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LTC4444IMS8E-5-PBF Datasheet(PDF) 8 Page - Linear Technology |
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LTC4444IMS8E-5-PBF Datasheet(HTML) 8 Page - Linear Technology |
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8 / 12 page  LTC4444-5 8 44445fa OPERATION Output Stage A simplified version of the LTC4444-5’s output stage is shown in Figure 1. The pull-up devices on the BG and TG outputs are NPN bipolar junction transistors (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 (M1 and M2) which pull BG and TG down to their negative rails, GND and TS. 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 the gate overdrive voltage (VGS − VTH). Rise/Fall Time The LTC4444-5’s rise and fall times are determined by the peak current capabilities of Q1 and M1. The predriver that drives Q1 and M1 uses a nonoverlapping transition scheme to minimize cross-conduction currents. M1 is fully turned off before Q1 is turned on and vice versa. Since the power MOSFET generally accounts for the majority of the power loss in a converter, it is important to quickly turn it on or off, thereby minimizing the transition time in its linear region. 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 (synchronous) 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 there is an insufficient pull-down on the BG pin, the voltage on the BG pin can rise above the threshold voltage of the low side power MOSFET, momentarily turning it back on. With both the high side and low side MOSFETs conducting, significant cross-conduction current will flow through the MOSFETs from VIN to ground and will cause substantial power loss. A similar effect occurs on TG due to the CGS and CGD capacitances of the high side MOSFET. The powerful output driver of the LTC4444-5 reduces the switching losses of the power MOSFET, which increase with transition time. The LTC4444-5’s high side driver is Figure 1. Capacitance Seen by BG and TG During Switching capable of driving a 1nF load with 8ns rise and 5ns fall times using a bootstrapped supply voltage VBOOST-TS of 12V while its low side driver is capable of driving a 1nF load with 6ns rise and 3ns fall times using a supply voltage VCC of 12V. Undervoltage Lockout (UVLO) The LTC4444-5 contains an undervoltage lockout detector that monitors VCC supply. When VCC falls below 3.55V, the output pins BG and TG are pulled down to GND and TS, respectively. This turns off both external MOSFETs. When VCC has adequate supply voltage, normal operation will resume. Adaptive Shoot-Through Protection Internal adaptive shoot-through protection circuitry moni- tors the voltages on the external MOSFETs to ensure that they do not conduct simultaneously. This feature improves efficiency by eliminating cross-conduction current from flowing from the VIN supply through both of the MOSFETs to ground during a switch transition. The adaptive shoot- through protection circuitry also monitors the level of the TS pin. If the TS pin stays high, BG will be turned on 150ns after TG is turned off. 6 BOOST LTC4444-5 8 TS TG 7 VIN UP TO 100V Q1 M1 CGS CGD 3 VCC 9 GND 4 BG Q2 M2 LOW SIDE POWER MOSFET HIGH SIDE POWER MOSFET CGS CGD LOAD INDUCTOR |
Similar Part No. - LTC4444IMS8E-5-PBF |
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Similar Description - LTC4444IMS8E-5-PBF |
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