Datasheet LTC3522 (Analog Devices) - 10

LTC3522
OPERATION Soft-Start
L The buck converter has an internal voltage mode soft-start PVIN1 A SW1A SW1B D VOUT1 circuit with a nominal duration of 600μs. The converter remains in regulation during soft-start and will therefore respond to output load transients which occur during B C this time. In addition, the output voltage rise time has minimal dependency on the size of the output capacitor LTC3522 PGND1 PGND2 or load current. 3522 F01
Figure 1. Buck-Boost Switch Topology Error Amplifi er and Compensation
The LT3522 buck converter utilizes an internal transcon- When the input voltage is signifi cantly greater than the ductance error amplifi er. Compensation of the feedback output voltage, the buck-boost converter operates in loop is performed internally to reduce the size of the buck mode. Switch D turns on continuously and switch application circuit and simplify the design process. The C remains off. Switches A and B are pulse width modu- compensation network has been designed to allow use of lated to produce the required duty cycle to support the a wide range of output capacitors while simultaneously output regulation voltage. As the input voltage decreases, ensuring rapid response to load transients. switch A remains on for a larger portion of the switching cycle. When the duty cycle reaches approximately 85%,
PGOOD2 Comparator
the switch pair AC begins turning on for a small fraction of the switching period. As the input voltage decreases The PGOOD2 pin is an open-drain output which indicates further, the AC switch pair remains on for longer durations the status of the buck converter. If the buck output volt- and the duration of the BD phase decreases proportionally. age falls 7.7% below the regulation voltage, the PGOOD2 As the input voltage drops below the output voltage, the open-drain output will pull low. The output voltage must AC phase will eventually increase to the point that there is rise 2.5% above the falling threshold before the pull-down no longer any BD phase. At this point, switch A remains on will turn off. In addition, there is a 60μs typical deglitch- continuously while switch pair CD is pulse width modu- ing delay in the fl ag in order to prevent false trips due lated to obtain the desired output voltage. At this point, to voltage transients on load steps. The PGOOD2 output the converter is operating solely in boost mode. will also pull low during overtemperature shutdown and undervoltage lockout to indicate these fault conditions. This switching algorithm provides a seamless transition The PGOOD2 output is only active if the buck converter between operating modes and eliminates discontinuities is enabled. in average inductor current, inductor current ripple, and loop transfer function throughout all three operational
BUCK-BOOST CONVERTER OPERATION
modes. These advantages result in increased effi ciency and stability in comparison to the traditional 4-switch
PWM Mode Operation
buck-boost converter. When the PWM pin is held high, the LTC3522 buck-boost
Error Amplifi er and Compensation
converter operates in a constant frequency PWM mode with voltage mode control. A proprietary switching algorithm The buck-boost converter utilizes a voltage mode error allows the converter to switch between buck, buck-boost amplifi er with an internal compensation network as shown and boost modes without discontinuity in inductor cur- in Figure 2. rent or loop characteristics. The switch topology for the Notice that resistor R2 of the external resistor divider buck-boost converter is shown in Figure 1. network plays an integral role in determining the frequency 3522fa 10