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MAX786SCAI Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX786SCAI Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 20 page Dual-Output Power-Supply Controller for Notebook Computers 10 ______________________________________________________________________________________ +5V Switch-Mode Supply The +5V output is produced by a current-mode, PWM step-down regulator, which is nearly identical to the +3.3V supply. The +5V supply’s dropout voltage, as configured in Figure 1, is typically 400mV at 2A. As V+ approaches 5V, the +5V output gracefully falls with V+ until the VL regulator output hits its undervoltage- lockout threshold at 4V. At this point, the +5V supply turns off. The default frequency for both PWM controllers is 300kHz (with SYNC connected to REF), but 200kHz may be used by connecting SYNC to GND or VL. +3.3V and +5V PWM Buck Controllers The two current-mode PWM controllers are identical except for different preset output voltages (Figure 3). Each PWM is independent except for being synchro- nized to a master oscillator and sharing a common ref- erence (REF) and logic supply (VL). Each PWM can be turned on and off separately via ON3 and ON5. The PWMs are a direct-summing type, lacking a tradi- tional integrator error amplifier and the phase shift associated with it. They therefore do not require any external feedback compensation components if the fil- ter capacitor ESR guidelines given in the Design Procedure are followed. The main gain block is an open-loop comparator that sums four input signals: an output voltage error signal, current-sense signal, slope-compensation ramp, and precision voltage reference. This direct-summing method approaches the ideal of cycle-by-cycle control of the output voltage. Under heavy loads, the controller operates in full PWM mode. Every pulse from the oscil- lator sets the output latch and turns on the high-side switch for a period determined by the duty cycle (approximately VOUT/VIN). As the high-side switch turns off, the synchronous rectifier latch is set and, 60ns later, the low-side switch turns on (and stays on until the beginning of the next clock cycle, in continuous mode, or until the inductor current crosses through zero, in discontinuous mode). Under fault conditions where the inductor current exceeds the 100mV current-limit threshold, the high-side latch is reset and the high-side switch is turned off. At light loads, the inductor current fails to exceed the 25mV threshold set by the minimum current comparator. When this occurs, the PWM goes into idle mode, skip- ping most of the oscillator pulses in order to reduce the switching frequency and cut back switching losses. The oscillator is effectively gated off at light loads because the minimum current comparator immediately resets the high-side latch at the beginning of each cycle, unless the FB_ signal falls below the reference voltage level. Soft-Start/SS_ Inputs Connecting capacitors to SS3 and SS5 allows gradual build-up of the +3.3V and +5V supplies after ON3 and ON5 are driven high. When ON3 or ON5 is low, the appropriate SS capacitors are discharged to GND. When ON3 or ON5 is driven high, a 4µA constant cur- rent source charges these capacitors up to 4V. The resulting ramp voltage on the SS_ pins linearly increas- es the current-limit comparator setpoint so as to increase the duty cycle to the external power MOSFETs up to the maximum output. With no SS capacitors, the circuit will reach maximum current limit within 10µs. Soft-start greatly reduces initial in-rush current peaks and allows start-up time to be programmed externally. Synchronous Rectifiers Synchronous rectification allows for high efficiency by reducing the losses associated with the Schottky rectifiers. When the external power MOSFET N1 (or N2) turns off, energy stored in the inductor causes its terminal volt- age to reverse instantly. Current flows in the loop formed by the inductor, Schottky diode, and load — an action that charges up the filter capacitor. The Schottky diode has a forward voltage of about 0.5V which, although small, represents a significant power loss, degrading efficiency. A synchronous rectifier, N3 (or N4), parallels the diode and is turned on by DL3 (or DL5) shortly after the diode conducts. Since the on resistance (rDS(ON)) of the synchronous rectifier is very low, the losses are reduced. The synchronous rectifier MOSFET is turned off when the inductor current falls to zero. Cross conduction (or “shoot-through”) occurs if the high-side switch turns on at the same time as the syn- chronous rectifier. The MAX786’s internal break-before- make timing ensures that shoot-through does not occur. The Schottky rectifier conducts during the time that nei- ther MOSFET is on, which improves efficiency by pre- venting the synchronous-rectifier MOSFET’s lossy body diode from conducting. The synchronous rectifier works under all operating condi- tions, including discontinuous-conduction and idle mode. Boost Gate-Driver Supply Gate-drive voltage for the high-side N-channel switch is generated with a flying-capacitor boost circuit as shown in Figure 4. The capacitor is alternately charged from the VL supply via the diode and placed in parallel with the high-side MOSFET’s gate-source terminals. On start- up, the synchronous rectifier (low-side) MOSFET forces LX_ to 0V and charges the BST_ capacitor to 5V. On the |
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