LM2636 National Semiconductor, LM2636 Datasheet - Page 12

• Manufacturer Part Number: LM2636
• Description: 5-Bit Programmable Synchronous Buck Regulator Controller
• Manufacturer: National Semiconductor

www.national.com
Applications Information
A possible drawback of the scheme in Figure 7 is slow tran-
sient recovery speed. Since the 5 k 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
resistor in Figure 8 above. The advantage of this implemen-
tation over the previous one is a much faster speed of V
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.
FIGURE 8. Dynamic Voltage Positioning by Using A
Stand-Alone Resistor
C
due to a less effective low pass filter.
(Continued)
core
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CORE
may
12
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
2. Try to keep gate traces short. However, do not make
3. When not employing dynamic voltage positioning, place
4. Start component placement with the power devices such
5. Do not place the LM2636 directly underneath the MOS-
6. If possible, keep the capacitors some distance away
7. When
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.
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.
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.
as MOSFETs and inductors.
FETs when when surface mount MOSFETs are used.
from the inductors so that the capacitors will have a
lower temperature environment.
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.
FIGURE 9. V
REF
pin can have many uses, such as in the watchdog
REF
implementing
is used to build a N-FET LDO controller. An
REF
Used in an N-FET LDO Supply
dynamic
voltage
positioning
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