LP2957_15 Datasheet, PDF (14/24 Pages) Texas Instruments – 5V Low-Dropout Regulator for μP Applications

LP2957, LP2957A
SNVS102C – JUNE 1998 – REVISED APRIL 2013
www.ti.com
The simplest approach is to assume a value for R3. Best results will typically be obtained using values between
about 20 kΩ 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_IN is measured at zero current and maximum current (600 mA) to determine the minimum allowable hysteresis.
V_IN is measured using an oscilloscope (both traces are shown on the same grid for clarity):
Figure 37. V_IN VOLTAGE WAVEFORMS
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 conditions 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_OFF = 5.5V V_ON = V _OFF + HYST = 5.5 + 1.5 = 7V R3 = 49.9k
Solving for R1:
(8)
Solving for R2:
(9)
14
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