Datasheet LT3505 (Analog Devices) - 8

LT3505 BLOCK DIAGRAM VIN VIN 3 C2 INT REG AND UVLO D2 ON OFF Σ BOOST 1 SLOPE R R3 COMP Q SHDN 4 C3 S Q DRIVER Q1 C4 L1 OSC SW 2 VOUT D1 C1 FREQUENCY FOLDBACK VC gm 780mV GND VC FB RT 5 8 7 6 R1 3505 BD R2 OPERATION
(Refer to Block Diagram)
The LT3505 is a constant frequency, current mode step- An internal regulator provides power to the control circuitry. down regulator. A resistor-programmed oscillator enables This regulator includes an undervoltage lockout to prevent an RS fl ip-fl op, turning on the internal 1.4A power switch switching when VIN is less than ~3.4V. The SHDN pin is Q1. An amplifi er and comparator monitor the current used to place the LT3505 in shutdown, disconnecting the fl owing between the VIN and SW pins, turning the switch output and reducing the input current to less than 2µA. off when this current reaches a level determined by the The switch driver operates from either the input or from voltage at the VC pin. An error amplifi er measures the the BOOST pin. An external capacitor and diode are used output voltage through an external resistor divider tied to to generate a voltage at the BOOST pin that is higher than the FB pin and servos the VC node. If the error amplifi er’s the input supply. This allows the driver to fully saturate output increases, more current is delivered to the output; the internal bipolar NPN power switch for effi cient opera- if it decreases, less current is delivered. An active clamp tion. (not shown) on the VC node provides current limit. The VC node is also clamped to the voltage on the SHDN pin; When the FB pin is low, the voltage at the RT pin decreases soft-start is implemented by generating a voltage ramp at to reduce the oscillator frequency. This frequency foldback the SHDN pin using an external resistor and capacitor. helps to control the output current during start-up and overload. 3505fc 8