2961

HIGH-CURRENT

HALF-BRIDGE

PRINTHEAD/MOTOR DRIVER

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

BB

V

V

SENSE

+

-

REF

CIRCUIT

BB

V

Dwg. EP-037

DRIVE CURRENT

RECIRCULATION (SLOW-DECAY MODE)

RECIRCULATION (FAST-DECAY MODE)

APPLICATIONS INFORMATION

The UDN2961B/W is a high current

half-bridge designed to drive a number of

inductive loads such as printer solenoids,

stepper motors, and dc motors. Load current

is sensed internally and is controlled by

pulse-width modulating (PWM) the output

driver(s) in a fixed off-time, variable-

frequency format. The peak current level is

set by the user’s selection of a reference

voltage. A slow current-decay mode

(chopping only the source driver) or a fast

current-decay mode (chopping both the

source and sink drivers) can be selected via

the MODE pin.

PWM CURRENT CONTROL

A logic low on the MODE pin sets the

current-control circuitry into the slow-decay

mode. The RS flip-flop is set initially, and

both the source driver and the sink driver are

turned ON when the INPUT pin is at a logic

low. As current in the load increases, it is

sensed by the internal sense resistor until the

sense voltage equals the trip voltage of the

comparator. At this time, the flip-flop is reset

and the source driver is turned OFF. Over the

current trip point transfer function is a direct

linear function of the reference voltage:

To ensure an accurate chop current level

(

±10%), an external 2940 Ω ±1% resistor

will be slightly higher than the trip point

(especially for low-inductance loads) because

of the internal logic and switching delays

(typically 1.5

µs). After the source driver turns

OFF, the load current decays, circulating

through an external ground clamp diode, the

load, and the sink transistor. The source

driver’s OFF time (and therefore the magni-

tude of the current decrease) is determined

by the one-shot’s external RC timing compo-

nents:

within the range of 20 k

Ω to 100 kΩ and 100 pF to 1000 pF. When the

one-shot times out, the flip-flop is set again, the source driver is re-

enabled, and the load current again is allowed to rise to the set peak

value and trip the comparator. This cycle repeats itself, maintaining the

average load current at the desired level.

ENABLE

INPUT

MODE

LOAD

CURRENT

RC

RC/2

I

TRIP

Dwg. WP-015