Datasheet LTC1702A (Analog Devices) - 10

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