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LTC3812-5 Datasheet(PDF) 11 Page - Linear Technology |
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LTC3812-5 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 34 page  LTC3812-5 11 38125fc Main Control Loop The LTC3812-5 is a current mode controller for DC/DC step-down converters. In normal operation, the top MOSFET is turned on for a fixed interval determined by a one-shot timer (OST). When the top MOSFET is turned off, the bottom MOSFET is turned on until the current comparator ICMP trips, restarting the one-shot timer and initiating the next cycle. Inductor current is determined by sensing the voltage between the PGND and SW pins using the bottom MOSFET on-resistance. The voltage on the ITH pin sets the comparator threshold corresponding to the inductor valley current. The fast 25MHz error amplifier EA adjusts this voltage by comparing the feedback signal VFB to the internal 0.8V reference voltage. If the load current increases, it causes a drop in the feedback voltage relative to the reference. The ITH voltage then rises until the average inductor current again matches the load current. The operating frequency is determined implicitly by the top MOSFET on-time and the duty cycle required to maintain regulation. The one-shot timer generates an on time that is proportional to the ideal duty cycle, thus holding frequency approximately constant with changes in VIN. The nominal frequency can be adjusted with an external resistor RON. Pulling the RUN/SS pin low forces the controller into its shutdown state, turning off both M1 and M2. Forcing a voltage above 1.5V will turn on the device. Pulse-Skipping Mode The LTC3812-5 can operate in one of two modes selectable with the FCB pin—pulse-skipping mode or forced continuous mode (see Figure 1). Pulse-skipping mode is selected when increased efficiency at light loads is desired (see Figure 2). In this mode, the bottom MOSFET is turned off when inductor current reverses to minimize efficiency loss due to reverse current flow and gate charge switching. At low load currents, ITH will drop below the zero current level (1.2V) shutting off both switches. Both switches will remain off with the output capacitor supplying the load current until the ITH voltage rises above the zero current Figure 1. Comparison of Inductor Current Waveforms for Pulse-Skipping Mode and Forced Continuous Operation OPERATION Figure 2. Efficiency in Pulse-Skipping/Forced Continuous Modes LOAD (A) 0.01 40 50 60 70 80 0.1 1 10 38125 F02 30 20 10 0 90 100 VIN = 12V VIN = 42V PULSE SKIP FORCED CONTINUOUS DECREASING LOAD CURRENT 38125 F01 PULSE SKIPPING MODE 0A 0A 0A 0A 0A 0A FORCED CONTINUOUS |
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Similar Description - LTC3812-5_15 |
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