The difference in input voltage measured at no load and

full load defines the amount of hysteresis required for

proper snap-on/snap-off operation (the programmed hys-

teresis must be greater than the difference in voltages).

CALCULATING RESISTOR VALUES:

The values of R1, R2 and R3 can be calculated assuming

the designer knows the hysteresis.

In most transformer-powered applications, it can be as-

sumed that V

for about 5.5V, since this allows about 500 mV across the

LP2957 to keep the output in regulation until it snaps off. V

ON

(the input voltage at turn on) is found by adding the hyster-

esis voltage to V

R1, R2 and R3 are found by solving the node equations for

the currents entering the node nearest the shutdown pin

(written at the turn-on and turn-off thresholds).

The shutdown pin bias current (10 nA typical) is not included

in the calculations:

Since these two equations contain three unknowns (R1, R2

and R3) one resistor value must be assumed and then the

remaining two values can be obtained by solving the equa-

tions.

The node equations will be simplified by solving both equa-

tions for R2, and then equating the two to generate an ex-

pression in terms of R1 and R3.

Setting these equal to each other and solving for R1 yields:

The same equation solved for R3 is:

A value for R1 or R3 can be derived using either one of the

above equations, if the designer assumes a value for one of

the resistors.

The simplest approach is to assume a value for R3. Best re-

sults will typically be obtained using values between about

20 k

Ω and 100 kΩ (this keeps the current drain low, but also

generates realistic values for the other resistors).

There is no limit on the minimum value of R3, but current

should be minimized as it generates power that drains the

source and does not power the load.

SUMMARY: TO SOLVE FOR R1, R2 AND R3:

1.

Assume a value for either R1 or R3.

2.

Solve for the other variable using the equation for R1 or

R3.

3.

Take the values for R1 and R3 and plug them back into

either equation for R2 and solve for this value.

DESIGN EXAMPLE #1:

A 5V regulated output is to be powered from a transformer

secondary which is rectified and filtered. The voltage V

measured at zero current and maximum current (600 mA) to

determine the minimum allowable hysteresis.

V

shown on the same grid for clarity):

The full-load voltage waveform from a transformer-powered

supply will have ripple voltage as shown. The correct point to

measure is the lowest value of the waveform.

The 1.2V differential between no-load and full-load condi-

tions means that at least 1.2V of hysteresis is required for

proper snap-on/snap-off operation (for this example, we will

use 1.5V ).

As a starting point, we will assume:

V

V

R3 = 49.9k

Solving for R1:

Turn-ON Transition

DS011340-11

Turn-OFF Transition

DS011340-12

FIGURE 6. Equivalent Circuits

DS011340-13

FIGURE 7. V

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