SPX2950 데이터 시트보기 (PDF) - Signal Processing Technologies

부품명

상세내역

일치하는 목록

SPX2950

150mA Low Dropout Voltage Regulators

Signal Processing Technologies 

APPLICATION HINTS

EXTERNAL CAPACITORS

The stability of the SPX2950/SPX2951 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 SPX2950, 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 SPX2951 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 SPX2951

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.

ERROR DETECTION COMPARATOR OUTPUT

The Comparator produces a logic low output whenever the

SPX2951 output falls out of regulation by more than around 5%.

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 SPX2951 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 SPX2951’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.

SPX2950/SPX2951

.

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

SPX2951

The SPX2951 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 SPX2951 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 SPX2950 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. 10/30/00

Share Link: