LP2950/2951

Micrel

3

February 1999

3-43

which V

OUT

= 4.75V). Since the LP2951’s dropout voltage is

load-dependent (see curve in Typical Performance

Characteristics), the input voltage trip point (about 5V) will

vary with the load current.

The output voltage trip point

(approximately 4.75V) does not vary with load.

The error comparator has an open-collector output which

requires an external pull-up resistor. Depending on system

requirements, this resistor may be returned to the 5V output

or some other supply voltage. In determining a value for this

resistor, note that while the output is rated to sink 400

µA, this

sink current adds to battery drain in a low battery condition.

Suggested values range from 100k to 1M

Ω. The resistor is not

required if this output is unused.

Programming the Output Voltage (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). Alternatively, it may be

programmed for any output voltage between its 1.235V

reference and its 30V maximum rating. An external pair of

resistors is required, as shown in Figure 2.

The complete equation for the output voltage is

V

REF

x { 1 + R

+ I

FB

R

2

where V

REF

is the nominal 1.235 reference voltage and I

FB

is

the feedback pin bias current, nominally 20 nA. The minimum

recommended load current of 1

µA forces an upper limit of 1.2

M

Ω on the value of R

2

, if the regulator must work with no load

(a condition often found in CMOS in standby), I

FB

will produce

a 2% typical error in V

OUT

which may be eliminated at room

temperature by trimming R

For better accuracy, choosing

R

2

= 100k

Ω reduces this error to 0.17% while increasing the

resistor program current to 12

µA. Since the LP2951 typically

draws 60

µA at no load with Pin 2 open-circuited, this is a small

price to pay.

Reducing Output Noise

In reference applications it may be advantageous to reduce

the AC noise present at the output. One method is to reduce

the regulator bandwidth by increasing the size of the output

capacitor. This is the only method by which noise can be

reduced on the 3 lead LP2950 and is relatively inefficient, as

increasing the capacitor from 1

µF to 220µF only decreases

the noise from 430

µV to 160µV rms for a 100kHz bandwidth

at 5V output.

Noise can be reduced fourfold by a bypass capacitor across

R

1

, since it reduces the high frequency gain from 4 to unity.

Pick

or about 0.01

µF. When doing this, the output capacitor must

be increased to 3.3

µF to maintain stability. These changes

reduce the output noise from 430

µV to 100µV rms 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.

Applications Information

External Capacitors

A 1.0

µF (or greater) capacitor is required between the LP2950/

LP2951 output and ground to prevent oscillations due to

instability. Most types of tantalum or aluminum electrolytics

will be adequate; film types will work, but are costly and

therefore not recommended. Many aluminum electrolytics

have electrolytes that freeze at about –30

°C, so solid tantalum

capacitors are recommended for operation below –25

°C. The

important parameters of the capacitor are an effective series

resistance of about 5

Ω or less and a resonant frequency

above 500kHz. The value of this capacitor may be increased

without limit.

At lower values of output current, less output capacitance is

required for output stability. The capacitor can be reduced to

0.33

µF for current below 10mA or 0.1µF for currents below

1mA. Using the 8-Pin versions at voltages below 5V runs the

error amplifier at lower gains so that more output capacitance

is needed. For the worst-case situation of a 100mA load at

1.23V output (Output shorted to Feedback) a 3.3

µF

(or greater) capacitor should be used.

The LP2950 will remain stable and in regulation with no load

in addition to the internal voltage divider, unlike many other

voltage regulators. This is especially important in CMOS

RAM keep-alive applications. When setting the output voltage

of the LP2951 version with external resistors, a minimum load

of 1

µA is recommended.

A 0.1

µF capacitor should be placed from the LP2950/LP2951

input to ground 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.

Stray capacitance to the LP2951 Feedback terminal (pin 7)

can cause instability. This may especially be a problem when

using high value external resistors to set the output voltage.

Adding a 100pF capacitor between Output and Feedback and

increasing the output capacitor to at least 3.3

µF will remedy

this.

Error Detection Comparator Output

A logic low output will be produced by the comparator whenever

the LP2951 output falls out of regulation by more than

approximately 5%. This figure is the comparator’s built-in

offset of about 60mV divided by the 1.235V reference voltage.

(Refer to the block diagram on Page 1). This trip level remains

“5% below normal” regardless of the programmed output

voltage of the LP2951. For example, the error flag trip level

is typically 4.75V for a 5V output or 11.4V for a 12V output.

The out of regulation condition may be due either to low input

voltage, current limiting, or thermal limiting.

Figure 1 is a timing diagram depicting the ERROR signal and

the regulated output voltage as the LP2951 input is ramped up

and down. The ERROR signal becomes valid (low) at about

1.3V input. It goes high at about 5V input (the input voltage at

≅

1

π