PBL 3776/1

5

Functional Description

Each channel of the PBL 3776/1

consists of the following sections:

•

An output section with four output

transistors, two sourcing and two

sinking, intended to drive the four

transistors in an external H-bridge.

Each transistor is capable of driving

up to 200 mA continuous current.

•

A logic section that controls the

output transistors.

•

An S-R flip-flop, and a comparator.

The clock-oscillator is common to

both channels.

Constant current control is achieved

by switching the output current to the

windings. This is done by sensing the

peak current through the winding via a

current-sensing resistor R

connected in series with the motor

winding. As the current increases, a

voltage develops across the sensing

resistor, which is fed back to the

comparator. At the predetermined level,

defined by the voltage at the reference

input V

flop, which turns off the sourcing output

transistor in the circuit. Consequently the

correspond-ing lower external transistor,

in the H-bridge, is turned off. The turn-off

of one channel is independent of the

other channel. The current decreases

until the clock oscillator triggers the flip-

flops of both channels simultaneously,

which turns on the output transistors

again, and the cycle is repeated.

To prevent erroneous switching due to

switching transients at turn-on, the PBL

3776/1 includes a digital filter. The clock

oscillator provides a blanking pulse

which is used for digital filtering of the

voltage transient across the current

sensing resistor during turn-on. Due to

the high output drive capability, this

transient might exceed the max. allowed

voltage on the C inputs and damage the

circuit. A resistor is placed in the

feedback loop in order to prevent this

transient from damaging the circuit.

The current paths during turn-on, turn-

off and phase shift are shown in figure 6.

Applications Information

Output current

The maximum peak output, sink/source,

current is 500 mA. But due to the power

handling capacity of the package this

Figure 5. Typical 5 A stepper motor driver application with PBL 3776/1. One channel shown.

current can only be used for a short

period of time (1mS). Recommended

max continuous output current is 200

mA/output transistor. This is practical

when driving MOS FET power

transistors, since a high peak output

current capability will rapidly charge/

discharge the gate capacitance, while

the continuous current usage is very

small.

Current control

The regulated output current level to the

motor winding is determined by the

voltage at the reference input and the

value of the sensing resistor, R

peak current through the sensing

resistor (and the motor winding) can be

expressed as:

I

[A]

With a recommended value of 0.1 ohm

for the sense resistor R

5 V referen-

ce voltage will produce an output current

of approximately 5 A. R

selected for maximum motor current.

Chopping frequency, winding inductance

and supply voltage also affect the

current, but to much less extent.

+5 V

+

RC

PBL

3776/1

S

R

Q

S

R Q

+

–

–

+

–

Phase 2

Phase 1

Dis

2

C2

SGND 2

Pwr GND2

T1BL

T1AU

T1AL

BB2

V

T2BU

T2AU

T2AL

T2BL

Dis1

C1

SGND 1

Pwr GND 1

24

18

23

21

20

22

19

16

17

14

15

12

13

10

11

89

7

1

2

4

5

3

6

390

Ω

270

Ω

270

Ω

390

Ω

0.11

Ω

R1

R2

R3

R4

Q1

270

Ω

390

Ω

R5

R6

R7

R8

Q3

Q4

IRFZ34

IRFZ34

Rs

Rt

Ct

12 k

Ω

4700 pF

Vmm

+

390

Ω

270

Ω

Q2

IRF9Z34

IRF9Z34

PHASE CH 2

DISABLE CH 2

REFERENCE VOLTAGE CH 2

R8

1k

Ω

T1BU

1000pF