Datasheet
SLUS318B – APRIL 1999 – REVISED JANUARY 2002
16
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APPLICATION INFORMATION
current sensing
The UCC3626 includes a differential current-sense amplifier with a fixed gain of five, along with an absolute
value circuit. The current-sense signal should be low pass filtered to eliminate leading-edge spikes. In order to
maximize performance, the input impedance of the amplifier should be balanced. If the sense voltage must be
trimmed for accuracy reasons, a low-value input divider or a differential divider should be used to maintain
impedance matching, as shown in Figure 14.
UDG–01123
R
F
R
ADJ
R
F
R
S
C
F
R
F
R
ADJ
R
F
R
S
C
F
SNS_NI
SNS_I
(a)
SNS_NI
SNS_I
(b)
R
F
R
ADJ
<< R
F
Figure 14. (a) Differential Divider and (b) Low-Value Divider
With four-quadrant chopping, motor current always flows through the sense resistor. However, during the
flyback period the polarity across the sense resistor is reversed. The absolute value amplifier cancels the
polarity reversal by inverting the negative sense signal during the flyback time, see Figure 15. Therefore, the
output of the absolute value amplifier is a reconstructed analog of the motor current, suitable for protection as
well as feedback loop closure.
UDG–01124
IPHASE
+ BEMF –
VMOT
IOFF
ION
S1
S3 S5
S2
S4
S6
X5
Is
If
Im
Is
If
Im
Ip
Ip
5*Ip
Figure 15. Current Sense Amplifier Waveform