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LTC4449IDCBTRPBF Datasheet(PDF) 8 Page - Linear Technology |
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LTC4449IDCBTRPBF Datasheet(HTML) 8 Page - Linear Technology |
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8 / 12 page  LTC4449 8 4449f OPERATION Rise/Fall Time Since the power MOSFETs generally account for the majority of power loss in a converter, it is important to quickly turn them on and off, thereby minimizing the transition time and power loss. The LTC4449’s peak pull- up current of 3.2A for both BG and TG produces a rapid turn-on transition for the MOSFETs. This high current is capable of driving a 3nF load with an 8ns risetime. It is also important to turn the power MOSFETs off quickly to minimize power loss due to transition time; however, an additional benefit of a strong pull-down on the driver outputs is the prevention of cross-conduction current. For example, when BG turns the low side power MOSFET off and TG turns the high side power MOSFET on, the voltage on the TS pin will rise to VIN very rapidly. This high frequency positive voltage transient will couple through the CGD capacitance of the low side power MOSFET to the BG pin. If the BG pin is not held down sufficiently, the voltage on the BG pin will rise above the threshold voltage of the low side power MOSFET, momentarily turning it back on. As a result, both the high side and low side MOSFETs will be conducting, which will cause significant cross-conduction current to flow through the MOSFETs from VIN to ground, thereby introducing substantial power loss. A similar effect occurs on TG due to the CGS and CGD capacitances of the high side MOSFET. The hysteresis between the corresponding VIH and VIL voltage levels eliminates false triggering due to noise during switch transitions; however, care should be taken to keep noise from coupling into the IN pin, particularly in high frequency, high voltage applications. Undervoltage Lockout The LTC4449 contains undervoltage lockout detectors that monitor both the VCC and VLOGIC supplies. When VCC falls below 3.04V or VLOGIC falls below 2.65V, the output pins BG and TG are pulled to GND and TS, respectively. This turns off both of the external MOSFETs. When VCC and VLOGIC have adequate supply voltage for the LTC4449 to operate reliably, normal operation will resume. Adaptive Shoot-Through Protection Internal adaptive shoot-through protection circuitry monitors the voltages on the external MOSFETs to ensure that they do not conduct simultaneously. The LTC4449 does not allow the bottom MOSFET to turn on until the gate-source voltage on the top MOSFET is sufficiently low, and vice-versa. This feature improves efficiency by eliminating cross-conduction current from flowing from the VIN supply through the MOSFETs to ground during a switch transition. Output Stage A simplified version of the LTC4449’s output stage is shown in Figure 2. The pull-up device on both the BG and TG outputs is an NPN bipolar junction transistor (Q1 and Q2) in parallel with a low resistance P-channel MOSFET (P1 and P2). This powerful combination rapidly pulls the BG and TG outputs to their positive rails (VCC and BOOST, respectively). Both BG and TG have N-channel MOSFET pull-down devices (N1 and N2) which pull BG and TG down to their negative rails, GND and TS. An additional NPN bipolar junction transistor (Q3) is present on BG to increase its pull-down drive current capacity. The rail-to-rail voltage swing of the BG and TG output pins is important in driving external power MOSFETs, whose RDS(ON) is inversely proportional to its gate overdrive voltage (VGS – VTH). BOOST LTC4449 TG TS CGS CGD HIGH SIDE POWER MOSFET VIN VCC BG N2 P2 Q2 GND CGS 4449 F02 CGD LOW SIDE POWER MOSFET LOAD INDUCTOR Q3 N1 P1 Q1 Figure 2. Capacitance Seen by BG and TG During Switching |
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