Datasheet

UC1625

UC2625

UC3625

SLUS353C –NOVEMBER 2003–REVISED JUNE 2013

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Power Stage Design

The UC3625 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 9 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

, through which the

transistor's currents are passed (Figure A, Figure B, and Figure C of Figure 9). In these cases, R

is not needed.

The low-side circulating diodes go to ground and the current sense terminals of the UC3625 (I

SENSE1

and I

SENSE2

)

are connected to R

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 kΩ

and matched.

The current that flows through R

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

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

in Figure D of

Figure 9) to measure current in the low-side circulating diodes, and operate in four quadrant mode (pin 22 high).

The negative voltage across R

is corrected by the absolute value current sense amplifier. Within the limitations

imposed by Table 1, the circuit of Figure B can also sense average current.

Product Folder Links: UC1625 UC2625 UC3625