LP2950ACN-L-3.3 Просмотр технического описания (PDF) - Signal Processing Technologies

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LP2950ACN-L-3.3

100mA Low Dropout Voltage Regulators

Signal Processing Technologies 

APPLICATION HINTS

EXTERNAL CAPACITORS

The stability of the LP2950/LP2951 requires a 1.0µF or greater

capacitor between output and ground. Oscillation could occur without

this capacitor. Most types of tantalum or aluminum electrolytic works

fine here. For operations below -25°C solid tantalum is recommended

since the many aluminum types have electrolytes that freeze at about

-30°C. The ESR of about 5Ω or less and resonant frequency above

500kHz are the most important parameters in the value of the

capacitor. The capacitors value may be increased without limit.

At lower values of output current, less output capacitance is required

for stability. For the currents below 10mA the value of the capacitor

can be reduced to 0.33µ F and 0.1µF for 1mA. More output

capacitance is needed for the 8-pin version at voltages below 5V

since it runs the error amplifier at lower gain. At worst case 3.3 µF or

greater must be used for the condition of 100mA load at 1.23V

output.

The LP2950/51 unlike other low dropout regulators will remain

stable and in regulation with no load in addition to the internal

voltage divider. This feature is especially important in applications

like CMOS RAM keep-alive. When setting the output voltage of the

LP2950/51 version with external resistors, a minimum load of 1µA is

recommended

If there is more than 10 inches of wire between the input and the AC

filter capacitor or if a battery is used as the input then a 1µA tantalum

or aluminum electrolytic capacitor should be placed from the input to

the ground.

Instability can occur if there is stray capacitance to the LP2951

feedback terminal (pin 7). This could cause more problems when

using a higher value of external resistors to set the output voltage.

This problem can be fixed by adding a 100pF capacitor between

output and feedback and increasing the output capacitor to at least 3.3

µF.

4.75V

OUTPUT

VOLTAGE

\

_______

ERROR*

+5.0V

+ INPUT

VOLTAGE 1.3V

* See Application Info.

Figure 1. ERROR Output Timing

+

+

ERROR DETECTION COMPARATOR OUTPUT

The Comparator produces a logic low output whenever the LP2951

output falls out of regulation by more than around 5%. This is around

60 mV offset divided by the 1.235 reference voltage. This trip level

LP2950/51

remains 5% below normal regardless of the programmed output

voltage of the regulator.

Figure 1 shows the timing diagram depicting the ERROR signal and

the regulator output voltage as the LP2951 input is ramped up and

down. The ERROR signal becomes low at around 1.3V input, and

goes high around 5V input (input voltage at which VOUT = 4.75).

Since the LP2951Õs dropout voltage is load dependent, the input

voltage trip point (around 5V) will vary with the load current. The

output voltage trip point (approx. 4.75V) does not vary with load.

The error comparator has an open-collector output, which requires an

external pull-up resistor. Depending on the system requirements the

resistor may be returned to 5V output or other supply voltage. In

determining the value of this resistor, note that the output is rated to

sink 400µA, this value adds to battery drain in a low battery

condition. Suggested values range from 100K to 1MΩ. If the output

is unused this resistor is not required.

PROGRAMMING THE OUTPUT VOLTAGE OF LP2951

The LP2951 may be pin-strapped for 5V using its internal voltage

divider by tying Pin 1 (output) to Pin 2 (sense) and Pin 7 (feedback)

to Pin 6 (5V Tap). Also, it may be programmed for any output

voltage between its 1.235V reference and its 30V maximum rating .

As seen in Figure 2, an external pair of resistors is required. Refer to

the below equation for the programming of the output voltage:

VOUT = VREF × (1 + R1/R2)+ IFBR1

The VREF is 1.235 and IFB is the feedback bias current, nominally -

20nA. The minimum recommended load current of 1 µA forces an

upper limit of 1.2MΩ on value of R2. If no load is presented the IFB

produces an error of typically 2% in VOUT which may be eliminated

at room temperature by trimming R1. To improve the accuracy

choose the value of R2 = 100k this reduces the error by 0.17% and

increases the resistor program current by 12µA. Since the LP2951

typically draws 60µA at no load with Pin 2 open-circuited this is a

small price to pay

REDUCING OUTPUT NOISE

It may be an advantage to reduce the AC noise present at the output.

One way is to reduce the regulator bandwidth by increasing the size

of the output capacitor. This is the only way that noise can be reduced

on the 3 lead LP2950 but is relatively inefficient, as increasing the

capacitor from 1µF to 220 µF only decreases the noise from 430µV

to 160µVRMS for a 100kHz bandwidth at 5V output. Noise could also

be reduced fourfold by a bypass capacitor across R1, since it reduces

the high frequency gain from 4 to unity. Pick

CBYPASS ≅ 1 / 2πR1 × 200 Hz

or choose 0.01µF. When doing this, the output capacitor must be

increased to 3.3µF to maintain stability. These changes reduce the

output noise from 430µV to 100µVRMS for a 100kHz bandwidth at

5V output. With the bypass capacitor added, noise no longer scales

with output voltage so that improvements are more dramatic at higher

output voltages.

Rev. 8/10/01

9

LP2950/51 100mA Low Dropout Voltage Regulators

8

©Copyright 2001 Sipex Corporation

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