Application Information

Resistive Divider Calculation

For the adjustable part, the voltage on the feedback pin is set

to 2.42V under regulation. This is achieved by means of a

resistive divider, as indicated in the Typical Applications for

the adjustable part. Designating the upper resistor as ’R

(connected to the output) and the lower resistor as ’R

(connected to ground), the following equation relates R

and the output voltage level V

Setting the lower resistor to 2.21k (which is a standard

resistance value), the upper resistor is chosen as 806 ohms

for a 3.3V output and as 2.37k for a 5V output. This should

suffice for most applications. However the more experienced

designer may like to know more about the rather overlooked

intricacy of selecting resistors especially in regard to the

resultant error in the output voltage. It is also helpful to

consider the other factors affecting the tolerance of the

output voltage. This is disussed under ’Tolerance of set

Output Voltage’ at the end of ’Application Information’. Note

that if the the lower resistor is set to 2.21k, the divider current

is greater than 1mA, so a 0.1µF boostrap will always suffice

(see Pin Descriptions for Pin 1 and Pin 15 above).

Inductor Selection

Inductor selection for buck converters is discussed in great

detail in AN-1197, to which the reader can refer to for a

deeper understanding. It must be understood that though the

scope of the above Application Note is limited to buck con-

verters that rely on voltage mode control, all the consider-

ations contained therein also apply to buck converters rely-

ing on current mode control, such as the LM1572. In fact,

with current mode control, there are additional consider-

ations that may apply which need to be discussed here.

The basic requirement for any converter is that it should be

able to deliver the required power without hitting the current

limit of the switch. This is ensured by having an inductance

large enough to limit the peak current (this is obviously not

feasible if the required load current is very close to or larger

than the current limit!). In the LM1572, a ’slope compensa-

tion’ ramp is also summed-in with the switch current ramp,

for duty cycles greater than 0.5. The reason for this slope

compensation will be explained later below, here it suffices

to realize that it affects the effective current limit for duty

cycles greater than 0.5. From the Electrical Characteristics it

can be seen that the current limit I

terms: one for D less than (or equal to) 0.5, and one for D =

0.8. Since the current limit falls off at high duty cycles/low

input voltage due to the slope compensation, a peak power

calculation should generally be done both at the highest and

the lowest input voltage, so as to ensure that the inductance

is large enough to cover the entire desired operating input

voltage range.

The overall strategy here is to determine various ’minimum

inductances’ based on all different considerations (as appli-

cable), and to then pick the largest of all the ’minimums’ so

as to satisfy each of the conditions.

It is noted here that there can also be an ’optimum’ value for

the inductance, one which offers a compromise solution for

reducing the overall size of the power converter, the mag-

netics and capacitors included. However, since the primary

reason for going to higher switching frequencies is to reduce

the size of the magnetics alone, ’optimization’ may be rel-

egated to a lower priority, as in the example to follow.

In (peak) current mode control, the main additional consid-

eration is the phenomenon of subharmonic instability (also

called alternate cycle or half-frequency oscillations). This is

fundamental to the topology, and no amount of ’tweaking’ the

compensation resistor/capacitor values will circumvent it.

The well known solution is to add a certain amount of ’slope

compensation’, the value of which is directly related to the

inductance being used. Higher inductance requires smaller

slope compensation. If the slope compensation is fixed, as

for the LM1572, it is the inductance that needs to be con-

trolled. Then higher slope compensation requires smaller

inductance. This defines a ’minimum’ value of inductance

required to avoid subharmonic instability. The value can

therefore be exceeded. If for example the first priority in a

given application is not the size of the inductor, but the

reduction of output ripple, a higher than the minimum induc-

tance may be selected. But too high an inductance, for a

given slope compensation (or equivalently too much of slope

compensation for a given inductance), will cause the loop

response to become more and more that of voltage mode

control, eventually making it slower and harder to compen-

sate. For any LM1572 design therefore, the maximum rec-

ommended inductance is 15µH, irrespective of input or out-

put conditions.

For the LM1572, the slope compensation can vary (from

device to device) over the range 0.42 to 0.75 A/µs. A little

thought will lead to the conclusion that any calculation for the

minimum inductance (required to avoid subharmonic insta-

bility), must be carried out at its ’worst-case’: which is the

lower limit of the slope compensation (i.e. 0.42 A/µs). This

also happens to be the value used for peak power calcula-

tion since it corresponds to the lower limit of current limit

(2A). The value of 0.75 A/µs can be used to check if the

slope compensation is not ’excessive’ in the sense dis-

cussed above.

The effective current limit, ’I

tics) is the sum of two terms. The first is the basic preset

current limit (the flat part) , which we call ’I

value given for ’I

≤ 0.5). Superimposed on this is

the effect of slope compensation. This causes the current

compensation can be expressed as ’m

From D = 0.5 to a projected value of D = 1 (a time interval of

1µs), the current limit would therefore fall exactly by m

C

Amps. At D = 0.8 the current limit falls by 3/5th of this i.e. by

m

Electrical Characteristics tables.

As mentioned, the inductance must be chosen to be higher

than the minimum value corresponding to the condition of

peak calculated switch current equal to the current limit. The

worst case must be used here: i.e. the ’min’ of current limit

values in the Electrical Characteristics (not ’typ’). Further, it

should be confirmed over the entire input voltage range (or

duty cycle) that the peak current does not attempt to exceed

the effective current limit. This is easily carried out using the

same general strategy: by calculating the minimum induc-

tance at both input voltage extremes, and then choosing the

greater of the two calculated ’minimum’ inductances.

It should also be remembered that subharmonic instability

can only occur when several conditions are simultaneously

satisfied: (peak) current mode control, duty cycle greater

than (or around) 0.5, and continuous conduction mode. Sub-

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