Datasheet LT1913 (Analog Devices) - 9

LT1913
OPERATION
The LT1913 is a constant frequency, current mode step- effi ciency. The RUN/SS pin is used to place the LT1913 down regulator. An oscillator, with frequency set by RT, in shutdown, disconnecting the output and reducing the enables an RS fl ip-fl op, turning on the internal power input current to less than 0.5μA. switch. An amplifi er and comparator monitor the current The switch driver operates from either the input or from fl owing between the VIN and SW pins, turning the switch the BOOST pin. An external capacitor and diode are used off when this current reaches a level determined by the to generate a voltage at the BOOST pin that is higher than voltage at VC. An error amplifi er measures the output the input supply. This allows the driver to fully saturate voltage through an external resistor divider tied to the FB the internal bipolar NPN power switch for effi cient opera- pin and servos the VC pin. If the error amplifi er’s output tion. increases, more current is delivered to the output; if it decreases, less current is delivered. An active clamp on the The oscillator reduces the LT1913’s operating frequency V when the voltage at the FB pin is low. This frequency C pin provides current limit. The VC pin is also clamped to the voltage on the RUN/SS pin; soft-start is implemented foldback helps to control the output current during startup by generating a voltage ramp at the RUN/SS pin using an and overload. external resistor and capacitor. The LT1913 contains a power good comparator which trips An internal regulator provides power to the control circuitry. when the FB pin is at 91% of its regulated value. The PG The bias regulator normally draws power from the V output is an open-collector transistor that is off when the IN pin, but if the BD pin is connected to an external voltage higher output is in regulation, allowing an external resistor to pull than 3V bias power will be drawn from the external source the PG pin high. Power good is valid when the LT1913 is (typically the regulated output voltage). This improves enabled and VIN is above 3.6V. 1913f 9