LM2633 datasheet(29/40 Pages) NSC

29 / 40 page

Current Limit Setting

What is actually monitored and limited is the peak drain-

source voltage of the top FET when it is conducting. The

equation for current limit resistor is as follows:

(16)

where I

load_lim is the desired load current limit level and

ilim_min is the minimum sink current at the ILIM1 pin. This

calculated R

ilim value guarantees that the minimum current

limit will not be less than I

load_lim.

Example: I

load_lim = 16A, Irip_max = 4.3A, Rds_max = 18mΩ,

j_max = 100˚C, Iilim_min = 8µA.

It is recommended that a 1% tolerant resistor be used and its

resistance should not be lower than the calculated value.

Input Capacitor Selection

In a typical buck regulator the power loss in the input capaci-

tors is much larger than that in the output capacitors. That is

because the current flowing through the input capacitors is of

square-wave shape and the peak-to-peak magnitude is

equal to load current. The result is a large ripple RMS current

in the input capacitors.

The fact that the two switching channels of the LM2633 are

180˚ out of phase helps reduce the RMS value of the ripple

current seen by the input capacitors. That will help extend

input capacitor life span and result in a more efficient sys-

tem. In a mobile CPU application, both the CPU core and

GTL bus voltages are rather low compared to the input

voltage. The corresponding duty cycles are therefore less

than 50%, which means there will be no over-lapping be-

tween the two channels’ input current pulses. The equation

for calculating the maximum total input ripple RMS current is

therefore:

(17)

where I

1 is maximum load current of Channel 1, I2 is the

maximum load current of Channel 2, D

1 is the duty cycle of

Channel 1, and D

2 is the duty cycle of Channel 2.

Example: I

load_max_1 = 6.8A, Iload_max_2 = 2A, D1 = 0.09,

and D2 = 0.1.

Choose input capacitors that can handle 1.97A ripple RMS

current at highest ambient temperature. The input capacitors

should also meet the voltage rating requirement. In this

case, a SANYO OSCON capacitor 25SP33M, or a Taiyo

Yuden ceramic capacitor TMK325BJ475, will meet both re-

quirements.

Comparison: If the two channels are operating in phase, the

ripple RMS value would be 2.52A. The equation for calculat-

ing ripple RMS current takes the same form as the one

above but the meanings of the variables change. I

1 is the

sum of the maximum load currents, D

1 is the smaller duty

cycle of the two, D

2 is the difference between the two duty

cycles, and I

2 is the maximum load current of the channel

that has larger duty cycle.

Figure 6 shows how the reduction of input ripple RMS cur-

rent brought by the 2-phase operation varies with load cur-

rent ratio and duty cycles. From the plots, it can be seen that

the benefit of the 2-phase operation tends to maximize when

the two load currents tend to be equal. Another conclusion is

that the ratio increases rapidly when one channel’s duty

cycle is catching up with the other channel’s and then be-

comes almost flat when the former exceeds the latter. So the

absolute optimal operating point in terms of input ripple is at

1 =D2 = 0.5 and Iload_max_1 =Iload_max_2, when the input

ripple current is zero for 2-phase operation.

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