Datasheet LT8608, LT8608B (Analog Devices) - 9

LT8608/LT8608B
PIN FUNCTIONS BST:
This pin is used to provide a drive voltage, higher
TR/SS (MSOP Only):
Output Tracking and Soft-Start Pin. than the input voltage, to the topside power switch. Place This pin allows user control of output voltage ramp rate a 0.1µF boost capacitor as close as possible to the IC. Do during start-up. A TR/SS voltage below 0.778V forces the not place a resistor in series with this pin. LT8608 to regulate the FB pin to equal the TR/SS pin volt-
SW:
The SW pin is the output of the internal power age. When TR/SS is above 0.778V, the tracking function switches. Connect this pin to the inductor and boost is disabled and the internal reference resumes control of capacitor. This node should be kept small on the PCB for the error amplifier. An internal 2μA pull-up current from good performance. INTVCC on this pin allows a capacitor to program out- put voltage slew rate. This pin is pulled to ground with a
INTVCC:
Internal 3.5V Regulator Bypass Pin. The inter- 300Ω MOSFET during shutdown and fault conditions; use nal power drivers and control circuits are powered from a series resistor if driving from a low impedance output. this voltage. INTVCC max output current is 20mA. Voltage There is no TR/SS pin in the LT8608 DFN or LT8608B and on INTVCC will vary between 2.8V and 3.5V. Decouple the node is internally floated. this pin to power ground with at least a 1μF low ESR ceramic capacitor. Do not load the INTV
PG:
The PG pin is the open-drain output of an internal CC pin with exter- nal circuitry. comparator. PG remains low until the FB pin is within ±8.5% of the final regulation voltage, and there are no
RT:
A resistor is tied between RT and ground to set the fault conditions. PG is valid when VIN is above 3.2V, switching frequency. When synchronizing, the RT resistor regardless of EN/UV pin state. should be chosen to set the LT8608 switching frequency equal to or below the lowest synchronization input.
VIN:
The VIN pin supplies current to the LT8608 internal circuitry and to the internal topside power switch. This pin
SYNC (MSOP Only):
External Clock Synchronization Input. must be locally bypassed. Be sure to place the positive Ground this pin for low ripple Burst Mode operation at low terminal of the input capacitor as close as possible to the output loads. Tie to a clock source for synchronization to VIN pins, and the negative capacitor terminal as close as an external frequency. Leave floating for pulse-skipping possible to the GND pins. mode with no spread spectrum modulation. Tie to INTVCC or tie to a voltage between 3.2V and 5.0V for pulse-skip-
EN/UV:
The LT8608 is shut down when this pin is low and ping mode with spread spectrum modulation. When in active when this pin is high. The hysteretic threshold volt- pulse-skipping mode, the I age is 1.05V going up and 1.00V going down. Tie to VIN Q will increase to several mA. There is no SYNC pin in the LT8608 DFN package. The if the shutdown feature is not used. An external resistor LT8608 DFN package internally ties SYNC to ground. The divider from VIN can be used to program a VIN threshold LT8608B DFN package internally floats SYNC. below which the LT8608 will shut down.
FB:
The LT8608 regulates the FB pin to 0.778V. Connect
GND:
Exposed Pad Pin. The exposed pad must be con- the feedback resistor divider tap to this pin. nected to the negative terminal of the input capacitor and soldered to the PCB in order to lower the thermal resistance. Rev. E For more information www.analog.com 9 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts