SMH4811AS Ver la hoja de datos (PDF) - Summit Microelectronics

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SMH4811AS

Distributed Power Hot-Swap Controller

Summit Microelectronics 

SMH4811A

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 between 20V and 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.

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:

( ) ( ) VDS ON THRESHOLD = VSENSE − ISENSE × RT − VDIODE ,

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 68kΩ

resistor for RT gives:

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

VDS

( ) ON

THRESHOLD

=

2.5V −

(15µA × 68kΩ) − 0.5V

=

1V .

The voltage drop across the MOSFET switch and sense

resistor, VDSS, is calculated from:

( ) VDSS = ID RS + RON ,

where ID is the MOSFET drain current, RS is the circuit

breaker sense resistor, and RON is the MOSFET on

resistance.

Note: Figures 5 through 8 — the *10Ω resistor must be located as close as possible to the MOSFET

SUMMIT MICROELECTRONICS, Inc.

2044 6.1 2/8/01

11

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