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SMH4804 Datasheet(PDF) 4 Page - Summit Microelectronics, Inc. |
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SMH4804 Datasheet(HTML) 4 Page - Summit Microelectronics, Inc. |
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4 / 41 page  Functional Description SMH4804 4 2050 3.7 10/30/02 Summit Microelectronics 2.5V for a period of at least tPDD (Pin Detect Delay time). The tPDD filter helps prevent spurious start-up sequences while the card is being inserted. If UV falls below 2.5V or OV rises above 2.5V, the PG[4:1]# and VGATE outputs are disabled immediately. Under-/Over-Voltage Filtering The SMH4804 can be configured so that an out-of- tolerance condition on UV/OV does not shut off the output immediately. Instead, a filter delay may be inserted so that only sustained under-voltage or over-voltage conditions can shut off the output. An out-of-tolerance condition on UV/OV for longer than the filter delay time (tUOFLTR in Figure 3) causes the VGATE and PG[4:1]# outputs to shut off when the UV/OV filter option is enabled using bits 2:1 of Register 4. The under-/over-voltage filtering feature is disabled (bits 2:1 = 00) in the default configuration. Refer to Register 4 - Address 0100 on page 33 for more information on the filter delay options. The UV and OV filters are enabled and disabled by programming bits 3 and 2 of Register 6 respectively. Refer to Register 6 - Address 0110 on page 36 for more information. Note that the delay values in Register 4 are only valid if the corresponding over or under voltage filtering is enabled using bits 3:2 of Register 6. Figure 3 shows the timing for the under-/over-voltage filter. Figure 3. Under-/Over-Voltage Filter Timing Under-/Over-Voltage Latching An additional option for an out-of-tolerance condition on UV/ OV is to latch the VGATE and PG[4:1]# outputs off such that a return to normal UV/OV operation does not turn them back on. In this case the FAULT# output is asserted. Under-Voltage Hysteresis The Under-Voltage comparator input may be configured with a programmable level of hysteresis using Register 7. The falling voltage compare level may be set in steps of 62.5mV below 2.5V. The rising voltage compare level is fixed at 2.5V. The default under-voltage hysteresis level is set to 62.5mV. In default conditions the SMH4804 is not in an under-voltage state once the UV voltage rises above 2.5V; and after that an under-voltage occurrence is not recognized until the UV voltage falls below 2.4375V (2.5V – 62.5mV). Soft Start Slew Rate Control Once all of the preconditions for powering up the DC/DC controllers have been met as explained in the previous sections, the SMH4804 provides a means to soft start the external power MOSFET. It is important to limit in-rush current to prevent damage to the add-in card or disruptions to the host power supply. For example, charging the filter capacitance too quickly (normally required at the input of the DC/DC controllers) may generate very high current. The VGATE output of the SMH4804 is current limited to IVGATE, allowing the slew rate to be easily modified using external passive components. The slew rate may be found by dividing IVGATE by the gate-to-drain capacitance placed on the external FET. Load Control — Sequencing the Secondary Supplies The PG1# through PG4# output pins are used to enable the external DC/DC controllers. Once the card is inserted, the SMH4804 samples the PD1# and PD2# pin detect input pins to determine if the card as been inserted properly. It then monitors the state of the UV and OV input pins to assure there is no under-voltage or over-voltage condition present. Once these conditions are met, and the EN/TS pin is greater than 2.5V, the SMH4804 asserts the VGATE output to turn on the external MOSFET. During the time it takes to turn the MOSFET on, the SMH4804 monitors the system for an over-current condition via the CBSENSE input pin. In addition, the device internally monitors the voltage level on the VGATE output pin. This is shown by the ‘VGATE Sense’ block in Figure 2. Once power has been ramped to the DC/DC controllers, two conditions must be met before the PG[4:1]# outputs can be enabled: • the DRAIN SENSE input voltage must be below 2.5V. • the VGATE voltage must be greater than VDD – VGT, where VGT is the gate threshold. The DRAIN SENSE input helps ensure that the power MOSFET is not absorbing too much steady state power from operating at a high VDS. This sensor remains active at all times (except when current regulation is enabled). The 2050 Fig07 OV / UV FAULT# tUOFLTR 2.5V |
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