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MAX767 Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX767 Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 20 page Gate-Driver Boost Supply Gate-drive voltage for the high-side N-channel switch is generated with the flying-capacitor boost circuit shown in Figure 4. The capacitor (C3) is alternately charged from the 5V input via the diode (D1) and placed in par- allel with the high-side MOSFET’s gate-source termi- nals. On start-up, the synchronous rectifier (low-side) MOSFET (N2) forces LX to 0V and charges the BST capacitor to 5V. On the second half-cycle, the PWM turns on the high-side MOSFET (N1); it does this by closing an internal switch between BST and DH, which connects the capacitor to the MOSFET gate. This pro- vides the necessary enhancement voltage to turn on the high-side switch, an action that “boosts” the 5V gate-drive signal above the input voltage. Ringing seen at the high-side MOSFET gates (DH) in discontinuous-conduction mode (light loads) is a natur- al operating condition. It is caused by the residual energy in the tank circuit, formed by the inductor and stray capacitance at the LX node. The gate-driver neg- ative rail is referred to LX, so any ringing there is direct- ly coupled to the gate-drive supply. Modes of Operation PWM Mode Under heavy loads—over approximately 25% of full load—the supply operates as a continuous-current PWM supply (see Typical Operating Characteristics). The duty cycle, %ON, is approximately: VOUT %ON = ________ VIN Current flows continuously in the inductor: first, it ramps up when the power MOSFET conducts; second, it ramps down during the flyback portion of each cycle as energy is put into the inductor and then discharged into the load. Note that the current flowing into the inductor when it is being charged is also flowing into the load, so the load is continuously receiving current from the inductor. This minimizes output ripple and maximizes inductor use, allowing very small physical and electrical sizes. Output ripple is primarily a function of the filter capacitor’s effective series resistance (ESR), and is typically under 50mV (see Design Procedure section). Idle-Mode Under light loads (<25% of full load), the MAX767 enhances efficiency by turning the drive voltage on and off for only a single clock period, skipping most of the clock pulses entirely. Asynchronous switching, seen as “ghosting” on an oscilloscope, is thus a normal operat- ing condition whenever the load current is less than approximately 25% of full load. At certain input voltage and load conditions, a transition region exists where the controller can pass back and forth from idle-mode to PWM mode. In this situation, short pulse bursts occur, which make the current wave- form look erratic but do not materially affect the output ripple. Efficiency remains high. Current Limiting The voltage between CS and FB is continuously moni- tored. An external, low-value shunt resistor is connect- ed between these pins, in series with the inductor, allowing the inductor current to be continuously mea- sured throughout the switching cycle. Whenever this voltage exceeds 100mV, the drive voltage to the exter- nal high-side MOSFET is cut off. This protects the MOS- FET, the load, and the input supply in case of short cir- cuits or temporary load surges. The current-limiting resistance is typically 20m Ω for 3A. 5V-to-3.3V, Synchronous, Step-Down Power-Supply Controller 10 ______________________________________________________________________________________ LEVEL TRANSLATOR PWM C1 VIN BST DH LX DL N1 D1 C3 N2 L1 VCC VCC MAX767 Figure 4. Boost Supply for High-Side Gate Driver |
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