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SC2440EVB Datasheet(PDF) 8 Page - Semtech Corporation |
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SC2440EVB Datasheet(HTML) 8 Page - Semtech Corporation |
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8 / 26 page  8 © 2005 Semtech Corp. www.semtech.com SC2440 POWER MANAGEMENT Operation The SC2440 is a 2-channel constant-frequency peak current-mode step-down switching regulator with integrated 2A power transistors. Both regulators of the SC2440 operate from a common input power supply and share the same voltage reference, the master oscillator and the synchronizing circuit. Turn-on of the power transistors are phase-shifted by 180 °. The two regulators are otherwise completely identical, independent and are capable of producing two separate outputs from the same input. The master oscillator of the SC2440 runs at twice the channel frequency. The free-running frequency of the master oscillator can be programmed with an external resistor from the ROSC pin to ground. Frequency adjustability makes switching regulator design flexible. Peak current-mode control is utilized for the SC2440. The double reactive poles of the output LC filter are reduced to a single real pole by the inner current loop, easing loop compensation. Fast transient response can be achieved with a simple Type-2 compensation network. Switch collector current is sensed with an integrated 7.7mΩ sense resistor. The sensed current is summed with slope- compensating ramp before it is compared with the transconductance error amplifier output. The PWM comparator tripping instant determines the switch turn- on pulse width (Figure 2). The current-limit comparator ILIM turns off the power switch when the sensed-signal exceeds the 20mV current-limit threshold. ILIM therefore provides cycle-by-cycle limit. Current-limit does not vary with duty-cycle. Driving the base of the power transistor above the input power supply rail minimizes the power transistor turn-on voltage and maximizes efficiency. An external charge pump (formed by the capacitor C 2 and the diode D3 in Figure 1) generates a voltage higher than the input rail at the BOOST pin. The bootstrapped voltage generated becomes the supply voltage for the power transistor driver. The SS pin is a multiple-function pin. An external capacitor connected from the SS pin to the ground together with the internal 1.8µA and 2.6µA current sources set the soft-start and overload shutoff times of the regulator (Figure 3). The SS pin can also be used to shut off the corresponding regulator. When either SS pin is pulled below 0.8V, that regulator is turned off. If both SS pins are pulled below 0.2V, then the SC2440 undergoes overall shutdown. The current draw from the input power supply reduces to 38µA. When either SS pin is released, the corresponding soft- start capacitor is charged with a 2µA current source (not shown in Figure 3). As either SS voltage exceeds 0.3V, the internal bias circuit of the SC2440 is enabled. The SC2440 draws 3.3mA from V IN. An internal fast charge circuit quickly charges the soft-start capacitor to 1V. At this juncture, the fast charge circuit turns off and the 1.8µA current source slowly charges the soft-start capacitor. The output of the error amplifier is forced to track the slow soft-start ramp at the SS pin. When the COMP voltage exceeds 1.1V, the switching regulator starts to switch. During soft-start, the current limit of the converter is gradually increased until the converter output comes into regulation. Hiccup overload protection is utilized in the SC2440. Overload shutdown is disabled during soft-start (V SS < 2V). In Figure 3 the reset input of the overload latch will remain high if the SS voltage is below 2V. Once the soft- start capacitor is charged above 2V, the overload shutdown latch is enabled. As the load draws more current from the regulator, the current-limit comparator will limit the peak inductor current. This is cycle-by-cycle current limiting. Further increase in load current will cause the output voltage to decrease. If the output voltage falls below 74% of its set point, then the overload latch will be set and the soft-start capacitor will be discharged with a net current of 0.8µA. The switching regulator is shut off until the soft-start capacitor is discharged below 1V. At this moment, the overload latch is reset. The soft-start capacitor is recharged and the converter again undergoes soft-start. The regulator will go through soft-start, overload shutdown and restart until it is no longer overloaded. Each regulator of the SC2440 has its own power good comparator. The open collector output of the power good comparator will be actively pulled low if the corresponding feedback voltage is below 0.9V. |
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Similar Description - SC2440EVB |
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