A possible drawback of the scheme in

Figure 7 is slow tran-

sient recovery speed. Since the 5 k

Ω resistor and the 0.1 µF

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 µF capacitor, V

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

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

Ω

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

circuitry and in an LDO controller.

Figure 9 shows an applica-

tion where V

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

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