Electronic Components Datasheet Search |
|
SMH4812G Datasheet(PDF) 9 Page - Summit Microelectronics, Inc. |
|
SMH4812G Datasheet(HTML) 9 Page - Summit Microelectronics, Inc. |
9 / 19 page 9 2055 4.0 12/22/00 SMH4812 SUMMIT MICROELECTRONICS, Inc. Preliminary Under-Voltage Hysteresis The Under-Voltage comparator input may be configured with a programmable level of hysteresis. The compare level may be set in steps (up to 15) of 62.5mV below 2.5V. The default under-voltage hysteresis level is set to 62.5mV. Soft Start Slew Rate Control Once all of the preconditions for powering up the DC/DC controllers have been met, the SMH4812 provides a means to soft start the external power FET. 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 (normally required at the input of the DC/DC controllers) too quickly may generate very high current. The VGATE output of the SMH4812 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. A complete design example is given in the Applications Section. Load Control — Sequencing the Secondary Sup- plies Once power has been ramped to the DC/DC controllers, two conditions must be met before the PG# output can be enabled: the Drain Sense voltage must be below 2.5V, and the VGATE voltage must be greater than V DD – VGT. The Drain Sense input helps ensure that the power MOS- FET is not absorbing too much steady state power from operating at a high V DS. This sensor remains active at all times (except during the current regulation period). The VGATE sensor makes sure that the power MOSFET is operating well into its saturation region before allowing the loads to be switched on. Once VGATE reaches V DD – VGT this sensor is latched. When the external MOSFET is properly switched on the PG# output may be enabled (if ENPG is high). Output PG# is activated after a t PGD delay. The delay time is program- mable from 50µs to 160ms. The PG# output has a 12V withstand capability, so high voltages must not be connected to this pin. A bipolar transistor or an opto-isolator can be used to boost the withstand voltage to that of the host supply. See Figure 9 for connections. Forced Shutdown — Secondary Feedback The Forced Shutdown signal (FS#) is an active low input that provides a method of receiving feedback from the secondary side of the DC/DC controllers. A built-in holdoff timer allows the SMH4812 to ignore the state of the FS# input until the timer period expires. The FS# input must be driven high by the end of this timer period. A low level on this input will cause a Fault condition, driving FAULT# low and shutting off the VGATE and PG# outputs. The purpose of the holdoff timer is to allow enough time for devices on the secondary side of the DC/DC controllers to power up and stabilize. This unique feature of the SMH4812 allows supervisory circuits such as an SMS44 to control the shutdown of the primary side soft start circuit, even though the secondary side initially has no power. The FS# input can be programmed to act as a second ENPG input controlling the PG# output. Circuit Breaker Operation The SMH4812 provides a number of circuit breaker func- tions to protect against over current conditions. A sus- tained over-current event could damage the host supply and/or the load circuitry. The board’s load current passes through a series resistor (R S) connected between the MOSFET source (which is tied to CBSENSE) and V SS. The breaker trips whenever the voltage drop across R S is greater than 50mV for more than t CBD (a programmable filter delay ranging from 10µs to 500µs). Quick-TripTM Circuit Breaker Additionally, the SMH4812 provides a Quick-Trip feature that will cause the circuit breaker to trip immediately if the voltage drop across R S exceeds VQCB. The Quick-Trip level may be set to 60mV, 100mV (default), 200mV, or the feature may be disabled. 2055 Fig04 CBSENSE VGATE <TCBD TFSTSHTDN 50mV VQCB Figure 4. Circuit Breaker Quick Trip Response |
Similar Part No. - SMH4812G |
|
Similar Description - SMH4812G |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.COM |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Datasheet Upload | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |