PRELIMINARY DATASHEET

May, 2000

6

PRELIMINARY

ML4435

SOFT START

The voltage at SOFT START (pin 19) sets the current limit

threshold. The ML4435 has an internal voltage divider

with a 1.1V supply voltage. This circuit is shown in Figure

3. The divider consists of two 225k

Ω resistors setting

the current limit threshold to approximately 0.55V. An

external voltage divider off of VCC or an external reference

can be used to override the default setting of SOFT START

by using a divider with 10 times the current draw of the

internal divider.

RSENSE

The function of RSENSE is to provide a voltage propor-

tional to the motor current, for current limiting. The default

trip voltage across RSENSE is 0.6V as set by the SOFT

START (pin 19). The current sense resistor should be a

low inductance resistor such as a carbon composition. For

resistors in the milli ohms range wire wound resistors tend

to have low values of inductance. RSENSE can be selected

using Figure 4. The power rating of RSENSE should be

sized to handle the power dissipation (I MAX squared

times RSENSE) seen at maximum current.

ISENSE FILTER

An RC lowpass filter is required at the ISENSE input pin

to remove the voltage spike on the leading edge of the

current sense signal caused by the diode reverse recovery

shoot through current. Absent the filter, false triggering of

the current limit could occur.

The recommended starting values for this circuit are

R = 1K

Ω and C = 1000pF a configuration that will filter

out spikes less than 1

µs long. It is recommended that the

capacitor value not be increased beyond 330pF.

Pulse-By-Pulse Current Limiting

When current limit is activated by the voltage on ISENSE

exceeding the voltage on SOFT START the current limit is

tripped, turning off LA, LB, and LC for the remainder of

the PWM period.

COMMUTATION CONTROL

A 3 phase Brushless DC motor requires electronic com-

Figure 3. SOFT START Function

0.5V

.V

CSOFT START

SOFT START

225k

Ω

225k

Ω

9

A capacitor to ground on the SOFT START pin can be used

to provide a soft ramping of the current limit on power up.

The ramp time can be selected using Figure 4.

Figure 4. SOFT START Ramp Time vs. CSOFTSTART

C SOFT START

F

050

00

30

5

0

Figure 5. ISENSE Filter Wave Forms

BEFORE FILTERIN

AFTER FILTERIN

FUNCTIONAL DESCRIPTION