Datasheet LT1936 (Analog Devices) - 7

LT1936
OPERATION (Refer to Block Diagram)
The LT1936 is a constant frequency, current mode step- An internal regulator provides power to the control circuitry. down regulator. A 500kHz oscillator enables an RS fl ip-fl op, This regulator includes an undervoltage lockout to prevent turning on the internal 1.9A power switch Q1. An ampli- switching when VIN is less than ~3.45V. The SHDN pin is fi er and comparator monitor the current fl owing between used to place the LT1936 in shutdown, disconnecting the the VIN and SW pins, turning the switch off when this output and reducing the input current to less than 2μA. current reaches a level determined by the voltage at VC. The switch driver operates from either the input or from An error amplifi er measures the output voltage through the BOOST pin. An external capacitor and diode are used an external resistor divider tied to the FB pin and servos to generate a voltage at the BOOST pin that is higher than the VC pin. If the error amplifi er’s output increases, more the input supply. This allows the driver to fully saturate current is delivered to the output; if it decreases, less the internal bipolar NPN power switch for effi cient opera- current is delivered. An active clamp (not shown) on the tion. VC pin provides current limit. The VC pin is also clamped to the voltage on the SHDN pin; soft-start is implemented The oscillator reduces the LT1936’s operating frequency by generating a voltage ramp at the SHDN pin using an when the voltage at the FB pin is low. This frequency external resistor and capacitor. foldback helps to control the output current during startup and overload. 1936fd 7