SPX1129M3-2.5 Просмотр технического описания (PDF) - Signal Processing Technologies

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SPX1129M3-2.5

500mA Low Dropout Voltage Regulator

Signal Processing Technologies 

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 1.2 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 SPX1129 typically draws 60 µA at no load with Pin 2 open-

circuited this is a small price to pay

SPX1129

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 SPX1129 but is relatively inefficient, as increasing the

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

160µV Vrms 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 100µV Vrms 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.

HEAT SINK REQUIREMENTS

Depending on the maximum ambient temperature and maximum

power dissipation a heat sink may be required with the SPX1129.

The junction temperature range has to be within the range

specified under Absolute Maximum Ratings under all possible

operating conditions. To find out if a heat sink is required, the

maximum power dissipation of the device needs to be calculated.

This is the maximum specific AC voltage that must be taken into

consideration at input. Figure 3 shows the condition and power

dissipation which should be calculated with the following

formula:

PTOTAL = (VIN - 5) * IL + (VIN) IG

Next step is to calculate the temperature rise TR (max). TJ (max)

maximum allowable junction temperature, TA (max) maximum

ambient temperature :

TR (max) = TJ (max) - TA (max)

Junction to ambient thermal resistance θ(J-A) can be calculated

after determining of PTOTAL & TR (max):

θ(J-A) = TR (max)/P(max)

If the θ(J-A) is 60°C/W or higher, the device could be operated

without a heat sink. If the value is below 60°C/W then the heat

sink is required and the thermal resistance of the heat sink can be

calculated by the following formula, θ(J-C) junction to case, θ(C-H)

case to heat sink, θ(H-A) heat sink to ambient:

θ(J-A) = θ(J-C) + θ(C-H) + θ(H-A)

VOUT

IIN

IN

5V

OUT

SPX1129

+

GND

+

2.2uF

IIN = IL + IG

IG

Figure 3. Fixed +5V Regulator Circuit

IL

LOAD

Rev. 10/25/00

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