lm2637mx National Semiconductor Corporation, lm2637mx Datasheet - Page 11

• 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

Applications Information
Notice however, that the r
temperature coefficient and it can increase by as much as
50% when heated up. Also the distribution of the r
be fairly wide, a 1.25 to 1.5 ratio is not uncommon. Consult
the MOSFET vendor for further information on the distribu-
tion of r
The designer should carefully choose the value of R
that even under the extreme case (largest r
est temperature) the current limit will not trigger below the
preset value.
To provide the greatest protection over the high-side FET,
cycle-by-cycle protection is implemented. The sampling of
the V
Whenever an over-current condition is detected, the high-
side FET is immediately turned off and the low-side FET
turned on. This status remains for the rest of the cycle. The
same procedure applies to the next switching cycle. The
blanking time of 250 ns is to avoid the switching noise that
occurs whenever the FET is turned on.
The resistor between CS− pin and the switching node
(source of the high-side FET) is important for minimizing the
noise and negative voltage present at the CS− pin. A resis-
tance of 100Ω to 300Ω is recommended.
Method 2 — Current Sense Resistor
This method uses a sense resistor in series with the output
inductor to detect the load current. SeeFigure 5. The voltage
across the sense resistor is proportional to load current. In
the case that the sense resistor is of discrete type (i.e., not a
PCB etch resistor) or the sense resistor value is optimized
for dynamic voltage positioning (see the Dynamic Position-
ing of Load Voltage section), it may be necessary to use two
signal level resistors, R
desired current limit.
FIGURE 5. Current Limit via Current Sense Resistor
DS
DS_ON
starts as early as 250 ns after the FET is turned on.
.
1
DS_ON
and R
2
of the FET has a positive
to appropriately set the
DS_ON
(Continued)
DS_ON
and high-
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IMAX
can
so
11
For a given current limit value, the minimum R
determined by:
where V
55 mV, see the Electrical Characteristic table. For example,
for a 20A current limit, the minimum R
3 mΩ sense resistor is used instead, use appropriate values
of R
the voltage across R
The discrete current sense resistor usually has a very good
temperature coefficient and tolerance. A temperature coeffi-
cient of
sense resistors.
A PCB etch resistor can also be used as the R
advantage of that approach is flexible resistance, which will
result in minimum power loss. R
eliminated. The drawback is too high a temperature coeffi-
cient, typically +4000 ppm/˚C, which will result in a much less
accurate current limit than a discrete sense resistor. The
copper thickness of a PCB is usually of 5% tolerance.
Linear Section — There is no current limit function in the
linear controllers. However, if there is ever a severe over-
load, the output voltage may drop below 0.63V, in which
case the under-voltage latch-off will provide the protection.
DESIGN CONSIDERATIONS
Control Loop Compensation
Switching Section — A switching regulator should be prop-
erly compensated to achieve a stable operation, tight regu-
lation and good dynamic performance. For a synchronous
buck regulator that needs to meet stringent load transient
requirement such as that of processor core voltage supply, a
2-pole-1-zero compensation network should suffice, such as
the one shown in Figure 6 (C
because the ESR zero of the typical output capacitors is low
enough to make the control-to-output transfer function a
single-pole roll-off.
As an example, let us figure out the values of the compen-
sation network components in Figure 6. Assume the follow-
ing parameters: R = 20Ω, R
C = 7.5 mF, V
kHz. Notice R
the on resistance of the FET’s.
The control-to-output transfer function is:
The ESR zero frequency is:
The double pole frequency is:
±
5%. Vishay Dale and IRC offer a broad range of discrete
1
and R
±
OCP
30 ppm/˚C is typical. Tolerance is usually
2
to make the voltage across R
is the over-current trip voltage and is typically
L
IN
is the sum of the inductor DC resistance and
= 5V, V
SENSE
m
= 2V and PWM frequency = 300
L
is 60 mV.
= 20 mΩ, R
1
, C
1
2
, R
and R
SENSE
C
1
1
= 9 mΩ, L = 2 µH,
and R
to be V
2
is 2.75 mΩ. If a
may also be
www.national.com
SENSE
2
OCP
). This is
SENSE
±
1% or
when
. The
(4)
(5)
(6)
is