LP2975 Datasheet, PDF (10/37 Pages) National Semiconductor (TI) – MOSFET LDO Driver/Controller

LP2975
SNVS006F – SEPTEMBER 1997 – REVISED APRIL 2013
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Transient Response
Transient response is defined as the change in output voltage which occurs after the load current is suddenly
changed. V_IN = 5.6V for this test.
The load resistor is connected to the regulator output using a switch so that the load current increases from 0 to
5A abruptly. The change in output voltage is shown in the scope photo below (the vertical scale is 200
mV/division and the horizontal scale is 10 µs/division). The regulator nominal output (5V) is located on the center
line of the photo.
The output shows a maximum change of about −600 mV compared to nominal. This is due to the relatively small
output capacitor chosen for this design. Increasing C_OUT greatly improves transient response (see Design #2 and
Design #3).
Figure 17. Transient Response for 0–5A Load Step
DESIGN #2: V_OUT = 3V @ 0.5A
(Refer to TYPICAL APPLICATION CIRCUITS, Adjustable Voltage Regulator)
Components
C_IN = 68 µF Tantalum
C_OUT = 2 X 68 µF Tantalum
C_C = 470 pF
R1 = 237 kΩ, 1%
R2 = NOT USED
R_SC = 0.1Ω
Tie feedback pin to V_OUT
P-FET = NDT452P
Heatsink: Tab of N-FET is soldered down to 0.6 in² copper area on PC board.
Output Voltage Adjustment: For this application, a 3.3V part is “trimmed” down to 3V by using a single external
237 kΩ resistor at R1, which parallels the internal 39.9 kΩ resistor (reducing the effective resistance to 34.2 kΩ).
Because the tempco of the external resistor will not match the tempco of the internal resistor (which is typically
3000 ppm), this method of adjusting V_OUT by using a single resistor is only recommended in cases where the
output voltage is adjusted ≤ 10% away from the nominal value.
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