lm2633mtd National Semiconductor Corporation, lm2633mtd Datasheet - Page 29

• Manufacturer Part Number: lm2633mtd
• Description: Advanced Two-phase Synchronous Triple Regulator Controller For Notebook Cpus
• Manufacturer: National Semiconductor Corporation

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:
where I
I
calculated R
limit will not be less than I
Example: I
T
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-
ilim_min
j_max
= 100˚C, I
load_lim
is the minimum sink current at the ILIM1 pin. This
load_lim
ilim
is the desired load current limit level and
value guarantees that the minimum current
ilim_min
= 16A, I
= 8µA.
load_lim
rip_max
.
= 4.3A, R
ds_max
= 18m ,
(16)
29
tween the two channels’ input current pulses. The equation
for calculating the maximum total input ripple RMS current is
therefore:
where I
maximum load current of Channel 2, D
Channel 1, and D
Example: I
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
sum of the maximum load currents, D
cycle of the two, D
cycles, and I
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
D
ripple current is zero for 2-phase operation.
1
= D
2
1
= 0.5 and I
is maximum load current of Channel 1, I
load_max_1
2
is the maximum load current of the channel
2
2
is the duty cycle of Channel 2.
load_max_1
is the difference between the two duty
= 6.8A, I
load_max_2
= I
load_max_2
1
1
is the duty cycle of
is the smaller duty
= 2A, D1 = 0.09,
, when the input
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2
1
is the
is the
(17)