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SC2443 Datasheet(PDF) 11 Page - Semtech Corporation |
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SC2443 Datasheet(HTML) 11 Page - Semtech Corporation |
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11 / 22 page  SC2443 Applications Information Description The SC2443 is a constant frequency 2-phase current-mode step-downPWMswitchingcontrollerdrivingallN-channel MOSFET. The two channels of the controller operate at 80 degrees out-of-phase from each other. Since input currents are interleaved in a two-phase converter, input ripple current is lower and smaller input capacitor can be used for filtering. Also, with lower inductor current and smaller inductor ripple current per phase, overall I2R losses are reduced. The SC2443 operates in synchronous continuous- conduction mode. It can be configured either as two independent step-down controllers producing two separate outputs or as a dual-phase single-output controller by tying the IN2- pin to VCC. In single output operation, the channel one error amplifier controls both channels and the channel two error amplifier is disabled. Soft-start and overload hiccup of both channels is synchronized to channel one. Frequency Setting and Synchronization The internal oscillator of the SC2443 runs at twice the phase frequency. The free-running frequency of the oscillator can be programmed with an external resistor from the ROSC pin to ground. The step-down controllers are capable of operating up to MHz. It is necessary to consider the operating duty-ratio before deciding the switching frequency. See Applications Information section for more details. Whensynchronizedexternally,theappliedclockfrequency should be twice the desired phase frequency. The synchronizing clock frequency should also be between 2 - 2.6 times the set free-running channel frequency. Control Loop The SC2443 uses peak current-mode control for fast transient response, ease of compensation and current sharing in single output operation. The low-side MOSFET of each channel is turned off at the falling-edge of the phase timing clock. After a brief non-overlapping time interval of 90ns, the high-side MOSFET is turned on. The phase inductor current ramps up. When the sensed inductor current reaches the threshold determined by the error amplifier output and compensation ramp, the high-side MOSFET is turned off. After a non-overlapping conduction time of 90ns, the low-side MOSFET is turned on. The supply voltages for the high-side gate drivers are obtained from two diode-capacitor bootstrap circuits. If the bootstrap capacitor is charged from VCC, the high- side gate drive voltage swing will be from approximately 2VCC to the ground. The power dissipated in the high- side gate driver is not higher with higher voltage swing because the gate-source voltage of the high-side MOSFET still swing from zero to VCC. The outputs of the low-side gate drivers swing from VCC to ground. The SC2443 has internal ramp-compensation to prevent sub-harmonic oscillation when operating above 50% duty cycle. There is enough ramp internally for a sensed voltage ripple between /4 to /3 of the full-scale sensed voltage limit of 75mV. The maximum sensed voltage limit is unaffected by the compensating ramp. Current-Sensing There are two ways to sense the inductor current for current-mode control with the SC2443. Since the peak inductor current corresponds to 75mV of sensed voltage (CS+ - CS-), resistor current sensing can be used at the output without resulting in excessive power dissipation. Although accurate and far easier to lay out than high-side resistor sensing, a pair of precision sense resistors adds cost to the converter. With proper RC filter, Inductor DCR sensing can also be used for SC2443 resulting in low cost and without extra power dissipation. Error Amplifiers In closed loop operation, the error amplifier output ranges from .V to 3.5V. The upper output operating range of either error amplifier is reserved for positive current- sense voltage (CS+ - CS-) and corresponds to positive (sourcing) output current. If the amplifier swings to its lower operating range, the amplifier will still modulate the high-side gate drive duty-ratio. However the peak current- sense voltage (hence the peak inductor current) will be limited to a negative value. The error amplifier output is about 2.2V when the peak sense-voltage is zero. The built-in offset in the current sense amplifier together with synchronous continuous-conduction mode of operation allows the SC2443 to regulate the output irrespective of the direction of the load current. |
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