LM2633 datasheet(41/47 Pages) TI1

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vout1 (> 2V)

external

op-amp

fb1

2.00V

LM2633

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SNVS139C – MAY 2004 – REVISED APRIL 2005

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 operation 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 23.

Figure 23. How to Make VOUT1 Higher Than 2V

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 feedback resistors and total tolerance still hold,

except that the reference voltage Vfb1 is 2.00V instead of 1.24V. Channel 1 can operate only in force-PWM mode

when it is configured as Figure 23.

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 18, the

inductance value can be calculated as follows:

(80)

Example: Vin_max = 21V, Vn = 1.6V, Iload_max = 10A.

(81)

The output capacitors can be chosen based on the output voltage ripple requirement. If there is no specific

requirement, then a ±1% ripple level may be a good starting point. The equation for determining the impedance

of the output capacitors is:

(82)

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 chosen by the ESR value. Otherwise, such as in

the case of ceramic capacitors, the output capacitors are chosen by the capacitance. The equation is:

(83)

Basically make sure that the product of the impedance of the capacitors and the ripple current does not exceed

the ripple voltage requirement.

Example: Vn = 1.6V, Irip = 3A.

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