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MAX746EPE Datasheet(PDF) 7 Page - Maxim Integrated Products |
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MAX746EPE Datasheet(HTML) 7 Page - Maxim Integrated Products |
7 / 16 page High-Efficiency, PWM, Step-Down, N-Channel DC-DC Controller _______________________________________________________________________________________ 7 Discontinuous-/Continuous- Conduction Modes The MAX746 is designed to operate in continuous-con- duction mode (CCM) but can also operate in discontinu- ous-conduction mode (DCM), making it ideal for variable- load applications. In DCM, the current starts at zero and returns to zero on each cycle. In CCM, the inductor current never returns to zero; it consists of a small AC component superimposed on a DC offset. This results in higher current capability because the AC component in the inductor cur- rent waveform is small. It also results in lower output noise, since the inductor does not exhibit the ringing that would occur if the current reached zero (see inductor waveforms in the Typical Operating Characteristics). To transfer equal amounts of energy to the load in one cycle, the peak cur- rent level for the discontinuous waveform must be much larger than the peak current for the continuous waveform. Slope Compensation Slope compensation stabilizes the inner current-feedback loop by adding a ramp signal to the current-sense amplifier output. Ideal slope compensation can be achieved by adding a linear ramp, with the same slope as the declining inductor current, to the rising inductor current-sense voltage. Under these conditions, the inductor must be scaled to the current-sense resistor value. Overcompensation adds a pole to the outer voltage feed- back-loop response, degrading loop stability. This may cause voltage-mode pulse-frequency-modulation instead of PWM operation. Undercompensation results in inner current feed- back-loop instability, and may cause the inductor current to staircase. Ideal matching between the sense resistor and inductor is not required; it can differ by ±30% or more. Oscillator and EXT Control The oscillator frequency is nominally 100kHz, and the duty cycle varies from 5% to 96%, depending on the input/out- put voltage ratio. EXT, which provides the gate drive for the external logic-level N-FET, is switched between HIGH and GND at the switching frequency. EXT is controlled by a unique two-comparator control scheme consisting of a PWM comparator and an idle-mode comparator (Figure 2). The PWM comparator determines the cycle-by-cycle peak cur- rent with heavy loads, and the idle-mode comparator sets the light-load peak current. As VOUT begins to drop, EXT goes high and remains high until both comparators trip. With heavy loads, the idle-mode comparator trips first and the PWM control comparator determines the EXT on-time; 2 3 4 6 5 11 LBI SS REF FB SHDN AGND V+ CP HIGH AV+ CS EXT CC OUT LB0 VIN 6V TO 15V GND C6 1.0 µF D2 1N914 * C8 0.1 µF D4 1N5817 D3 1N914 * D1 NSQ03A03 5V AT 3A L1 39 µH C1 430 µF N C9 4.7 µF 14 13 8 10 12 7 9 1 R2 R1 RSENSE 40m Ω R3 100k C7 2.7nF C4 0.1 µF C2 100 µF C3 0.1 µF C5 0.1 µF Q1 Si9410DY 15 16 * SEE TABLE 2 FOR DIODE SELECTION. MAX746 Figure 1a. 5V Standard Application Circuit (15W) |
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