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| 型号： | AN-53 |
| 厂商： | Fairchild Semiconductor Corporation |
| 元件分类： | DC/DC变换器 |
| 英文描述： | Implementing an RC5051 DC-DC Converter on Pentium II Motherboards |
| 中文描述： | 推行RC5051的DC - DC转换器的奔腾II主板 |
| 文件页数： | 14/20页 |
| 文件大小： | 143K |
| 代理商： | AN-53 |
AN53
APPLICATION NOTE
14
rent , you should limit the junction temperature to less than
130
°
C. You can find the required thermal resistance using the
equation for maximum junction temperature:
Assuming that the ambient temperature is 50
°
C,
Thus, you need to provide a heat sink that gives the low-side
MOSFET a thermal resistance of 38
°
C/W or lower to protect
the device during an indefinite short.
In summary, with proper heat sink, the low-side MOSFET is
not over-stressed during an over-current condition.
Schottky Diode Selection
The application circuit diagram of Figure 3 shows a Schottky
diode, D2. D2 is used as a flyback diode to provide a current
path for the inductor current during the dead-time when both
the high-side and low-side MOSFETs are briefly both turned
off. Table 11 shows the characteristics of several Schottky
diodes. Note that MBRB2515L has a very low
forward voltage drop even at high current. Although it is not
necessary to use a high-current diode for this application,
selecting a higher current schottky will provide improved
efficiency at slightly higher cost.
Table 11. Schottky Diode Selection Table
Output Filter Capacitors
Correct calculation of the output capacitance is crucial to the
performance of the DC-DC converter. The output capacitor
determines the overall loop stability, output voltage ripple,
and, most importantly, load transient response. Because the
control loop response of the controller is not instantaneous,
the initial load transient must be supplied entirely by the out-
put capacitors. The initial voltage deviation is determined by
the total ESR of the capacitors used and the parasitic resis-
tance of the output traces. For a detailed analysis of capacitor
requirements in a high-end microprocessor system, please
refer to Application Bulletin 14.
Input Filter
The DC-DC converter may include an input inductor
between the system +5V supply and the converter input
as described below. This inductor serves to isolate the +5V
supply from the noise in the switching portion of the DC-DC
converter, and to limit the inrush current into the input capac-
itors during power up. A value of 2.5
m
H is typical, as illus-
trated in Figure 13; details on selection of an input inductor
may be found in Applications Bulletin AB-16.
The number of input capacitors required for a converter is
determined by the capacitors’ ripple current rating. The
ripple current is given by:
Thus, for example, a Deschutes processor running at 2.0V
out from 5.0V in has a DC = 2.0/5.0 = .4; if it pulls 14.2A, its
I
rms
= 7A.
Table 12 shows some typical input capacitors’ current rat-
ings; the current rating increases as temperature decreases.
Although exceeding these ratings will not cause capacitor
damage, it will reduce their life, and thus the converter’s
MTBF.
Figure 13. Typical Input Filter
Table 12. Input Capacitor Selection Guide
Manufacturer
Model #
Motorola
1N5817
Motorola
1N5820
Motorola
MBR2015CTL
Motorola
MBRB2515L
Conditions
IF = 1A; Tj = 25
°
C
Forward
Voltage V
F
<.45v
IF = 3A; Tj = 25
°
C
<.475v
IF = 20A; Tj = 25
°
C
IF = 20A;Tj = 150
°
C
IF = 19A; Tj = 70
°
C
< 0.58v
< 0.48v
< 0.28v
P
D
T
-------------------------------
T
A
–
R
Q
JA
=
R
Q
JA
T
-----------P
T
A
–
D
130
50
–
2.1
38
°
C W
=
=
=
Manufacturer
Sanyo
Part #
Irms
10MV1200GX
2.0A @
65
°
C
1.2A @
105
°
C
1.2A @
105
°
C
United
Chemicon
Panasonic
LXZ10VB122M10X20
EEUFA10122
I
rms
I
OUT
DC
DC
2
–
=
1000
μ
F, 10V
Electrolytic
0.1
μ
F
65-5051-09
5V
Vin
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