參數(shù)資料
型號(hào)： CS5166
廠商： ZF Electronics Corporation
英文描述： 5-Bit Synchronous CPU Controller with Power-Good and Current Limit
中文描述： 5位同步CPU，具有功率控制器，準(zhǔn)確和電流限制
文件頁(yè)數(shù)： 15/22頁(yè)
文件大?。?/td> 435K
代理商： CS5166
15
rating of the external power components as these are the
first to fail during an overload condition. The MOSFET
continuous and pulsed drain current rating at a given case
temperature has to be accounted for when setting the cur-
rent limit trip point. For example the IRL 3103S (D
2
PAK)
MOSFET has a continuous drain current rating of 45A at
V
GS
= 10V and T
C
= 100
C. Temperature curves on MOS-
FET manufacturers’ data sheets allow the designer to
determine the MOSFET drain current at a particular V
GS
and T
J
(junction temperature). This, in turn, will assist the
designer to set a proper current limit, without causing
device breakdown during an overload condition.
For a 300MHz Pentium
II CPU the full load is 14.2A. The
internal current sense comparator current limit voltage
limits are: 55mV < V
TH
< 130mV. Also, there is a 29% total
variation in R
SENSE
as discussed in the previous section.
We select the value of the current sensing element (embed-
ded PCB trace) for the minimum current limit setpoint:
R
SENSE(MAX)
=
R
SENSE
×
1.29 =
R
SENSE
×
1.29 = 3.87m
R
SENSE
= 3m
We calculate the range of load currents that will cause the
internal current sense comparator to detect an overload
condition.
From the overcurrent detection data section (pg 3),
Nominal Current Limit Setpoint
V
TH(TYP)
= 76mV.
I
CL(NOM) =
Maximum Current Limit Setpoint
Therefore , I
CL(NOM)
=
= 25.3A
V
TH(MAX)
= 110mV.
Therefore,
I
CL(MAX)
=
=
=
= 51.6A
Therefore, the range of load currents that will cause the
internal current sense comparator to detect an overload
condition through a 3m
embedded PCB trace is: 14.2A <
I
CL
< 51.6A, with 25.3A being the nominal overload condi-
tion.
There may be applications whose layout will require the
use of two extra filter components, a 510
resistor in series
with the I
SENSE
pin, and a 0.1μF capacitor between the
I
SENSE
and V
FB
pins. These are needed for proper current
limit operation and the resistor value is layout dependent.
This series resistor affects the calculation of the current
limit setpoint, and has to be taken into account when
determining an effective current limit.
The calculations below show how the current limit set-
point is determined when this 510
is taken into consider-
ation.
V
TRIP
= V
TH
+ (I
SENSE
×
R
ISENSE
) – (R
FB
×
I
FB
)
Where V
TRIP
= voltage across the droop resistor that trips
the I
SENSE
comparator
V
TH
= internal I
SENSE
comparator threshold
I
SENSE
= I
SENSE
bias current
R
ISENSE
= I
SENSE
pin 510
filter resistor
R
FB
= V
FB
pin 3.3K filter resistor
I
FB
= V
FB
bias current
Minimum current sense resistor (droop resistor) voltage
drop required for current limit when R
ISENSE
is used
V
TRIP(MIN)
= 55mV + (13μA
×
510) – (3.3K
×
1μA) = 55mV +
6.6mV – 3.3mV = 58.3mV
Nominal current sense resistor (droop resistor) voltage
drop required for current limit when R
ISENSE
is used
V
TRIP(NOM)
= 76mV + (30μA
×
510) – (3.3K
×
0.1μA) =
76mV + 15.3mV – 0.33mV = 90.97mV
Maximum current sense resistor (droop resistor) voltage
drop required for current limit when R
ISENSE
is used
V
TRIP(MAX)
= 110mV + (50μA
×
510) = 110mV + 25.5mV =
135.5mV
The value of R
SENSE
(current sense PCB trace) is then calcu-
lated:
R
SENSE(MAX)
=
= 4.1m
14.2A
R
SENSE(NOM)
=
=
= 3.18m
The range of load currents that will cause the internal cur-
rent sense comparator to detect an overload condition is as
follows:
Nominal Current Limit Setpoint
I
CL(NOM)
= V
TRIP(NOM)
/ R
SENSE(NOM)
Therefore,
I
CL(NOM)
= 90.97mV / 3.18m
= 28.6A
4.1m
m
1.29
R
SENSE(MAX)
1.29
58.3mV
110mV
3m
× 0.7
1
110mV
R
SENSE
× 0.7
1
110mV
R
SENSE(MIN)
76mV
3m
V
TH(TYP)
R
SENSE(NOM)
55mV
14.2A
V
TH(MIN)
I
CL(MIN)
Application Information: continued
C
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