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参数资料
| 型号： | LM2636MTCX/NOPB |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 2000 kHz SWITCHING FREQ-MAX, PDSO20 |
| 封装： | PLASTIC, TSSOP-20 |
| 文件页数： | 4/17页 |
| 文件大小： | 372K |
| 代理商： | LM2636MTCX/NOPB |
Applications Information (Continued)
A possible drawback of the scheme in
Figure 7 is slow tran-
sient recovery speed. Since the 5 k
resistor and the 0.1 F
capacitor have a large time constant, the settling of point C
to its steady state value during a load transient may take a
few milliseconds. Depending on the interaction between the
compensation network and the 0.1 F capacitor, V
core may
take different routes to reach its steady state value. This is
undesired when the load transients happens more than 1000
times per second. Reducing the time constant will result in a
more fluctuating V
C due to a less effective low pass filter.
Fine tuning the parameters may balance the tradeoffs.
Another way to implement the dynamic voltage positioning is
through the use of a stand-alone resistor, such as the 4 m
resistor in
Figure 8 above. The advantage of this implemen-
tation over the previous one is a much faster speed of V
CORE
from transient level to steady state level. The disadvantage
is less efficiency. The total power loss can be 0.78W at 14A
of load current. The cost of the resistor can be minimized by
implementing it through a PCB trace.
REFERENCE VOLTAGE
The V
REF pin can have many uses, such as in the watchdog
circuitry and in an LDO controller.
Figure 9 shows an applica-
tion where V
REF is used to build a N-FET LDO controller. An
appropriate compensation network is necessary to tailor the
dynamic performance of the whole power supply.
PCB LAYOUT CONSIDERATIONS
There are several points to consider.
1.
Try to use 2 oz. copper for the ground plane if tight load
regulation is desired. In the case of dynamic voltage po-
sitioning, this may not be a concern because the loose
load regulation is desired anyway. However, do not for-
get to take into consideration the voltage drop caused by
the ground plane when calculating dynamic voltage po-
sitioning parameters.
2.
Try to keep gate traces short. However, do not make
them too short or else the LM2636 may stay too close to
the MOSFETs and get heated up by them. For the same
reason, do not use wide traces, 10 mil traces should be
enough.
3.
When not employing dynamic voltage positioning, place
the feedback point at the VRM connector pins to have a
tight load regulation. If it is an on-board power supply,
place the feedback point at Slot I connector or wherever
is closest to the MPU.
4.
Start component placement with the power devices such
as MOSFETs and inductors.
5.
Do not place the LM2636 directly underneath the MOS-
FETs when when surface mount MOSFETs are used.
6.
If possible, keep the capacitors some distance away
from the inductors so that the capacitors will have a
lower temperature environment.
7.
When
implementing
dynamic
voltage
positioning
through a PCB trace, be aware that the PCB trace is a
heat source and try to avoid placing the trace directly un-
derneath the LM2636.
DS100834-27
FIGURE 8. Dynamic Voltage Positioning by Using A
Stand-Alone Resistor
DS100834-28
FIGURE 9. V
REF Used in an N-FET LDO Supply
LM2636
www.national.com
12
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