Datasheet LTC7138 (Analog Devices) - 6

LTC7138
pin FuncTions SW (Pin 1):
Switch Node Connection to Inductor and
ISET (Pin 11):
Peak Current Set Input. A resistor from this Catch Diode Cathode. This pin connects to the drain of pin to ground sets the peak current comparator threshold. the internal power MOSFET switch. Leave floating for the maximum peak current (610mA typi-
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cal) or short to ground for minimum peak current (170mA
IN (Pin 3):
Main Supply Pin. A ceramic bypass capacitor should be tied between this pin and GND. typical). The valley current is typically 60% of the peak current set by this pin. The maximum output current is
FBO (Pin 5):
Feedback Comparator Output. Connect to the 75% of the peak current. The 5µA current that is sourced VFB pins of additional LTC7138s to combine the output out of this pin when switching is reduced to 1µA in sleep. current. The typical pull-up current is 20µA. The typical pull- Optionally, a capacitor can be placed from this pin to GND down impedance is 70Ω. See Applications Information. to trade off efficiency for light load output voltage ripple.
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See Applications Information.
PRG2, VPRG1 (Pins 6, 7):
Output Voltage Selection. Short both pins to ground for a resistive divider programmable
OVLO (Pin 12):
Overvoltage Lockout Input. Connect to output voltage. Short VPRG1 to SS and short VPRG2 to the input supply through a resistor divider to set the over- ground for a 5V output voltage. Short VPRG1 to ground voltage lockout level. A voltage on this pin above 1.21V and short VPRG2 to SS for a 3.3V output voltage. Short disables the internal MOSFET switch. Normal operation both pins to SS for a 1.8V output voltage. resumes when the voltage on this pin decreases below
GND (Pin 8, Exposed Pad Pin 17):
Ground. The exposed 1.10V. Exceeding the OVLO lockout threshold triggers a pad must be soldered to the PCB ground plane for rated soft-start reset, resulting in a graceful recovery from an electrical and thermal performance. input supply transient. This pin must be grounded if the OVLO is not used.
VFB (Pin 9):
Output Voltage Feedback. When configured for an adjustable output voltage, connect to an external
RUN (Pin 14):
Run Control Input. A voltage on this pin resistive divider to divide the output voltage down for above 1.21V enables normal operation. Forcing this pin comparison to the 0.8V reference. For the fixed output below 0.7V shuts down the LTC7138, reducing quiescent configuration, directly connect this pin to the output. current to approximately 1.4µA. Optionally, connect to the input supply through a resistor divider to set the
SS (Pin 10):
Soft-Start Control Input. A capacitor to undervoltage lockout. ground at this pin sets the output voltage ramp time. A 50µA current initially charges the soft-start capacitor until
ANODE (Pin 16):
Catch Diode Anode Sense. This pin is the switching begins, at which time the current is reduced to anode connection for the catch diode. An internal sense its nominal value of 5µA. The output voltage ramp time resistor is connected between this pin and the exposed from zero to its regulated value is 1ms for every 6.25nF pad ground. of capacitance from SS to GND. If left floating, the ramp time defaults to an internal 1ms soft-start. 7138f 6 For more information www.linear.com/LTC7138 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