LP2950AZ View Datasheet(PDF) - Bay Linear

Part Name

Description

Manufacturer

LP2950AZ

100mA Low Dropout Voltage Regulator

Bay Linear 

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 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 capacitor value can

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 0.5µF and 0.15µF 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 5µF or greater must be used for the condition of 150mA load

at 1.23V output.

The LP2950, unlike other low dropout regulators will remain

stable 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

LP2951 version with external resistors, a minimum load of 1uA 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

3.3µF.

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%.

LP2950/LP2951

This is around 60mV offset divided by the 1.235 reference voltage.

This trip level 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

LP2951’s 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

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

upper limit of 1.2 MΩ on value of R2. If no load 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 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.

Bay Linear, Inc 2478 Armstrong Street, Livermore, CA 94550 Tel: (925) 989-7144, Fax: (925) 940-9556

www.baylinear.com

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