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LTC3813IG-PBF Datasheet(PDF) 11 Page - Linear Technology |
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LTC3813IG-PBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 32 page  LTC3813 11 3813fb Figure 1. Floating TG Driver Supply and Negative BG Return Main Control Loop The LTC3813 is a current mode controller for DC/DC step- up 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 bot- tom 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 SENSE– and SENSE+ pins using a sense resistor or the bottom MOSFET on-resistance. The voltage on the ITH pin sets the comparator threshold cor- responding to the inductor peak current. The fast 25MHz error amplifier EA adjusts this voltage by comparing the feedback signal VFB to the internal 0.8V reference volt- age. 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 (tOFF) and the duty cycle required to maintain regulation. The one-shot timer generates a top MOSFET on-time that is inversely proportional to the IOFF current and proportional to the VOFF voltage. Connecting VOUT to IOFF and VIN to VOFF with a resistive divider keeps the frequency approximately constant with changes in VIN. The nominal frequency can be adjusted with an external resistor ROFF. For applications with stringent constant-frequency require- ments, the LTC3813 can be synchronized with an external clock. By programming the nominal frequency the same as the external clock frequency, the LTC3813 behaves as a con- stant-frequency part against the load and supply variations. Pulling the SHDN 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. Fault Monitoring/Protection Constant off-time current mode architecture provides accurate cycle-by-cycle current limit protection—a fea- ture that is very important for protecting the high volt- age power supply from output overcurrent conditions. The cycle-by-cycle current monitor guarantees that the inductor current will never exceed the value programmed on the VRNG pin. Overvoltage and undervoltage comparators OV and UV pull the PGOOD output low if the output feedback voltage exits a ±10% window around the regulation point after the internal 125μs power bad mask timer expires. Furthermore, in an overvoltage condition, M1 is turned off and M2 is turned on immediately and held on until the overvoltage condition clears. The LTC3813 provides two undervoltage lockout com- parators—one for the INTVCC/DRVCC supply and one for the input supply VIN. The INTVCC UV threshold is 6.2V to guarantee that the MOSFETs have sufficient gate drive volt- age before turning on. The VIN UV threshold (UVIN pin) is 0.8V with 10% hysteresis which allows programming the VIN threshold with the appropriate resistor divider con- nected to VIN. If either comparator inputs are under the UV threshold, the LTC3813 is shut down and the drivers are turned off. Strong Gate Drivers The LTC3813 contains very low impedance drivers capable of supplying amps of current to slew large MOSFET gates quickly. This minimizes transition losses and allows paral- leling MOSFETs for higher current applications. A 100V floating high side driver drives the top side MOSFET and a low side driver drives the bottom side MOSFET (see Figure 1). The bottom side driver is supplied directly from the DRVCC pin. The top MOSFET drivers are biased from floating bootstrap capacitor CB, which normally is recharged during each off cycle through an external diode OPERATION BOOST TG SW BG M2 + + VIN CIN VOUT COUT DB CB 3813 F01 M1 L BGRTN DRVCC DRVCC LTC3813 0V TO –5V |
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Similar Description - LTC3813IG-PBF |
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