SPX2700AM3-3.3 Просмотр технического описания (PDF) - Signal Processing Technologies

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SPX2700AM3-3.3

700mA low dropout voltage regulators with shutdown

Signal Processing Technologies 

APPLICATION HINTS

EXTERNAL CAPACITORS

The stability of the SPX2700/01/02 requires a 10µF 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 500 kHz are the most important

parameters in the value of the capacitor. The capacitor 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 22µF 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 22µF or

greater must be used for the condition of 750mA load at 1.23V

output.

The SPX2700/01/02, 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

SPX2700/01/02 version with external resistors, a minimum load of

5mA 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.

ERROR DETECTION COMPARATOR OUTPUT

4.75V

OUTPUT

VOLTAGE

_______

ERROR*

+5.0V

+ INPUT

VOLTAGE 1.3V

+

+

* See Application Info. _______

Figure 1. ERROR Output Timing

The Comparator produces a logic low output whenever the

SPX2700/01/02 output falls out of regulation by more than around

5%. This is around 75mV 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 SPX2700/01/02 input is ramped up and down. The

ERROR signal becomes low at around 1.3V input, and goes

SPX2700/01/02

high around 5V input (input voltage at which Vout = 4.75 ). Since

the SPX2700/01/02’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 250µA, 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

SPX2700/01/02

The SPX2700/01/02 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 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

SPX2700/01/02 typically draws 100µA at no load with

SHUTDOWN 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 SPX2700/01/02 but is relatively inefficient, as

increasing the capacitor from 1µF to 220µF only decreases the noise

from 430µV to 160µVRMS for a 100 kHz 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 100 kHz 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.

SHUTDOWN

VOUT

VIN

SPX2703

GND

ADJ

Fig.2 Adjustable Voltage Regulator

Rev. 10/25/00

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