Datasheet LT3694, LT3694-1 (Analog Devices) - 9

LT3694/LT3694-1
OPERATION
Unless specifically noted, this data sheet refers to both the the current through the inductor to the output. The internal LT3694 and the LT3694-1 generically as the LT3694. error amplifier regulates the output voltage by continually The LT3694 is a constant-frequency, current mode, buck adjusting the VC1 pin voltage. The threshold for switching regulator with an internal power switch plus two low on the VC1 pin is 0.75V and an active clamp of 2V limits dropout linear regulator controllers. The three regulators the output current. share common circuitry including input source, voltage Overcurrent protection is provided by the DA comparator. reference, undervoltage lockout, and enable, but are oth- The DA comparator senses the catch diode current and erwise independent. Operation can be best understood by will delay the switch-on cycle if the diode current is too referring to the Block Diagram (Figure 1). high at the beginning of a cycle. If the EN/UVLO pin is below 0.35V (min), the LT3694 is The TRK/SS pins override the 0.75V reference for the FB shut down and draws <2µA from the input source tied to pins when the TRK/SS pins are below 0.75V. This allows VIN1. If the EN/UVLO pin is driven above 0.5V (typ), the either coincident or ratiometric supply tracking on start-up internal bias circuits turn on, including the internal regulator, as well as a soft-start capability. reference and master oscillator. The switching regulator The switch driver operates either from V will only begin to operate when the EN/UVLO pin reaches IN or from the BST >1.20V (typ). The EN/UVLO pin can be driven from a logic pin. An external capacitor is used to generate a voltage at gate or can be used as an undervoltage lockout by using the BST pin that is higher than the input supply. This al- a resistor divider to V lows the driver to saturate the internal bipolar NPN power IN. switch for efficient operation. The switcher is a current mode regulator. Instead of directly modulating the duty cycle of the power switch, the feedback The BIAS pin allows the internal circuitry to draw its cur- loop controls the peak current in the switch during each rent from a voltage supply lower than VIN, reducing power cycle. Compared to voltage mode control, current mode dissipation and increasing efficiency. If the voltage on the control improves loop dynamics and provides cycle-by- BIAS pin falls below 2.7V, then its quiescent current will cycle current limit. flow from VIN. A pulse from the oscillator sets the RS flip-flop and turns The LDO regulator uses an external NPN pass transistor to on the internal NPN bipolar power switch. Current in the form a linear regulator. The loop is internally compensated switch and the external inductor begins to increase. When to be stable with a minimum load capacitance of 2.2 µF. The this current exceeds a level determined by the voltage LDO also has a foldback current limiter available to protect at V the external transistor under overload conditions C1, the current comparator resets the RS flip-flop, turning off the switch. The current in the inductor flows The overvoltage detection shuts down the LT3694 if the through the external, Schottky, catch diode, and begins to input voltage goes above 38V. This will prevent the switch decrease. The cycle begins again at the next pulse from the from turning on under high voltage conditions and allows oscillator. In this way, the voltage on the VC1 pin controls the LT3694 to survive transient input voltages up to 70V. 36941fb 9 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