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LTC1702AIGN Datasheet(PDF) 10 Page - Linear Technology |
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LTC1702AIGN Datasheet(HTML) 10 Page - Linear Technology |
10 / 36 page 10 LTC1702A 1702afa Figure 2. Floating TG Driver Supply amp of continuous current with peak currents up to 5A to slew large MOSFET gates quickly. The external MOSFETs are connected with the drain of QT attached to the input supply and the source of QT at the switching node SW. QB is the synchronous rectifier with its drain at SW and its source at PGND. SW is connected to one end of the inductor, with the other end connected to VOUT. The output capacitor is connected from VOUT to PGND. When a switching cycle begins, QB is turned off and QT is turned on. SW rises almost immediately to VIN and the inductor current begins to increase. When the PWM pulse finishes, QT turns off and one nonoverlap interval later, QB turns on. Now SW drops to PGND and the inductor current decreases. The cycle repeats with the next tick of the master clock. The percentage of time spent in each mode is controlled by the duty cycle of the PWM signal, which in turn is controlled by the feedback amplifier. The master clock generates a 1VP-P, 550kHz sawtooth waveform and turns QT once every 1.8 µs. In a typical application with a 5V input and a 1.6V output, the duty cycle will be set at 1.6/ 5 × 100% or 32% by the feedback loop. This will give roughly a 575ns on-time for QT and a 1.22 µs on-time for QB. This constant frequency operation brings with it a couple of benefits. Inductor and capacitor values can be chosen with a precise operating frequency in mind and the feed- APPLICATIONS INFORMATION back loop components can be similarly tightly specified. Noise generated by the circuit will always be in a known frequency band with the 550kHz frequency designed to leave the 455kHz IF band free of interference. Subharmonic oscillation and slope compensation, common headaches with constant frequency current mode switchers, are absent in voltage mode designs like the LTC1702A. During the time that QT is on, its source (the SW pin) is at VIN. VIN is also the power supply for the LTC1702A. How- ever, QT requires VIN + VGS(ON) at its gate to achieve minimum RON. This presents a problem for the LTC1702A— it needs to generate a gate drive signal at TG higher than its highest supply voltage. To get around this, the TG driver runs from floating supplies, with its negative supply attached to SW and its power supply at BOOST. This allows it to slew up and down with the source of QT. In combination with a simple external charge pump (Fig- ure 2), this allows the LTC1702A to completely enhance the gate of QT without requiring an additional, higher supply voltage. The two channels of the LTC1702A run from a common clock, with the phasing chosen to be 180 ° from side 1 to side 2. This has the effect of doubling the frequency of the switching pulses seen by the input bypass capacitor, sig- nificantly lowering the RMS current seen by the capacitor and reducing the value required (see the 2-Phase section). + TG BOOST SW BG PGND PVCC DCP CIN + COUT 1702A F02 VOUT LEXT VIN QT QB CCP 1 µF LTC1702A |
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