Full-Bridge PWM Gate Driver

A89505

7

Allegro MicroSystems

955 Perimeter Road

Manchester, NH 03103-3353 U.S.A.

www.allegromicro.com

FUNCTIONAL DESCRIPTION

Device Operation

The A89505 is designed to operate DC motors. The output

drivers are capable of 50 V with gate-driver capability for an all

N-channel external MOSFET H-bridge. Control logic includes

synchronous rectification to reduce power dissipation. Cur-

rent limit is regulated by fixed off-time pulse-width-modulated

(PWM) control circuitry.

Internal PWM Current Control

Peak current is regulated by monitoring the voltage on an exter-

nal sense resistor.

I=

PEAK

V

REF

(10 × R)

SENSE

When the peak current is exceeded, the source driver turns off

for a fixed period tOFF to chop the current. The outputs operate

in mixed-decay mode during tOFF. Refer to the Fixed Off-Time

Setting section to set tOFF.

The internal current-sense circuit is ignored for tBLANK after

PWM transitions. The comparator output is blanked to prevent

false overcurrent detection due to reverse recovery currents of the

clamp diodes, or switching transients related to the capacitance of

the load, or both. Refer to the Blank-Time Setting section to set

tBLANK.

Brake

It is important to note that the internal PWM current-control

circuit will not limit the current when braking, since the current

does not flow through the sense resistor. The maximum current

can be approximated by VBEMF / RMOTOR. Care should be taken

to ensure that the maximum ratings of the external MOSFET are

not exceeded in worst-case braking situations of high speed and

high inertial loads.

ISET

A resistor from ISET terminal to ground sets the magnitude of

the gate current. The sink and source current ratios are fixed at

approximately 2-to-1 where the pull-down current is approxi-

mately two times the pull-up current. RISET should be between

15 and 150 kΩ.

The formula for determining the gate drive is:

I

(mA) =

GATE_HS

900

R(k )

ISET

1.9 +

I

(mA) = 3.5 +

GATE_LS

1700

R(k )

ISET

RC

The RC terminal is used to set both fixed off-time and blank-time

for the internal PWM current control. Refer to the following three

sections to select RC component values.

Fixed Off-Time Setting

The internal PWM current-control circuitry uses a one-shot to

control the time the drivers remain off. The one-shot off-time

(tOFF) is determined by the selection of an external resistor and

capacitor connected from the RC timing terminal to ground. The

off-time, over a range of values of CRC = 470 to 1500 pF and

RRC = 12 to 100 kΩ, is approximated by:

t

OFF

RC

RC

= R× C+ dead time

Blank-Time Setting

This circuit blanks the output of the current-sense comparator

when the outputs are switched by the internal current-control

circuitry or by an external PWM chop command. The comparator

blanking time, tBLANK, is determined by the selection of an exter-

nal resistor and capacitor connected from the RC timing terminal

to ground, and is approximated by:

t

BLANK

RC

= 2.6 µs × C (nF) × e

(3.6/R

(k))

RC

Slow Decay

In slow-decay mode, the low-side switch stays on and the

high-side switch turns off. Due to the synchronous rectification

feature, the complementary low-side switch turns on after a dead-

time.

Fast Decay SR

In fast-decay mode, the high-side and low-side switches turn off,

and the complementary pair of switches is turned on, effectively

reversing the voltage polarity across the motor winding.