SPX2955 datasheet(6/9 Pages) SIPEX

6 / 9 page

Rev. 11/21/00

SPX2955/56/57

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 M

Ω 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 12

µA. Since

the SPX2955/56/57 typically draws 60

µA 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 3 lead SPX2955/56/57 but is relatively inefficient, as

increasing the capacitor from 1

µF to 220µF only decreases the noise

from 430

µV 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 0.01

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 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

SPX2955/56/57. 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)

2.2 uF

SPX2955

Iin

Vin

OUT

GND

LOAD

IIN = IL + IG

Figure 3. 5V Regulator Circuit

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