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MAX782C/D Datasheet(PDF) 22 Page - Maxim Integrated Products |
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MAX782C/D Datasheet(HTML) 22 Page - Maxim Integrated Products |
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22 / 32 page  Triple-Output Power-Supply Controller for Notebook Computers 22 ______________________________________________________________________________________ Figure 8 shows a more complex example of power sequencing. On power-up, the Intel 486SL computer requires the +5V supply to come up before the +3.3V supply. A power-ready signal is required ≥50ms later. This circuit’s ON3 output connects to the MAX782’s ON3 pin, and can be wire-OR connected with an open- drain output to enable another circuit to turn the +3.3V supply off. PCMCIA Slot +3.3V/+5V VCC Switching The MAX782 contains level shifters that simplify driving external power MOSFETs to switch PCMCIA card VCC to 3.3V and 5V. While a PCMCIA card is being inserted into the socket, the VCC pins on the card edge should be powered down to 0V so “hot insertion” does not damage the PCMCIA card. The simplest way to do this is to use a mechanical switch that has to be physically opened before the PCMCIA card can be inserted. The switch, which disconnects VCC, can be closed only when the card has been fitted snugly into its socket. Figure 9’s circuit illustrates this approach and correctly shows the connections to both MOSFETs: N2 appears to be inserted with drain and source the wrong way around, but this is necessary to prevent its body diode from pulling the +3.3V supply up to 5V when VCC is connected to the +5V supply. Figure 10’s circuit provides an alternative method of connecting the VCC supply to the PCMCIA slot. While it avoids using a mechanical switch, it does not provide the security of a physical interlock. Placing the two MOSFETs N1 and N2 with their body diodes facing in opposite directions allows VCC to be shut down to 0V without using a mechanical switch, and allows VCC to be driven to 5V without the +3.3V supply being pulled up to 5V. P N HYST1 C4 100nF VCC R3 1M R6 1M C3 100nF POWER READY ON3 C1 22nF OUT2 R2 390k R1 1M R4 1M R5 820k C2 100 nF +5V SUPPLY +3.3V SUPPLY SET1 SET2 GND 1.3V ICL7665 Figure 8. Power-Up Sequencing for the Intel 486SL PCMCIA 2.0 DIGITAL CONTROLLER VCC_EN0 VCC_EN1 FB5 FB3 Q2 D1 D2 VDD VH +5V SUPPLY MECHANICAL SWITCH SLOT VCC +3.3V SUPPLY MAX782 d s g N2 Q1 d s g N1 NOTE: MOSFET BODY DIODES SHOWN FOR CLARITY. Figure 9. Simple Switching for PCMCIA Slot VCC PCMCIA 2.0 DIGITAL CONTROLLER VCC_EN0 VCC_EN1 FB5 FB3 Q2 D1 D2 +5V +3.3V MAX782 N2 Q1 N3 N1 VDD VH 100 µF SLOT VCC NOTE: MOSFET BODY DIODES SHOWN FOR CLARITY. Figure 10. Using the Level Shifters to Switch PCMCIA Slot VCC |
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