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LTC4442 Datasheet(PDF) 9 Page - Linear Technology |
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LTC4442 Datasheet(HTML) 9 Page - Linear Technology |
9 / 14 page LTC4442/LTC4442-1 9 4442fb OPERATION 8 BOOST LTC4442 TG N1 Q1 1 TS CGS CGD HIGH SIDE POWER MOSFET VIN UP TO 38V 2 7 VCC BG N2 Q2 Q3 3 GND CGS 4442 F02 CGD LOW SIDE POWER MOSFET 4 LOAD INDUCTOR Figure 2. Capacitance Seen by BG and TG During Switching 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 LTC4442’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). The BG and TG outputs are pulled up to within an NPN VBE (~0.7V) of their positive rails (VCC and BOOST, respectively).BothBGandTGhaveN-channelMOSFETpull- 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 large 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). Rise/Fall Time Since the power MOSFET generally accounts for the major- ity of power loss in a converter, it is important to quickly turn it on and off, thereby minimizing the transition time and power loss. The LTC4442’s peak pull-up current of 2.4A for both BG and TG (Q1 and Q2) produces a rapid turn-on transition for the MOSFETs. This high current is capable of driving a 3nF load with a 12ns rise time. 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 volt- age 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 volt- age 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 LTC4442’s powerful parallel combination of the N-channel MOSFET (N2) and NPN (Q3) on the BG pull-down generates a phenomenal 5ns fall time on BG while driving a 3nF load. Similarly, the 1Ω pull-down MOSFET (N1) on TG results in a rapid 8ns fall time with a 3nF load. These powerful pull-down devices minimize the power loss as- sociated with MOSFET turn-off time and cross-conduction current. |
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Similar Description - LTC4442 |
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