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参数资料
| 型号： | LM2637MWC |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 1000 kHz SWITCHING FREQ-MAX, UUC |
| 封装： | WAFER |
| 文件页数： | 3/20页 |
| 文件大小： | 404K |
| 代理商： | LM2637MWC |
Applications Information (Continued)
For a given current limit value, the minimum R
SENSE is deter-
mined by:
(4)
where V
OCP is the over-current trip voltage and is typically
55 mV, see the
Electrical Characteristic table. For example,
for a 20A current limit, the minimum R
SENSE is 2.75 m.Ifa
3m
sense resistor is used instead, use appropriate values
of R
1 and R2 to make the voltage across R1 to be VOCP when
the voltage across R
SENSE is 60 mV.
The discrete current sense resistor usually has a very good
temperature coefficient and tolerance. A temperature coeffi-
cient of ±30 ppm/C is typical. Tolerance is usually ±1% or
±5%. Vishay Dale and IRC offer a broad range of discrete
sense resistors.
A PCB etch resistor can also be used as the R
SENSE. The
advantage of that approach is flexible resistance, which will
result in minimum power loss. R
1 and R2 may also be elimi-
nated. The drawback is too high a temperature coefficient,
typically +4000 ppm/C, which will result in a much less ac-
curate current limit than a discrete sense resistor. The cop-
per thickness of a PCB is usually of 5% tolerance.
Linear Section — There is no current limit function in the lin-
ear 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 regula-
tion and good dynamic performance. For a synchronous
buck regulator that needs to meet stringent load transient re-
quirement 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
1,C2,R1 and R2). This is be-
cause 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 compensa-
tion network components in
Figure 6. Assume the following
parameters: R = 20
,R
L = 20 m,RC = 9m,L = 2 H, C
= 7.5 mF, V
IN = 5V, Vm = 2V and PWM frequency = 300 kHz.
Notice R
L is the sum of the inductor DC resistance and the
on resistance of the FET’s.
The control-to-output transfer function is:
(5)
The ESR zero frequency is:
(6)
The double pole frequency is:
(7)
The corresponding Bode plots are shown in
Figure 7.
Notice since the ESR zero frequency is so low that the phase
doesn’t even go beyond 90. This makes the compensation
easier to do.
Since the DC gain and cutoff frequency (0 dB frequency) are
too low, some compensation is needed. Otherwise the low
DC gain will cause a poor line regulation, and the low cutoff
frequency may hurt transient response performance.
The transfer function for the 2-pole-1-zero compensation
network shown in
Figure 6 is:
(8)
where
(9)
One of the poles is located at origin to help achieve the high-
est DC gain. So there are three parameters to determine, the
position of the zero, the position of the second pole, and the
constant A. To determine the cutoff frequency and phase
margin, the loop bode plots need to be generated. The loop
transfer function is:
TF = TF1 x TF2
(10)
By choosing the zero close to the double pole position and
the second pole to half of the switching frequency, the closed
loop transfer function turns out to be very good.
DS100848-9
FIGURE 5. Current Limit via Current Sense Resistor
www.national.com
11
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