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SMP402

5

POLARITY

current

threshold.

Regulation of the output voltage is

detected by a comparator which looks

for error amplifier saturation in the output

high (maximum duty cycle) state.

Power supply turn on and turn off with a

slowly changing input voltage is shown

in Figure 4 as measured using the circuit

shown in Figure 6. Also shown are the

level and polarity outputs. The input

voltage must rise above 32 V before the

SMP402 will turn on.

At this input

voltage the current through R1 into the

SENSE+ input exceeds the threshold

current (typically 23

µA), the LEVEL

and POLARITY outputs go high, and

the power supply turns on. As the input

voltage goes down, LEVEL will go low

at approximately 30 V, but POLARITY

will stay high. As the input voltage

continues to drop, POLARITY goes low

at approximately 12 V and the converter

loses regulation at approximately 10 V

and turns off. The LEVEL output will

go high and the power supply turns on

when the input voltage reaches -32 V.

POLARITY stays low to indicate that

the input voltage has reversed polarity.

As the negative input voltage falls toward

zero, LEVEL goes low at approximately

-30 V and the converter loses regulation

and turns off at approximately -10 V.

Enable

The power supply can be shut down by

pulling the ENABLE pin low.

It is

internally pulled up to V

(nominal) current source. However, it is

recommended that this pin be tied to V

S

if it is unused.

P-Channel Output Transistor

The output MOSFET is a 90 V pass

transistor capable of supplying >200

mA. To minimize switching noise and

EMI, it is important to keep the path

from OUT through the output diode, the

input storage capacitor, and into V

short as possible.

Functional Description (cont.)

Figure 5. LEVEL/POLARITY Truth Table. The LEVEL and POLARITY signals are only valid when the output voltage is in regulation.

INPUT VOLTAGE CONDITION

POLARITY

LEVEL

Negative voltage, level too low

0

0

Negative voltage, correct level

0

1

Positive voltage, level too low

1

0

Positive voltage, correct level

1

1