Datasheet

Power Stage Design
UC2625-EP
www.ti.com .................................................................................................................................................................................................. SLUS802 – MARCH 2008
The UC2625 is useful in a wide variety of applications, including high-power in robotics and machinery. The
power output stages used in such equipment can take a number of forms, according to the intended performance
and purpose of the system. Figure 8 show four different power stages with the advantages and disadvantages of
each.
For high-frequency chopping, fast recovery circulating diodes are essential. Six are required to clamp the
windings. These diodes should have a continuous current rating at least equal to the operating motor current,
since diode conduction duty-cycle can be high. For low-voltage systems, Schottky diodes are preferred. In higher
voltage systems, diodes such as Microsemi UHVP high voltage platinum rectifiers are recommended.
In a pulse-by-pulse current control arrangement, current sensing is done by resistor R
S
, through which the
transistor's currents are passed (Fig. A, B, and C). In these cases, R
D
is not needed. The low-side circulating
diodes go to ground and the current sense terminals of the UC2625 (I
SENSE1
and I
SENSE2
) are connected to R
S
through a differential RC filter. The input bias current of the current sense amplifier causes a common mode
offset voltage to appear at both inputs, so for best accuracy, keep the filter resistors below 2 k Ω and matched.
The current that flows through R
S
is discontinuous because of chopping. It flows during the on time of the power
stage and is zero during the off time. Consequently, the voltage across R
S
consists of a series of pulses,
occurring at the PWM frequency, with a peak value indicative of the peak motor current.
To sense average motor current instead of peak current, add another current sense resistor (R
D
in Fig. D) to
measure current in the low-side circulating diodes, and operate in four quadrant mode (pin 22 high). The
negative voltage across R
D
is corrected by the absolute value current sense amplifier. Within the limitations
imposed by Table 1 , the circuit of Fig. B can also sense average current.
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