LM2635MX National Semiconductor, LM2635MX Datasheet - Page 12

• Manufacturer Part Number: LM2635MX
• Description: IC REG SYNCH BUCK 5-BIT 20-SOIC
• Manufacturer: National Semiconductor

Specifications of LM2635MX

• Applications: Power Supplies
• Current - Supply: 2.5mA
• Voltage - Supply: 4.5 V ~ 5.5 V
• Operating Temperature: 0°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 20-SOIC (7.5mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LM2635MX; LM2635MXTR

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Applications Information
connected between the FB pin and ground to increase the
no load output voltage to close to the upper limit of the
window.
A possible drawback of the scheme in Figure 7 is slow
transient recovery speed. Since the 5 kΩ 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 mΩ
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.
REFERENCE VOLTAGE
The V
circuitry and in an LDO controller. Figure 9 shows an appli-
cation where V
An appropriate compensation network is necessary to tailor
the dynamic performance of the whole power supply.
FIGURE 8. Dynamic Voltage Positioning by Using A
REF
pin can have many uses, such as in the watchdog
REF
Stand-Alone Resistor
C
is used to build a N-FET LDO controller.
due to a less effective low pass filter.
(Continued)
core
CORE
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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 LM2635 directly underneath the MOS-
6. If possible, keep the capacitors some distance away
7. When
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.
them too short or else the LM2635 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
underneath the LM2635.
FIGURE 9. V
implementing
REF
Used in an N-FET LDO Supply
dynamic
voltage
positioning
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