NCP5318 ON Semiconductor, NCP5318 Datasheet - Page 29

• Manufacturer Part Number: NCP5318
• Description: Two/Three/Four-Phase Buck CPU Controller
• Manufacturer: ON Semiconductor

10. Current Limit Setting
block diagram) exceeds the voltage on the I
will latch off. For inductive sensing, the I
should be set based on the inductor’s maximum resistance
(R
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
voltage should be set based on the maximum value of the
sense resistor.
voltage at the I
R
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
20 kW, and C
compensation would require V
faster than ideal RC time constant, there is an overshoot of
50% and the overshoot decays with a 200 ms time constant.
With this compensation, the I
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
CSx
When the output of the current sense amplifier (COx in the
For the overcurrent protection to avoid false tripping, the
LMAX
x C
step with fast RC time constant (50 ms/div)
). The design must consider the inductor’s
CSx
time constant is set faster than the L
CSx
LIM
= 0.01 mF. In this case, ideal current signal
pin should be set even higher if the
LIM
CSx
pin threshold must be set
to be 31 k. Due to the
L
SENSE
= 1.6 mW, R
LIM
LIM
), the I
pin, the part
pin voltage
O
/ R
LIM
http://onsemi.com
CSx
L
time
pin
=
29
where:
from the R
1.0 V. This allows the user to determine the resistor divider
above by:
Where R
V ILIM + (I RIPP−P (2
The proper I
This voltage can be programmed by a resistor divider
When the NCP5318 is powered up, the R
Figure 37. Programming the Current Limit
I
R
g
I
#PH
T
OS
R2 = R
R1 = R
L
RIPP−P
TOTAL
L
L
(T L * 25))
ILIM
OSC
R
OSC
TOTAL
TOTAL
LIM
pin, as shown in Figure 37.
= maximum converter current (A)
= maximum 25°C sense element
= maximum current sense to I
= peak−to−peak phase ripple current (A)
= number of phases
= inductor temperature at overload (°C)
= maximum I
is determined as in Section 1 above.
resistance (W)
(see tabulated specs)
(see tabulated specs) (V)
pin voltage can be calculated by:
I
LIM
x V
− R2
g ) OS ILIM
#PH) ) I L )
LIM
LIM
/ 1.0 V
R1
R2
offset
R L
(1 ) 0.004
OSC
LIM
pin will be
gain