UC1625-SP

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SLUSAG8A

– SEPTEMBER 2011 – REVISED SEPTEMBER 2011

DEVICE INFORMATION

Terminal Functions

TERMINAL

DESCRIPTION

NAME

NO.

The position decoder logic translates the Hall signals and the DIR signal to the correct driver

signals (PUs and PDs). To prevent output stage damage, the signal on DIR is first loaded

into a direction latch, then shifted through a two-bit register.

As long as SPEED-IN is less than 250 mV, the direction latch is transparent. When

SPEED-IN is higher than 250 mV, the direction latch inhibits all changes indirection.

SPEED-IN can be connected to TACH-OUT through a filter, so that the direction latch is only

transparent when the motor is spinning slowly, and has too little stored energy to damage

power devices.

Additional circuitry detects when the input and output of the direction latch are different, or

DIR, SPEED-IN

6, 7

when the input and output of the shift register are different, and inhibits all output drives

during that time. This can be used to allow the motor to coast to a safe speed before

reversing.

The shift register ensures that direction can not be changed instantaneously. The register is

clocked by the PWM oscillator, so the delay between direction changes is always going to be

between one and two oscillator periods. At 40 kHz, this corresponds to a delay of between

25

μs and 50 μs. Regardless of output stage, 25 μs deadtime should be adequate to ensure

no overlap cross-conduction. Toggling DIR causes an output pulse on TACH-OUT

regardless of motor speed.

E/A IN(+) and E/A IN(

–) are not internally committed to allow for a wide variety of uses. They

can be connected to the ISENSE, to TACH-OUT through a filter, to an external command

voltage, to a D/A converter for computer control, or to another op amp for more elegant

feedback loops. The error amplifier is compensated for unity gain stability, so E/A OUT can

be tied to E/A IN(

–) for feedback and major loop compensation.

E/A IN(+), E/A IN(

–), E/A

1, 28, 27, 26

E/A OUT and PWM In drive the PWM comparator. For voltage-mode PWM systems, PWM In

OUT, PWM IN

can be connected to RC-OSC. The PWM comparator clears the PWM latch, commanding

the outputs to chop.

The error amplifier can be biased off by connecting E/A IN(

–) to a higher voltage than /EA

IN(+). When biased off, E/A OUT appears to the application as a resistor to ground. E/A OUT

can then be driven by an external amplifier.

GND

15

All thresholds and outputs are referred to the GND pin except for the PD and PU outputs.

The three shaft position sensor inputs consist of hysteresis comparators with input pullup

resistors. Logic thresholds meet TTL specifications and can be driven by 5-V CMOS, 12-V

CMOS, NMOS, or open-collectors.

Connect these inputs to motor shaft position sensors that are positioned 120 electrical

degrees apart. If noisy signals are expected, zener clamp and filter these inputs with 6-V

H1, H2, H3

8, 9, 10

zeners and an RC filter. Suggested filtering components are 1 k

Ω and 2 nF. Edge skew in

the filter is not a problem, because sensors normally generate modified gray code with only

one output changing at a time, but rise and fall times must be shorter than 20

μs for correct

tachometer operation. Motors with 60 electrical degree position sensor coding can be used if

one or two of the position sensor signals is inverted.

The current sense amplifier has a fixed gain of approximately two. It also has a built-in level

shift of approximately 2.5 V. The signal appearing on ISENSE is:

differential signal applied can be as high as

±0.5 V before saturation.

If spikes are expected on ISENSE1 or ISENSE2, they are best filtered by a capacitor from

ISENSE to ground. Filtering this way allows fast signal inversions to be correctly processed

ISENSE1, ISENSE2,

by the absolute value circuit. The peak-current comparator allows the PWM to enter a

3, 4, 5

ISENSE

current-limit mode with current in the windings never exceeding approximately 0.2 V /

off until the high current condition is removed. It is often essential to use some filter driving

ISENSE1 and ISENSE2 to reject extreme spikes and to control slew rate. Reasonable

starting values for filter components might be 250-

Ω series resistors and a 5-nF capacitor

between ISENSE1 and ISENSE2. Input resistors should be kept small and matched to

maintain gain accuracy.

This input can be used as an over-voltage shut-down input, as a coast input, or both. This

OV-COAST

23

input can be driven by TTL, 5-V CMOS, or 12-V CMOS.

Copyright

© 2011, Texas Instruments Incorporated

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