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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： | 10/20页 |
| 文件大小： | 143K |
| 代理商： | AN-53 |
AN53
APPLICATION NOTE
10
When using these formulae, special care must be taken
regarding the MOSFETs’ transition times: the rise and fall
refer to the MOSFETs’ drain-source voltage, NOT the gate-
source. Using the datasheet values (rather than measured val-
ues) can also result in serious overestimation of the losses,
since the transition is being driven by an inductive source,
not a resistor.
Selecting the Inductor
The inductor is one of the most critical components to be
selected for a DC-DC converter application. The critical param-
eters of the inductor are its inductance (L), maximum DC cur-
rent (I
O
), and DC coil resistance
(R
l
).
The inductor’s inductance helps determine two key parame-
ters of a converter, its ripple current and its transient
response. On the one hand, making the inductance large
reduces the ripple current, and thus the output ripple voltage.
On the other hand, a large inductance provides a slow
response to load transients. For Pentium II supplies, the tran-
sient response is paramount, and thus the inductance is typi-
cally chosen to be in the 1-5
m
H range.
Most inductors’ inductance also depends on current, that is,
increasing the current through the inductor decreases the
inductance. It is thus vital to specify the DC current when
procuring an inductor. The one type of inductor which does
not change inductance with current is the rod-core inductor,
but this type may have significant EMI (noise) problems. For
further information, refer to Applications Bulletin AB-12.
The resistance of the winding of the inductor is also impor-
tant, as it is directly responsible for much of the losses in the
inductor. Minimizing the resistance will help improve the
converter’s efficiency.
Implementing Over-current Protection
Intel currently requires all power supply manufacturers to
provide continuous protection against short circuit condi-
tions that may damage the CPU. To address this requirement,
Fairchild Semiconductor has implemented a current sense
methodology to limit the power delivered to the load in the
event of over-current. The voltage drop created by the output
current across a sense resistor is presented to one terminal of
an internal comparator with hysteresis. The other comparator
terminal has the threshold voltage, nominally of 120mV.
Table 7 states the limits for the comparator threshold of the
Switching Regulator.
Table 7. RC5051 Over-current Comparator
Threshold Voltage
When designing the external current sense circuitry, pay
careful attention to the output limitations during normal
operation and during a fault condition. If the over-current
protection threshold current is set too low, the DC-DC con-
verter may not be able to continuously deliver the maximum
CPU load current. If the threshold level is too high, the out-
put driver may not be disabled at a safe limit and the result-
ing power dissipation within the MOSFETs may rise to
destructive levels. The following is the design equation used
to set the over-current threshold limit:
Where I
pk
is defined as in Figure 7, and I
load, max
= maximum
output load current. Figure 7 illustrates the inductor current
waveform for the RC5051 DC-DC converter at maximum load.
Figure 7. Typical DC-DC Converter Inductor
Current Waveform
The calculation of the ripple current is as follows:
I
2
where:
V
IN
= input voltage to converter,
T
S
= the switching period of the converter = 1/f
S
, and
f
S
= switching frequency.
As an example, for an input voltage of 5V, output voltage of
2.8V @ 14A, L equal to 1.3
m
H and a switching frequency of
285KHz (using C
EXT
= 100pF), the peak inductor current can
be calculated as :
Therefore, the over-current detection threshold must be at
least 16A. The next step is to determine the value of the
sense resistor. Including sense resistor tolerance, the sense
resistor value can be determined as
Where TF = Tolerance Factor for the sense resistor. Table 8
describes tolerance, size, power capability, temperature coef-
ficient and cost of various type of sense resistors.
Short Circuit Comparator
V
threshold
(mV)
120
100
140
Typical
Minimum
Maximum
I
PK
I
LOAD MAX
=
I
2
-------------------
+
t
I
T
s
T
ON
I
PK
I
MIN
T
OFF
I
LOAD, MAX
65-AP53-04
-------------------
V
----------2L
S
V
OUT
–
V
IN
-V
=
I
PK
14A
2.8V
1.3
m
H
–
′
2
285kHz
′
5V
′
+
15.7A
=
=
R
SENSE
V
I
SC
1
(
TF
+
)
-----------------------------
=
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