May, 2000

PRELIMINARY DATASHEET

5

PRELIMINARY

ML4435

COMPONENT SELECTION

Selecting external components for the ML4435 requires

calculations based on the motor’s electrical and mechani-

cal parameters. The following is a list of the motor param-

eters needed to for these calculations:

The maximum DC motor supply voltage V MOTOR (V)

The maximum operating current I MAX (A)

The winding resistance measured line to line Rl-l

Ω

The number of magnetic poles N (Unitless)

The Back EMF constant Ke (V s/RAD)

The torque constant K

τ (N m/A) of the motor (Kτ = Ke

in SI units)

The maximum speed of operation RPMMAX (RPM)

load

The viscous damping factor

ζ (Unitless) of the motor and

its load

If one or more of the above values is not known, it is

still possible to pick components for the ML4435, but

some experimentation may be necessary to determine the

optimal values. All quantities are in SI units unless other-

wise specified. The following formulas and component

selection graphs should be considered as a starting point

from which to optimize the application. All calculations

for capacitors and resistors should be used as the first

approximation for selecting the closest standard value.

SUPPLY VOLTAGE AND ON-CHIP VOLTAGE

REFERENCE

VCC

The supply voltage at VCC (pin 10) is nominally

12V ± 10%. A bypass capacitor of 0.1µF to ground as

close as possible to VCC (pin 10) is recommended.

RT

An internal 6V reference is generated inside the ML4435.

The reference appears on RT (pin 6). A resistor to ground

on RT sets the PWM frequency. This resistor can be

replace with a potentiomenter for use in setting the speed

command. This topic is discussed under the PWM SPEED

CONTROL section. Note: Buffer this pin with an op amp

with at least a 1M

Ω input impedance if external circuits

are necessary.

OUTPUT DRIVERS

The output drivers LA, LB, LC,

HA, HB, and HC provide

totem pole output drive signals for a 3 phase bridge power

stage. All control functions in the ML4435 translate to

outputs at these pins. LA, LB, LC provide the low side drive

signals for phases A, B, and C of the 3 phase power stage

and are 12V active high signals.

HA, HB, and HC provide

the high side signals for phases A, B, and C of the 3 phase

power stage and are 12V active low signals.

CURRENT LIMITING IN THE 3 PHASE BRIDGE

POWER STAGE

A current sense resistor RSENSE shown in Figure 1 is

installed in the 3 phase power stage to regulate the maxi-

mum current in the power stage and the BLDC motor.

Current regulation is accomplished by shutting off the

output drivers LA, LB, and LC for the remainder of the

PWM period if the voltage across RSENSE exceeds the

current limit threshold set by the SOFT START (pin 19).

The maximum power dissipated in RSENSE is shown in

Figure 2.

Figure 1. Current Limit with RSENSE

Figure 2. RSENSE Power vs. Motor Current

RSENSE

ISENSE

R

C

IMAX [MOTOR] A

02

4

60

2

3

4

5

6

0

FUNCTIONAL DESCRIPTION