PBL3770ASOT datasheet(5/8 Pages) ERICSSON

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PBL 3770A

Functional Description

The PBL 3770A is intended to drive a

bipolar constant current through one

winding of a 2-phase stepper motor.

Current control is achieved through

switched-mode regulation, see figure 5

and 6.

Three different current levels and zero

current can be selected by the input

logic.

The circuit contains the following

functional blocks:

•

Input logic

•

Current sense

•

Single-pulse generator

•

Output stage

Input logic

Phase input. The phase input

determines the direction of the current in

the motor winding. High input forces the

current from terminal M

A to MB and low

input from terminal M

B to MA. A Schmitt

trigger provides noise immunity and a

delay circuit eliminates the risk of cross

conduction in the output stage during a

phase shift.

Half- and full-step operation is

possible.

Current level selection. The status of I

and I

1 inputs determines the current level

in the motor winding. Three fixed current

levels can be selected according to the

table below.

Motor current

High level

100%

Medium level

60%

Low level

20%

Zero current

The specific values of the different

current levels are determined by the

reference voltage V

R together with the

value of the sensing resistor R

The peak motor current can be

calculated as follows:

m = (VR • 0.080) / RS [A],

at 100% level

The motor current can also be

continuously varied by modulating the

voltage reference input.

Current sensor

The current sensor contains a reference

voltage divider and three comparators

for measuring each of the selectable

current levels. The motor current is

sensed as a voltage drop across the

current sensing resistor, R

S, and

compared with one of the voltage

references from the divider. When the

two voltages are equal, the compara-tor

triggers the single-pulse generator. Only

one comparator at a time is activa-ted by

the input logic.

Single-pulse generator

The pulse generator is a monostable

multivibrator triggered on the positive

edge of the comparator output. The

multivibrator output is high during the

pulse time, t

off , which is determined by

the timing components R

T and CT.

off = 0.69 • RT • CT

The single pulse switches off the

power feed to the motor winding,

causing the winding to decrease during

off .

If a new trigger signal should occur

during t

off , it is ignored.

Output stage

The output stage contains four

transistors and two diodes, connected in

an H-bridge. Note that the upper

recirculation diodes are connected to the

circuit externally. The two sinking

transistors are used to switch the power

supplied to the motor winding, thus

driving a constant current through the

winding. See figures 5 and 6.

Overload protection

The circuit is equipped with a thermal

shut-down function, which will limit the

junction temperature. The output current

will be reduced if the maximum permis-

sible junction temperature is exceeded.

It should be noted, however, that it is not

short circuit protected.

Operation

When a voltage V

MM is applied across

the motor winding, the current rise

follows the equation:

m = (VMM / R) • (1 - e

-(R • t ) / L )

R = Winding resistance

L = Winding inductance

= time

(see figure 6, arrow 1)

The motor current appears across the

external sensing resistor, R

S, as an

analog voltage. This voltage is fed

through a low-pass filter, R

CCC, to the

voltage comparator input (pin 10). At the

moment the sensed voltage rises above

the comparator threshold voltage, the

monostable is triggered and its output

turns off the conducting sink transistor.

The polarity across the motor winding

reverses and the current is forced to

circulate through the appropriate upper

protection diode back through the source

transistor (see figure 6, arrow 2).

After the monostable has timed out,

the current has decayed and the analog

Figure 4. Definition of terms.

| V

– V

1/2

off

f =

off

Normalized

D =

off

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