Datasheet LT3988 (Analog Devices) - 8

LT3988
operaTion
The LT3988 is a dual, constant frequency, current mode Each switcher contains an extra, independent oscillator to regulator with internal power switches. Operation can perform frequency foldback during overload conditions. be best understood by referring to the Block Diagram in This slave oscillator is normally synchronized to the master Figure 1. oscillator. A comparator senses when VFB is less than 50% If the EN/UVLO pin is pulled low, the LT3988 is shut down of its regulated value and switches the regulator from the and draws minimal current from the input source(s) tied master oscillator to a slower slave oscillator. VFB is less than to the V 50% of its regulated value during start-up, short-circuit, IN pins. If the EN/UVLO pin exceeds 0.5V (typ), the internal bias circuits turn on, including the internal and overload conditions. Frequency foldback helps limit regulator, reference and master oscillator. The switching switch current under these conditions. regulators will only begin to operate when the EN/UVLO The TRACK/SS pins override the 0.75V reference of the pin exceeds 1.2V (typ). FB pins when the TRACK/SS pins are below 0.75V. This The switcher is a current mode regulator. Instead of directly allows either coincident or ratiometric supply tracking on modulating the duty cycle of the power switch, the feedback start-up as well as a soft-start capability. loop controls the peak current in the switch during each The switch drivers operate either from VIN or from the cycle. Compared to voltage mode control, current mode BOOST pin. An external capacitor and internal Schottky control improves loop dynamics and provides cycle-by- diode are used to generate a voltage at the BOOST pin that cycle current limit. is higher than the input supply. This allows the driver to An oscillator enables an RS flip flop, turning on the internal obtain a low VCE across the internal bipolar NPN power power switch. An amplifier and comparator monitor the switch for efficient operation. current flowing between the VIN and SW pins, turning the The BD pin serves two purposes. The voltage at BD deter- switch off when this current reaches a level determined by mines the BOOST1 and BOOST2 levels over the VIN1 and the voltage at VC. An error amplifier measures the output VIN2 supply voltages, and allows the internal circuitry to voltage through an external resistor divider tied to the draw its current from a lower voltage supply than VIN1. FB pin and servos the VC voltage. If the error amplifier’s This reduces power dissipation and increases efficiency. output increases, more current is delivered to the output; If the voltage at BD falls below 3V, then quiescent current if it decreases, less current is delivered. An active clamp will flow from VIN1. on the VC voltage provides a current limit. The overvoltage and undervoltage detection shuts down The switching frequency is set either by the resistance to the LT3988 if the input voltage on VIN1 goes above or GND at the RT pin or by the frequency of the logic-level below thresholds. The overvoltage detector shuts down signal driving the SYNC pin. A detection circuit monitors the regulators when VIN1 exceeds 60V. An undervoltage for the presence of a SYNC signal on the pin and switches detector monitoring VIN1 disables both regulators when between the two modes upon detection of a clock applied VIN1 is under 3.7V, an undervoltage detector monitoring to the SYNC pin. Use of the SYNC pin as a frequency input VIN2 shuts down channel 2 when VIN2 is under 2.5V. The requires the use of an RT resistor as well. This requirement higher voltage is required on VIN1 to accomodate internal is detailed in the Switching Frequency section. Onboard bias circuits. Additionally, tying the EN/UVLO pin to a volt- circuitry generates the appropriate slope compensation age divider from VIN1 to ground allows a programmable ramps and generates the 180° out-of-phase clocks for undervoltage threshold. the two channels. 3988f 8 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