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SC415EVB Datasheet(PDF) 11 Page - Semtech Corporation |
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SC415EVB Datasheet(HTML) 11 Page - Semtech Corporation |
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11 / 30 page  SC415 11 Applications Information (continued) The voltages available are shown: FB Internal Voltage Selection FB = RTN FB = +5V Side1 1.5V 1.8V Side2 1.05V 1.25V Enable/Psave Inputs Each converter has a separate Enable pin. Each EN input operates as follows: EN = GND. This turns the converter off. EN = open (float). This turns the converter on with psave mode disabled (continuous conduction mode). In this case, the EN pin will float to approximately 2V due to an internal 1.5Meg/1Meg resistor divider from the +5V supply at VDDx to ground. EN = high (3.1V min). This turns the converter on with psave mode enabled. At light loads, the converter will operate in psave mode. Note that the two EN pins are separate, so each output can be disabled or operated with or without psave independently. If both EN1 and EN2 are grounded, the device is placed into the lowest-power state, drawing typically 10μA from the +5V supply. PSAVE Operation Each output provides automatic PSAVE operation at light loads if the ENx pin is set high. The internal Zero- Cross comparator looks for inductor current (via the voltage across the lower MOSFET) to fall to zero on eight consecutive switching cycles. Once observed, the controller enters PSAVE mode and turns off the low-side MOSFET on each subsequent cycle when the current crosses zero. To add hysteresis, the on-time is also increased by 25% when PSAVE mode is active, for that output only; it does not affect the other converter. The efficiency improvement at light loads more than offsets the disadvantage of slightly higher output ripple. If the inductor current does not cross zero on any switching cycle, the controller immediately exits PSAVE. Once PSAVE is exited, it requires 8 switching cycles at light load to re- enter PSAVE. Since the controller counts zero crossings, the converter can sink current as long as the current does not cross zero on eight consecutive cycles. This allows the output voltage to recover quickly in response to negative load steps. When operating in PSAVE mode at light loads, the LX waveform will not have the typical square wave shape seen when operating in continuous conduction mode. Shortly after DL drives low, and both MOSFETs are off, the LX voltage will show ringing. This ringing is caused by the LC circuit formed by the inductor and device capacitance of the MOSFETs and low-side diode. When the low- side MOSFET turns off the inductor current falls toward zero. When it reaches zero, the inductance and MOSFET capacitances will tend to ring freely. This is normal PSAVE operation as shown in Figure 3: VOUT VIN COUT L CQ1 CQ2 Q1 Q2 DH DL DL LX VOUT VIN Typical ringing at LX Figure 3 |
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