Datasheet LT8630 (Analog Devices) - 10

LT8630
operaTion
The LT8630 is a monolithic, variable frequency, current Comparators monitoring the FB pin voltage will pull the mode step-down DC/DC converter. When the voltage on PG pin low if the output voltage varies more the ±7.5% the EN/UV pin is below its 1.19V threshold, the LT8630 is from the feedback reference voltage. The PG comparators shutdown and draws less than 5µA from the input supply. have 1.9% of hysteresis. When the EN/UV pin is driven above 1.19V, the internal Provided that the output voltage is in regulation (as bias circuits turn on generating an internal regulated volt- determined by the PG comparators), the LT8630 maxi- age, 0.808V feedback reference, a 4.5µA soft-start current mizes efficiency across a wide range of input voltages reference, and a power on reset (POR) signal. by employing a boundary mode switching scheme that During power-up the POR signal is set and in turn sets the minimizes switching losses. The boundary mode switch- soft-start latch. When the soft-start latch is set, the TR/SS ing cycle is comprised of three stages: a top-switch ON pin will be discharged to ground to ensure proper start-up phase, a bottom-switch ON phase, and a discontinuous operation. When the TR/SS pin drops below 50mV, the ring phase. Figure 2 shows an example of boundary mode soft-start latch is reset. Once the latch is reset the soft-start operation. At the beginning of a switch cycle, the internal capacitor starts to charge with a typical value of 4.5µA. top side power switch is turned on at the peak of the SW node discontinuous ring. The current then flows from The error amplifier is a transconductance amplifier that VIN, through the top switch, inductor, and internal sense compares the FB pin voltage to the lowest voltage present resistor to the output. Once the voltage drop across the at either the TR/SS pin or an internal 0.808V reference. internal sense resistor exceeds a predetermined level set Since the TR/SS pin is driven by a constant current source, by the voltage on the internal VC node, the top switch a single capacitor on the soft-start pin will generate a is turned off. Inductor current ceases to flow through controlled linear ramp on the output voltage. The voltage the top switch; instead, inductor current discharges the on the output of the error amplifier (internal VC node in switch node capacitance. The switch node voltage is Figure 1) sets the peak current of each switch cycle and quickly driven below ground and caught by the bottom also determines when to enable low quiescent current switch body diode. The bottom-switch ON phase begins Burst Mode operation. with the internal low side power switch turning on with When the voltage on the VC node rises above the switch- nearly zero drain-source voltage. Energy is delivered to ing threshold, the internal clock set-pulse sets the driver the output as the inductor current decreases from its peak flip-flop, which turns on the internal top power switch. This down to zero. Once the current through the internal sense causes current from VIN, through the top switch, inductor, resistor reaches zero, the bottom switch turns off and the and internal sense resistor, to increase. When the voltage switch node discontinuous ring phase begins. The inductor drop across the internal sense resistor exceeds a prede- resonates with the stray capacitance on the switch node termined level set by the voltage on the internal VC node, and causes a discontinuous ring. If the VC voltage is still the flip-flop is reset and the internal top switch is turned above the switching threshold, the top power switch is off. Once the top switch is turned off the inductor will drive turned on again at the peak of the discontinuous ring and the voltage on the SW pin low. The synchronous power another cycle commences. The switching frequency during switch will turn on, decreasing the current in the inductor boundary mode operation is determined by the inductor until the reverse current comparator trips, indicating that value, input voltage, output voltage, and output current. the inductor current is close to zero. If the VC voltage is The regulator's maximum output current occurs when the still above the switching threshold, the top power switch internal VC node is driven to its maximum clamp value is turned on again and another cycle commences. by the error amplifier. The value of the typical maximum 8630f 10 For more information www.linear.com/LT8630 Document Outline Features Description Applications 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