SMH4803BHKP datasheet(12/22 Pages) SUMMIT

12 / 22 page

SMH4803

2041 8.4 6/15/00

SUMMIT MICROELECTRONICS

Dropper Resistor Selection

The SMH4803 is powered from the high-voltage supply

via a dropper resistor, Rd. The dropper resistor must

provide the SMH4803 (and its loads) with sufficient oper-

ating current under minimum supply voltage conditions,

but must not allow the maximum supply current to be

exceeded under maximum supply voltage conditions.

The dropper resistor value is calculated from:

RD =

Where Vsmin is the lowest operating supply voltage,

Vddmax is the upper limit of the SMH4803 supply voltage,

Idd is minimum current required for the SMH4803 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

dropper 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 SMH4803 so that it becomes the primary regulator.

MOSFET VDS(ON) Threshold

The drain sense input on the SMH4803 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 value the switch is considered to be ON. The

VDS(ON) 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. An inexpensive 1N4148 diode offers protection up

to 100V. The VDS(ON) threshold is calculated from:

VDS = VSENSE – (ISENSE

× RT) – VDIODE – (IRS × RS)

Where VDIODE is the forward voltage drop of the protection

diode, and IRS is the current flowing through the circuit

breaker sense resistor RS. 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:

VDS(ON) threshold =

2.5V – (15µA x 68k

Ω) – VDIODE =

2.5 – 1.0 – 0.5 = 1.0V

APPLICATIONS CIRCUITS

Reversing Polarity of the Power Good Outputs

The open-drain Power Good outputs on the SMH4803 are

active LO. The output polarity may be changed to active

HI, when required, with a minor circuit change around the

high-voltage buffer transistor, see Figure 6. The 1N4148

blocking diode must be included to prevent high-voltage

damage to the SMH4803.

Expanding Enable/Monitoring on the SMH4803

The 2.5V reference and 5V outputs on the SMH4803

make it easy to expand the number of enable or monitor-

ing inputs. The circuit in Figure 8 illustrates how a quad

low-voltage comparator expands the EN/TS input to four

enable inputs. The comparators draw power from the 5V

output on the SMH4803 and use the 2.5V reference for

the switching threshold. EN1 to EN4 inputs can accept

either analog or CMOS logic level signals between Vss

and +5V. The comparator outputs are ANDed together

and drive the EN/TS input. A 1M

Ω resistor adds hysteresis

around the comparators to prevent oscillation near the trip

point.

(

VS min – VDD max)

(

IDD +

Iload)

SMH4803BHKP Datasheet(PDF) 12 Page - Summit Microelectronics, Inc.

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