Datasheet LT8642S (Analog Devices) - 10

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PIN FUNCTIONS BIAS (Pin 1):
The internal regulator will draw current from external resistor divider from VIN can be used to program BIAS instead of VIN when BIAS is tied to a voltage higher a VIN threshold below which the LT8642S will shut down. than 3.1V. For output voltages of 3.3V and above this pin
RT (Pin 18):
A resistor is tied between RT and ground to should be tied to VOUT. If this pin is tied to a supply other set the switching frequency. than VOUT use a 1µF local bypass capacitor on this pin. If no supply is available, tie to GND. However, especially for
CLKOUT (Pin 19):
In forced continuous mode, spread high input or high frequency applications, BIAS should be spectrum, and synchronization modes, the CLKOUT pin tied to output or external supply of 3.3V or above. will provide a ~200ns wide pulse at the switch frequency. The low and high levels of the CLKOUT pin are ground and
INTVCC (Pin 2):
Internal 3.4V Regulator Bypass Pin. The INTV internal power drivers and control circuits are powered CC respectively, and the drive strength of the CLKOUT pin is several hundred ohms. In Burst Mode operation, from this voltage. Do not load the INTVCC pin with exter- the CLKOUT pin will be low. Float this pin if the CLKOUT nal circuitry. INTVCC current will be supplied from BIAS function is not used. if BIAS > 3.1V, otherwise current will be drawn from VIN. Voltage on INTVCC will vary between 2.8V and 3.4V when
SYNC/MODE (Pin 20):
For the LT8642S, this pin BIAS is between 3.0V and 3.6V. This pin should be floated. programs four different operating modes: (1) Burst Mode operation. Tie this pin to ground for Burst Mode
GND (Pins 3, 16, Exposed Pad Pins 25–28):
Ground. operation at low output loads—this will result in low Place the negative terminal of the input capacitor as close quiescent current. (2) Forced Continuous mode (FCM). to the GND pins as possible. The exposed pads should This mode offers fast transient response and full be soldered to the PCB for good thermal performance. If frequency operation over a wide load range. Float this necessary due to manufacturing limitations Pins 25 to 28 pin for FCM. When floating, pin leakage currents should may be left disconnected, however thermal performance be <1µA. See Block Diagram for internal pull-up and pull- will be degraded. down resistance. (3) Spread spectrum mode. Tie this pin
VIN (Pins 4, 5, 6, 13, 14, 15):
The VIN pins supply cur- high to INTVCC (or >3V) for forced continuous mode with rent to the LT8642S internal circuitry and to the internal spread-spectrum modulation. 4) Synchronization mode. topside power switch. These pins must be tied together Drive this pin with a clock source to synchronize to an and be locally bypassed with a capacitor of 4.7µF or more. external frequency. During synchronization the part will Be sure to place the positive terminal of the input capaci- operate in forced continuous mode. tor as close as possible to the VIN pins, and the negative
SS (Pin 21):
Output Tracking and Soft-Start Pin. This pin capacitor terminal as close as possible to the GND pins. allows user control of output voltage ramp rate during
BST (Pin 7):
This pin is used to provide a drive voltage, start-up. A SS voltage below 1V forces the LT8642S to higher than the input voltage, to the topside power switch. regulate the FB pin to a function of the SS pin voltage. See This pin should be floated. plot in the Typical Performance Characteristics section.
SW (Pins 8–12):
The SW pins are the outputs of the inter- When SS is above 1V, the tracking function is disabled nal power switches. Tie these pins together and connect and the internal reference resumes control of the error them to the inductor. This node should be kept small on amplifier. An internal 1.9µA pull-up current from INTVCC the PCB for good performance and low EMI. on this pin allows a capacitor to program output voltage slew rate. This pin is pul ed to ground with an internal
EN/UV (Pin 17):
The LT8642S is shut down when this 200Ω MOSFET during shutdown and fault conditions; use pin is low and active when this pin is high. The hyster- a series resistor if driving from a low impedance output. etic threshold voltage is 0.99V going up and 0.95V going This pin may be left floating if the soft-start feature is not down. Tie to VIN if the shutdown feature is not used. An being used. Rev 0 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Electrical Characteristics Pin Configuration Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Typical Applications Related Parts