LP2957AIS National Semiconductor, LP2957AIS Datasheet - Page 10

• Manufacturer Part Number: LP2957AIS
• Description: 5V Low-Dropout Regulator for P Applications
• Manufacturer: National Semiconductor

www.national.com
Application Hints
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
(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.
OFF
FIGURE 6. Equivalent Circuits
(the input voltage at turn-off) should be set
OFF
Turn-OFF Transition
Turn-ON Transition
.
(Continued)
DS011340-11
DS011340-12
ON
10
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 and 100 k (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
3. Take the values for R1 and R3 and plug them back into
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:
IN
OFF
ON
R3.
either equation for R2 and solve for this value.
is measured using an oscilloscope (both traces are
= V
= 5.5V
FIGURE 7. V
OFF
+ HYST = 5.5 + 1.5 = 7V
IN
VOLTAGE WAVEFORMS
DS011340-13
IN
is