lm2633mtd National Semiconductor Corporation, lm2633mtd Datasheet - Page 36

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

www.national.com
Control Loop Design
amplifier, and I
V
respectively.
Example: V
To calculate the total system tolerance, use the following
equation:
where
and
Example: V
tolerance.
That means the 3.3V output voltage will have a
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
can operate only in force-PWM mode when it is configured
as Figure 18 .
fb2
FIGURE 18. How to Make V
and I
is the tolerance of the resistors.
is the tolerance of the Channel 2 reference voltage,
fb2
out2
out2
have a typical value of 1.24V and 18 nA
fb2
= 1.5V, R
= 3.3V, feedback resistors have a
is the current drawn by the FB2 pin. The
fb1
is 2.00V instead of 1.24V. Channel 1
1
= 10 k .
OUT1
(Continued)
Higher Than 2V
200008B6
±
2.96%
±
(51)
(52)
(53)
1%
36
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:
Example: V
The output capacitors can be chosen based on the output
voltage ripple requirement. If there is no specific require-
ment, then a
The equation for determining the impedance of the output
capacitors is:
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:
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
If ceramic capacitors are preferred, then the minimum ca-
pacitance is:
in_max
n
±
= 1.6V, I
1% ripple level may be a good starting point.
= 21V, V
rip
= 3A.
n
= 1.6V, I
load_max
= 10A.
(54)
(55)
(56)
(57)
(58)