lm2637mx National Semiconductor Corporation, lm2637mx Datasheet - Page 16

• Manufacturer Part Number: lm2637mx
• Description: Motherboard Power Supply Solution With A 5-bit Programmable Switching Controller And Two Linear Regulator Controllers
• Manufacturer: National Semiconductor Corporation

www.national.com
Applications Information
A possible drawback of the scheme in Figure 9 is slow
transient recovery speed. Since the 5 kΩ resistor and the 0.1
µF capacitor have a large time constant, the settling of node
C to its steady state value during a load transient may take a
few milliseconds. Depends on the interaction between the
compensation network and the 0.1 µF capacitor, V
take different routes to reach its steady state value. This is
undesired when the load transient happens more than 1000
times per second. Reducing the time constant will result in a
more fluctuating V
Fine tuning the parameters may generate an acceptable
design.
Another way to implement the dynamic voltage ppsitioning is
through the use of a separate resistor, such as the 4 mΩ
resistor in Figure 10 above. The advantage of this implemen-
tation over the previous one is a much faster recovery speed
of V
tuned compensation network will give good response as
shown in Figure 11. The disadvantage is additional power
loss. The total power loss can be 0.78W at 14A of load
current. The cost of the resistor can be minimized by using a
PCB etch resistor.
FIGURE 11. Load Transient Response with DVP: 0A to
FIGURE 10. Dynamic Voltage Positioning by Using a
CORE
14A, ESR = 9.4 mΩ, Droop Resistor = 4 mΩ
from transient level to steady state level. A fine-
Stand-Alone Resistor
C
, due to a less effective low pass filter.
(Continued)
CORE
10084829
10084828
may
16
PCB Layout Considerations
There are several points to consider.
1. Try to use 2 oz. copper for the ground plane if tight load
2. Try to keep gate drive traces short. However, do not
3. When not employing dynamic voltage positioning, place
4. Start component placement with the power devices such
5. Do not place the LM2637 directly underneath the FETs
6. If possible, keep the capacitors some distance away
7. When
8. Try to place a ceramic capacitor as close as possible to
9. If it is a MPU core supply, try to place the output bulk
regulation is desired. In the case of dynamic voltage
positioning, this may not be a concern because the
loose load regulation is desired anyway. However, do
not forget to take into consideration the voltage drop
caused by the ground plane when calculating dynamic
voltage positioning parameters.
make them too short or else the LM2637 may be placed
too close to the FETs and get heated up by them. For the
same reason, do not use wide traces, 10 mil traces
should be enough.
the feedback point at the VRM connector pins so as to
have a tight load regulation. If it is an embedded power
supply, place the feedback point at Slot I connector or
wherever closest to the MPU.
as FETs, and inductors.
(on the other side of the PCB) when surface mount FETs
are used. Also try to avoid staying too close to the output
inductor, especially when using an open core inductor.
from the inductors and FET heatsinks so that the capaci-
tors will have a better thermal environment. Keep in
mind that the input capacitors are usually much hotter
than output capacitors.
through a PCB trace, keep in mind that the PCB trace is
a heat source and try to avoid placing the trace directly
underneath the LM2637.
the V
capacitors fairly close to the MPU for lower inductance.
DD
implementing
pin.
dynamic
voltage
positioning