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參數(shù)資料
| 型號(hào)： | NCP5318FTR2 |
| 廠商： | ON SEMICONDUCTOR |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Two/Three/Four−Phase Buck CPU Controller |
| 中文描述： | SWITCHING CONTROLLER, 1000 kHz SWITCHING FREQ-MAX, PQFP32 |
| 封裝： | LQFP-32 |
| 文件頁(yè)數(shù)： | 29/31頁(yè) |
| 文件大?。?/td> | 384K |
| 代理商： | NCP5318FTR2 |
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NCP5318
http://onsemi.com
29
10. Current Limit Setting
When the output of the current sense amplifier (COx in the
block diagram) exceeds the voltage on the I
LIM
pin, the part
will latch off. For inductive sensing, the I
LIM
pin voltage
should be set based on the inductor’s maximum resistance
(R
LMAX
). The design must consider the inductor’s
resistance increase due to current heating and ambient
temperature rise. Also, depending on the current sense
points, the circuit board may add additional resistance. In
general, the temperature coefficient of copper is +0.39% per
°
C. If using a current sense resistor (R
SENSE
), the I
LIM
pin
voltage should be set based on the maximum value of the
sense resistor.
For the overcurrent protection to avoid false tripping, the
voltage at the I
LIM
pin should be set even higher if the
R
CSx
x C
CSx
time constant is set faster than the L
O
/ R
L
time
constant. A step load change may cause the current signal to
appear larger than the actual inductor current and trip the
current limit at a lower level than desired. The waveforms in
Figure 36 show a simulation of the current sense signal and
the actual inductor current during a positive step in load
current with values of L = 500 nH, R
L
= 1.6 m , R
CSx
=
20 k , and C
CSx
= 0.01 F. In this case, ideal current signal
compensation would require V
CSx
to be 31 k. Due to the
faster than ideal RC time constant, there is an overshoot of
50% and the overshoot decays with a 200 s time constant.
With this compensation, the I
LIM
pin threshold must be set
more than 50% above the full load current to avoid
triggering current limit during a large output load step.
Figure 36. Inductive sensing waveform during a load
step with fast RC time constant (50
s/div)
The proper I
LIM
pin voltage can be calculated by:
VILIM
(IRIPP
P(2
(TL
#PH)
IL)
RL
(1
0.004
25))
g
OSILIM
where:
I
L
R
L
= maximum converter current (A)
= maximum 25
°
C sense element
resistance ( )
= maximum current sense to I
LIM
gain
(see tabulated specs)
I
RIPP
P
= peak
to
peak phase ripple current (A)
#PH
= number of phases
T
L
= inductor temperature at overload (
°
C)
OS
ILIM
= maximum I
LIM
offset
(see tabulated specs) (V)
This voltage can be programmed by a resistor divider
from the R
OSC
pin, as shown in Figure 37.
g
Figure 37. Programming the Current Limit
R
OSC
I
LIM
R1
R2
When the NCP5318 is powered up, the R
OSC
pin will be
1.0 V. This allows the user to determine the resistor divider
above by:
R2 = R
TOTAL
x V
LIM
/ 1.0 V
R1 = R
TOTAL
R2
Where R
TOTAL
is determined as in Section 1 above.
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