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SMH4803DJMP Datasheet(PDF) 11 Page - Summit Microelectronics, Inc. |
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SMH4803DJMP Datasheet(HTML) 11 Page - Summit Microelectronics, Inc. |
11 / 22 page 11 2041 8.4 6/15/00 SMH4803 SUMMIT MICROELECTRONICS after another delay PGD. The delays built into the SMH4803 allow correct sequencing of power to the loads, e.g. +3V supply must come up before +5V supply. The delay times are factory programmed. PG2# and PG3# can be disabled using the ENPGA and ENPGB inputs. When these inputs are low they override the enable function produced when the SMH4803 sees a power good condition. The PG1#, PG2#, and PG3# outputs have a 12V with- stand capability so high voltages must not be connected to these pins. Inexpensive bipolar transistors will boost the withstand voltage to that of the host supply, see figure 5 for connections. Output Slew-Rate Control The SMH4803 provides a current limited Vgate turn-on. A fast turn-off is performed by internally shorting Vgate to Vss. Changing the passive components around the power MOSFET switch will modify the turn-on slew-rate. Operating at High Voltages The breakdown voltage of the external active and passive components limits the maximum operating voltage of the SMH4803 hot-swap controller. Components that must be able to withstand the full supply voltage are: the input and output decoupling capacitors, the protection diode in series with DrainSense pin, the power MOSFET switch and capacitor connected between its drain and gate, the high-voltage transistors connected to the power good outputs, and the dropper resistor connected to the controller’s Vdd pin. Over-Voltage and Under-Voltage Resistors In the following examples, the three resistors, R1, R2, and R3, connected to the OV and UV inputs must be capable of withstanding the maximum supply voltage which can be several hundred volts. The trip voltage of the UV and OV inputs is +2.5V relative to Vss. As the input resis- tances of UV and OV are very high, high value resistors can be used in the resistive divider. The divider resistors should be high stability, 1% metal-film resistors to keep the under-voltage and over-voltage trip points accurate. Telecom Design Example A hot-swap telecom application uses a 48V power supply with a –25% to +50% tolerance, i.e. the 48V supply can vary from 36V to 72V. The formulae for calculating R1, R2, and R3 are shown below. 1) First select the peak current, IDmax, allowed through the resistive divider, say 250µA. The value of current is arbitrary; however, if the current is too high, self- heating in R3 may become a problem (especially in high voltage systems), and if the current is too low the value of R3 becomes very large and may be expensive at 1% tolerance. R1 is calculated from: R1 = VOV is the over-voltage trip point, i.e. 2.5V, therefore: R1 = =10k Ω 2) The minimum current that flows through the resistive divider, IDmin, is easily calculated from the ratio of maximum and minimum supply voltages: ID min = Therefore: ID min = = 125 µA 3) The value of R3 is now calculated using IDmin. R3 = Where Vuv is the under-voltage trip point, also 2.5V, therefore: R3 = = 268k Ω The closest standard 1% resistor value is 267k Ω 4) R2 may be calculated using: (R1 + R2) = R2 = – R1 Or R2 = –10k Ω = (20kΩ – 10kΩ) = 10kΩ Vuv ID min Vuv ID min 2.5 V 125 µA ID max x VS min VS max 250 µA x 36V 72 V ( VS min – Vuv) ID min (36 V – 2.5V) 125 µA Vov ID max 2.5 V 250 µA |
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