start the two channels, to allow one channel to tion, switching is disabled until the DA pin current falls track the other output, or to allow both channels to track below 1.32A. another output. For tracking, tie a resistor divider to this
EN/UVLO:
This pin is used to shut down the LT3988. It can pin from the tracked output. For soft-start, tie a capacitor be driven from a logic level, tied directly to the input, or to this pin. An internal –1.2μA soft-start current charges used as an undervoltage lockout by connecting a resistor the capacitor to create a voltage ramp at the pin. Leave divider from V these pins disconnected if unused. IN1.
FB1, FB2:
The LT3988 regulates each feedback pin to 0.750V.
VIN1:
The VIN1 pin supplies current to the LT3988 internal Connect the feedback resistor divider taps to these pins. circuitry and to the internal power switch connected to SW1 and must be locally bypassed. VIN1 must be greater
GND:
The exposed pad metal of the package provides both than 3.9V (typ) for channel 1 or channel 2 to operate. electrical contact to ground and good thermal contact to the printed circuit board. The exposed pad must be soldered
VIN2:
The VIN2 pin supplies current to the internal power to the circuit board for proper operation. switch connected to SW2 and must be locally bypassed. Connect this pin directly to VIN1 unless power for channel
RT:
The RT pin is used to set the internal oscillator fre- 2 is coming from a different source. VIN2 must be greater quency. Tie a resistor from RT to GND for the desired than 2.6V (typ) and VIN1 must be greater than 3.9V (typ) switching frequency. for channel 2 to operate. If VIN2 is driven above 60V, VIN2
SYNC:
To synchronize the part to an external frequency, must be connected to VIN1. drive the SYNC pin with a logic-level signal with positive 3988f 6 Document Outline Features Applications Description 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 Typical Application Related Parts