LM2633 datasheet(36/40 Pages) NSC

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Control Loop Design (Continued)

amplifier, and I

fb2 is the current drawn by the FB2 pin. The

fb2 and Ifb2 have a typical value of 1.24V and 18 nA

respectively.

Example: V

out2 = 1.5V, R1 =10 kΩ.

(51)

To calculate the total system tolerance, use the following

equation:

(52)

where

φ is the tolerance of the Channel 2 reference voltage,

and

σ is the tolerance of the resistors.

Example: V

out2

= 3.3V, feedback resistors have a ±1%

tolerance.

(53)

That means the 3.3V output voltage will have a ±2.96%

tolerance over the (LM2633 die) temperature range of 0˚C to

125˚C.

Channel 2 output voltage should not go above 6V in

pulse-skip mode. That is because the SENSE2 pin cannot

take a voltage higher than 6V. However, if force-PWM op-

eration is the chosen operating mode, then the SENSE2 pin

can be grounded and there will be no limitation to Channel 2

output voltage.

If the desired Channel 1 voltage is higher than 2V, an

op-amp and a voltage divider can be used to expand the

voltage range, as shown in

Figure 18.

It is recommended that the VIDx pins be all tied to ground so

that the DAC is set at 2.00V. That will reduce the total

tolerance. The equations used to calculate Channel 2’s feed-

back resistors and total tolerance still hold, except that the

reference voltage V

fb1 is 2.00V instead of 1.24V. Channel 1

can operate only in force-PWM mode when it is configured

Figure 18.

Since an op-amp is an active device, pay close attention to

its start up and shut down behavior. Make sure that it does

not create a problem during those times.

Designing a Power Supply without a Load Transient

Specification

Many times the load transient response of a buck regulator is

not a critical issue. In that case, the selection of the power

stage components can start from the inductor ripple current.

Choosing the peak-to-peak ripple current to be 30% of the

maximum load current is often a good starting point. Then

the inductance value can be determined by ripple, switching

frequency and input and output voltages. By rearranging

Equation (13), the inductance value can be calculated as

follows:

(54)

Example: V

in_max = 21V, Vn = 1.6V, Iload_max = 10A.

The output capacitors can be chosen based on the output

voltage ripple requirement. If there is no specific require-

ment, then a ±1% ripple level may be a good starting point.

The equation for determining the impedance of the output

capacitors is:

(55)

If the ESR zero frequency of the capacitor is lower than the

switching frequency, such as the case of aluminum, tantalum

and OSCON capacitors, then the output capacitors are cho-

sen by the ESR value. Otherwise, such as in the case of

ceramic capacitors, the output capacitors are chosen by the

capacitance. The equation is:

(56)

Basically make sure that the product of the impedance of the

capacitors and the ripple current does not exceed the ripple

voltage requirement.

Example: V

n = 1.6V, Irip = 3A.

(57)

If ceramic capacitors are preferred, then the minimum ca-

pacitance is:

(58)

200008B6

FIGURE 18. How to Make V

OUT1 Higher Than 2V

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