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LTC1693-3CMS8 Datasheet(PDF) 8 Page - Linear Technology |
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LTC1693-3CMS8 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 20 page  8 LTC1693 APPLICATIONS INFORMATION Overview The LTC1693 single and dual drivers allow 3V- or 5V-based digital circuits to drive power MOSFETs at high speeds. A power MOSFET’s gate-charge loss increases with switch- ing frequency and transition time. The LTC1693 is capable of driving a 1nF load with a 16ns rise and fall time using a VCC of 12V. This eliminates the need for higher voltage supplies, such as 18V, to reduce the gate charge losses. The LTC1693’s 360 µA quiescent current is an order of magnitude lower than most other drivers/buffers. This improves system efficiency in both standby and switching operation. Since a power MOSFET generally accounts for the majority of power loss in a converter, addition of the LT1693 to a high power converter design greatly improves efficiency, using very little board space. The LTC1693-1 and LTC1693-2 are dual drivers that are electrically isolated. Each driver has independent opera- tion from the other. Drivers may be used in different parts of a system, such as a circuit requiring a floating driver and the second driver being powered with respect to ground. Input Stage The LTC1693 employs 3V CMOS compatible input thresh- olds that allow a low voltage digital signal to drive standard power MOSFETs. The LTC1693 incorporates a 4V internal regulator to bias the input buffer. This allows the 3V CMOS compatible input thresholds (VIH = 2.6V, VIL = 1.4V) to be independent of variations in VCC. The 1.2V hysteresis between VIH and VIL eliminates false triggering due to ground noise during switching transitions. The LTC1693’s input buffer has a high input impedance and draws less than 10 µA during standby. Output Stage The LTC1693’s output stage is essentially a CMOS in- verter, as shown by the P- and N-channel MOSFETs in Figure 1 (P1 and N1). The CMOS inverter swings rail-to- rail, giving maximum voltage drive to the load. This large voltage swing is important in driving external power MOSFETs, whose RDS(ON) is inversely proportional to its gate overdrive voltage (VGS – VT). P1 CGD VDRAIN POWER MOSFET LEQ (LOAD INDUCTOR OR STRAY LEAD INDUCTANCE) CGS OUT GND LTC1693 1693 F01 N1 V + VCC Figure 1. Capacitance Seen by OUT During Switching The LTC1693’s output peak currents are 1.4A (P1) and 1.7A (N1) respectively. The N-channel MOSFET (N1) has higher current drive capability so it can discharge the power MOSFET’s gate capacitance during high-to-low signal transitions. When the power MOSFET’s gate is pulled low by the LTC1693, its drain voltage is pulled high by its load (e.g., a resistor or inductor). The slew rate of the drain voltage causes current to flow back to the MOSFETs gate through its gate-to-drain capacitance. If the MOSFET driver does not have sufficient sink current capability (low output impedance), the current through the power MOSFET’s Miller capacitance (CGD) can momentarily pull the gate high, turning the MOSFET back on. Rise/Fall Time Since the power MOSFET generally accounts for the ma- jority of power lost in a converter, it’s important to quickly turn it either fully “on” or “off” thereby minimizing the tran- sition time in its linear region. The LTC1693 has rise and fall times on the order of 16ns, delivering about 1.4A to 1.7A of peak current to a 1nF load with a VCC of only 12V. The LTC1693’s rise and fall times are determined by the peak current capabilities of P1 and N1. The predriver, shown in Figure 1 driving P1 and N1, uses an adaptive method to minimize cross-conduction currents. This is done with a 6ns nonoverlapping transition time. N1 is fully turned off before P1 is turned-on and vice-versa using this 6ns buffer time. This minimizes any cross-conduction currents while N1 and P1 are switching on and off yet is short enough to not prolong their rise and fall times. |
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Similar Description - LTC1693-3CMS8 |
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