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SMH4811A Datasheet(PDF) 11 Page - Summit Microelectronics, Inc. |
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SMH4811A Datasheet(HTML) 11 Page - Summit Microelectronics, Inc. |
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11 / 15 page  11 2044 6. 6 03/27/09 SMH4811A SUMMIT MICROELECTRONICS, Inc. Preliminary where VSMIN is the lowest operating supply voltage, VDDMAX is the upper limit of the SMH4811A supply voltage, IDD is minimum current required for the SMH4811A to operate, and ILOAD is any additional load current from the 2.5V and 5V outputs and between VDD and VSS. The min/max current limits are easily met using the drop- per resistor, except in circumstances where the input voltage may swing over a very wide range (e.g., input varies from 20V to 100V). In these circumstances it may be necessary to add an 11V zener diode between VDD and VSS to handle the wide current range. The zener voltage should be below the nominal regulation voltage of the SMH4811A so that it becomes the primary regulator. MOSFET VDS(ON) Threshold The drain sense input on the SMH4811A monitors the voltage at the drain of the external power MOSFET switch with respect to VSS. When the MOSFET’s VDS is below the user-defined threshold the MOSFET switch is considered to be ON. The VDS(ON)THRESHOLD is adjusted using the resistor, RT, in series with the drain sense protection diode. This protection, or blocking, diode prevents high voltage breakdown of the drain sense input when the MOSFET switch is OFF. A low leakage MMBD1401 diode offers protection up to 100V. For high voltage applications (up to 500V) the Central Semiconductor CMR1F-10M diode should be used. The VDS(ON)THRESHOLD is calcu- lated from: () ( ) DS SENSE SENSE T DIODE THRESHOLD VON V I R V =− × − , where VDIODE is the forward voltage drop of the protection diode. The VDS(ON)THRESHOLD varies over temperature due to the temperature dependence of VDIODE and ISENSE. The calculation below gives the VDS(ON)THRESHOLD under the worst case condition of 85°C ambient. Using a 68k Ω resistor for RT gives: () ( ) DS THRESHOLD V ON 2.5V 15 A 68k 0.5V 1V µ =− × Ω −= . The voltage drop across the MOSFET switch and sense resistor, VDSS, is calculated from: () DSS D S ON VI R R =+ , where ID is the MOSFET drain current, RS is the circuit breaker sense resistor, and RON is the MOSFET on resistance. Soft Start Slew Rate Control The –48V turn on time is controlled by the SM4811A and by the values of R4, C1 and C2. The turn on time is approximately 10ms with the component values shown in Figure 7. Increasing the capacitance reduces the output slew rate and increases the turn on time. The capacitors prevent the MOSFET from turning on simultaneously with the application of –48V. Resistor R4 is specified to limit the current into and the rate of charge of C1. The ratio of C1 to C2 (10:1) limits the MOSFETs VGS to approxi- mately 5V once the –48V supply is connected and C1 is fully charged. |
Similar Part No. - SMH4811A_09 |
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Similar Description - SMH4811A_09 |
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