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參數(shù)資料
| 型號(hào)： | UCC2882- |
| 廠商： | Texas Instruments, Inc. |
| 英文描述： | Average Current Mode Synchronous Controller With 5-Bit DAC |
| 中文描述： | 平均電流模式同步控制器，5位DAC |
| 文件頁(yè)數(shù)： | 8/14頁(yè) |
| 文件大?。?/td> | 579K |
| 代理商： | UCC2882- |
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8
UCC2882/-1
UCC3882/-1
Choosing R
SENSE
to Set the Current Limit
R
SENSE
is chosen to limit the maximum (short circuit)
current of the power supply. The short circuit current
equation for the UCC3882 is:
1.37V
RSENSE 16
ISC =
and therefore, the value of the sense resistor, for a cho-
sen short circuit current is:
1.37V
ISC 16
RSENSE =
The short circuit current limit does vary slightly as a func-
tion of the switching regulator’s output inductor value and
operating frequency because a high value of ripple cur-
rent will reduce the average short circuit current limit.
Figure 5 shows the variation in Isc given common values
for the UCC3882. The UCC3882 is nominally configured
so that a 0.005m
resistor will set the current limit to ap-
proximately 17A.
The UCC3882 incorporates short circuit current foldback,
as shown in Figure 6. When the output of the power sup-
ply is short circuited, the output voltage falls. When the
output voltage reaches 1/2 of its nominal voltage (COM-
MAND/2) then the output current is reduced. This feature
reduces the amount of current in the MOSFETs and ca-
pacitors, and insures high reliability.
Choosing VDRVLO, VDRVHI and VIN
The UCC3882 requires a nominal 12V input supplied at
VIN. VDRVLO and VDRVHI can be set to any voltage
less than 18.5V, and may be set individually. A power
supply deriving its power from +5V should use +12V at
the VDRVHI pin, but may use either +5V or +12V de-
pending on the drive requirements of the synchronous
low-side MOSFET. A power supply deriving its power
from +12V should use +18V at VDRVHI in order to pro-
vide adequate voltage (6V) gate drive to the high-side
MOSFET. VIN must be less than +15V.
Input Capacitors
The input capacitors are chosen primarily based on their
switching frequency RMS current handling capability and
their voltage rating. The input capacitors must handle vir-
tually all of the RMS current at the switching frequency,
even if the circuit does not have an input inductor. The
switching current in the input capacitors appears as
shown in Figure 7.
Aluminum or tantalum capacitors can be used. The
amount of RMS current in an Electrolytic capacitor has a
strong impact on the reliability and lifetime of the capaci-
tor. Other factors which affect the life of an input capaci-
tor are internal heat rise, external airflow, the amount of
time that the circuit operates at maximum current and
the operating voltage. The curves in Figure 8 show the
RMS current handled by the total input capacitance in
typical VRM circuits powered from 5V or from 12V.
APPLICATION INFORMATION
0
100
200
300
400
500
600
700
800
10
15
20
25
RT [kW]
F
1.0nF
1.2nF
1.8nF
2.7nF
2.2n
3.9nF
5.6nF
Figure 3. Programming UCC3882 oscillator
frequency.
Figure 2. Buck vs. synchronous buck regulator.
UDG-97049
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