Intersil HIP4086, Руководство пользователя

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Application Note 1829
8
AN1829.0
March 14, 2013
Please refer to the HIP4086 datasheet for additional application
information.
Current Monitor and Current Limit
There are two current control features in the HIP4086DEMO1Z. A
linear current monitor op amp, U2, amplifies the voltage across
R23 and R24. This op amp is configured as a true differential
amplifier to allow Kelvin connections across the current sensing
resistors (see Figure 11). R15 and R3, each 32.4k
Ω
, have a
Thevinen equivalent value that is the parallel value of R15 and
R3 (or 1/2 of 32.4k
Ω
). The Thevinen equivalent voltage also is
1/2 of the bias voltage of 5V. Consequently, the output of the
differential amplifier is offset by +2.5V (see Figure 12).
The current monitor output, I
MOTOR
, digitized by the
microcontroller, can be used to control the torque of the motor or
to limit the battery recharging current during regenerative
braking. Because of the offset voltage on the current monitor
output, signals above 2.5VDC represents positive motor current
and signals less that 2.5VDC represent negative motor current.
(Note that this hardware feature is provided for customer use but
is not implemented in the microcontroller firmware.)
The output voltage of the differential amplifier is:
where I
M
is the bridge current (motor current), R12||R14 =
R15||R3, and (R17+R21) = (R18+R22) (as required for the
differential amp topology).
Using the defaults values of the HIP4086DEMO1Z:
For 20A, Vout
CS
= 4.878V. For -20A, Vout
CS
= 0.122V.
The I
motor
signal is monitored by two comparators (see
Figure 13). The output of the upper U3 comparator is biased to
go low when the motor current > 20A. Conversely, the output of
the lower comparator is biased to go low when the motor current
is
≤ 20A.
The OR’ed outputs of these two comparators is monitored by the
microcontroller. Pulse-by-pulse current limiting is provided on
each negative transition. After 256 consecutive pulse limits, all
the bridge FETs are permanently turned off and the current limit
alarm LED (led3) is turned on.
There are two different methods to change the pulse-by-pulse
current limit. The easiest method is to change the value of the
current sensing resistors R23 and R24. For example, removing
R24 halves the pulse by pulse current limit to ± 10A while not
affecting the full scale I
motor
output signal.
Equation 3 calculates the value of the current sensing resistors to
set the pulse-by-pulse current limit at the desired level without
changing the full scale output voltage swing of the I
MOTOR
signal.
FIGURE 11. DIFFERENTIAL CURRENT MONITOR AMPLIFIER
FIGURE 12. THEVINEN EQUIVALENT DIFFERENTIAL AMPLIFIER
U2
R17 R21
R18 R22
R14
R12
R15
R3
5V
ISL28246FUZ
32.4k
FILTER CAPACITORS
ARE NOT SHOWN.
R23 R24
FROM
BRIDGE
0.015 0.015
I
MOTOR
+
-
Ω Ω
32.4k
Ω
32.4k
Ω
32.4k
Ω
511
Ω
511
Ω
511
Ω
511
Ω
U2
R17+R21
R18+R22
R12||R14
R15||R3
2.5VTHEV
ISL28246FUZ
Note that resistors labeled Rx||Ry
represent a parallel equivalent resistor
of Rx and Ry. Rx+Ry represents the
series combination of Rx and Ry.
R23||R24
FROM
BRIDGE
0.0075
I
MOTOR
+
-
1022
Ω
Ω
1022
Ω
16.2k
Ω
16.2k
Ω
FIGURE 13. PULSE-BY-PULSE CURRENT LIMIT COMPARATORS
(EQ. 1)
Vout
CS
=
[(R12||R14)) / (R17+R21)] x I
M
x (R23||R24)+ R3 / (R3+R15) x 5V
(EQ. 2)
Vout
CS
= [(16.2k
Ω
))/(1022)] x Im x (0.0075) + 32.4k
Ω
/(64.8k
Ω
) x 5V
or
Vout
CS
= 0.119 x I
M
+2.5V
U3
R4
R1
R12A
ISL28246FUZ
+
-
R38
5V
U3
R11
R39
R12B
ISL28246FUZ
+
-
R11B
5V
I
MOTOR
TO
MICROCONTROLLER
10k
Ω
10k
Ω
10k
Ω
10k
Ω
249
Ω
1M
Ω
1M
Ω
249
Ω