The available tolerances should also be checked out as they

govern what can be considered a ’standard’ value for a

certain requirement. Therefore for example, if 1% resistors

are required, it will be almost impossible to find such a

resistor in the E12 or E24 series, for which 10% and 5%

respectively are more commonly available tolerances. E48 is

normally 2%, E96 is 1%, and E192 is 0.5%/ 0.1%. Further,

each series is usually ’devoted’ to its tolerance.Therefore a

value like 221

Ω which exists in E96 and E192 may not be

readily available as a 2% resistor, simply because in E48

(which is usually for 2% resistors), the nearest standard

values are 215

Ω and 226Ω. One could always pay more for

better resistors than required, but the question is one of

’optimum’

design

here.

’Optimum’

means

a

correct

consideration/compromise between several factors: cost,

tolerance of the resistors, and the output error (from all

related sources). All these indicate that the task of correctly

selecting a resistive divider is usually under-estimated or

down-played.

Looking at the standard value series, it should also be noted

that every series is a subset of the next higher series.

However there are two distinct sets. Therefore E6 is a subset

of E12 and E24, and E48 is a subset of E96 and E192.

However no E24 value can be found in E48 (or higher). As

an example, the ’well-known’ resistance value of 220

Ω is

available in E6, E12 and E24. But this value does not exist in

the higher (more modern) series.There the closest ’standard’

values are 215

Ω and 226Ω in E48, and also 221Ω in the

more expensive series as discussed above. The following

example will make these considerations clearer.

Example: It is required to set an output voltage of 5V using

the adjustable part. R

standard resistor value (for a 0.25% error expected on ac-

count of the feedback pin current). So using the basic divider

equation

This is, as expected, not a standard resistance value. The

closest is 4.32k (E96). Using that value for R

additional error of (4.32−4.286)/4.286 = 0.8% (on top of the

errors due to other causes). If this is unacceptable, an

additional resistor can be placed in parallel with the 4.32k.

Checking to see if a ’standard’ 560k resistor would do the

4.287k, which is almost exactly the required value of 4.286k!

More on the choice of the 560k will follow below.

Final recommended values here are

R

Note that the effect of other tolerances have not been con-

sidered, including the possible spread on V

adds another +,- 1.25% of error as indicated by the Electrical

Characteristics tables). Another significant source of error to

consider is the tolerance of the resistors themselves. For a

majority of applications these are generally chosen to be of

1% tolerance. But they can be chosen to be 0.5% or 0.1% in

critical applications, or even 2% in less sensitive applica-

tions.

The percentage error in the output voltage is 2*(V

V

output to 5V, the maximum error on the output on account of

the resistor tolerance is 2*(5-2.42)*1/5=1%. (This is actually

+,-1% since the tolerance of the resistors to start with was

also +,-1%). For a 3.3V output, the error is about +,-0.5%

with 1% resistors. Similarly, the output error (on this account)

is reduced by a factor of 10, if 0.1% resistors are used

instead.

As for the shunt resistor of 560k (in parallel to R

really necessary to have a very tight tolerance for this resis-

tor. Since the entire effect of this resistor is to add a slight

’trim’ to the output voltage, the effect of its tolerance on the

output is proportionally small too. The proportionality factor

here is the ratio R

resistor in almost all applications. This was actually kept in

mind well in advance, when the value 560k was initially

proposed. Because 560k happens to be a standard value in

the 5% series (E24), though it does not exist in the higher

(expensive) series. Cost was clearly of concern here.

Concluding this discussion, there is a last observation to

make concerning the fixed voltage part. Here the output is

normally connected to the feedback pin directly. The resis-

tive divider is therefore internal. The relevant design infor-

mation here is that for the 5V part the effective resistance

(R

part this resistance is 6.6k. This gives a current of 0.5mA

passing through the divider. This is a satisfactory choice,

since it was seen to limit the contribution of the feedback

node current on the output to less than 0.3%. The error due

to the ’tolerance’ of the resistive divider is almost negligible

for fixed voltage parts. To understand this, the reason for the

error from an external divider, as used in an adjustable part,

must first be clarified. There the worst case situation is

where one resistor is at the lower end of its tolerance band,

while at the same time the other resistor is at the upper end

of the tolerance band. This gives the worst case error on the

output. However if both the upper and lower resistors were

simultaneously say ’x%’ higher (or lower) than their nominal,

their ’ratio’ would remain unchanged. It was earlier indicated

that if the ratio of the resistances in a resistive divider is

maintained, then theoretically there is no change in the

output voltage. This is the situation in the case of the internal

resistive divider. Because the two resistors are in the same

package, any drift or tolerance will affect both of them almost

equally. It is expected, and borne out, that their ’relative

tolerance’ is typically almost 10 times better than the (abso-

lute) tolerance of each. Therefore, the effect of the tolerance

of the resistors in an internal resistive divider, as in fixed

voltage parts, can be ignored.

Bill of Material for LM1572 Evaluation Board

Designator

Description

Manufacturer

Part Number

Qty.

U1

LM1572-5.0

National Semiconductor

LM1572-5.0

1

D1

2A/30V Schottky

International Rectifier

20BQ030

1

D2

SS diode

General Semiconductor

1N4448W

1

L1

8.2µH

Gowanda

1

C1

22µF/35V

Vishay-Sprague

595D226X0035R2T

1

www.national.com

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