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ML4425CS

Sensorless BLDC Motor Controller

Fairchild Semiconductor 

PRODUCT SPECIFICATION

ML4425

Current Limiting in the Power Stage

The current sense resistor (RSENSE) shown in Figure 1 regu-

lates the maximum current in the power stage and the BLDC

motor. Current regulation is accomplished by shutting off the

output drivers LA, LB, and LC for a fixed amount of time if

the voltage across RSENSE exceeds the current limit thresh-

old.

ILIMIT

The voltage on the ILIMIT pin sets the current limit threshold.

The ML4425 has an internal voltage divider from VREF that

sets a default current limit threshold of 2.3V (see Figure 2).

An external voltage divider referenced to VREF can be used

to override the default ILIMIT setting. The external divider

should have at least 10 times the current flow of the internal

divider.

RSENSE

The function of RSENSE is to provide a voltage proportional

to the motor current to set the current limit trip point. The

default trip voltage across RSENSE is 460mV, set by the

internal ILIMIT divider ratio. The current sense resistor

should be a low inductance resistor such as a carbon compo-

sition. For resistors in the milliohms range, wire-wound

resistors tend to have low values of inductance. RSENSE

should be sized to handle the power dissipation (IMAX2 ↔

RSENSE).

ISENSE Filter

The ISENSE RC lowpass filter is placed in series with the cur-

rent sense signal as shown in Figure 2. The purpose of this

filter is to remove the diode reverse recovery shootthrough

current. This current causes a voltage spike on the leading

edge of the current sense signal which may falsely trigger the

current limit. The current sense voltage waveform is shown

before and after filtering in Figure 3. The recommended

starting values for this circuit are R = 1kΩ and C = 330pF.

This gives a time constant of 330ns, and will filter out spikes

of shorter duration. C can be increased to as much as 2.2nF,

but should not exceed a time constant of more than a few

microseconds.

CIOS

When ISENSE exceeds 0.2 ↔ ILIMIT, the current limit one-

shot is activated, turning off LA, LB, and LC for a fixed

amount of time (tOFF). tOFF is set by the amount of capaci-

tance connected to CIOS. CIOS is usually set for a fixed off

time equal to or less than the PWM period. For a 25kHz

PWM frequency, the PWM period is 40µs; tOFF should be

between 20µs and 40µs. The lower limit of tOFF is dictated

by the minimum on time of the power stage; a safe approxi-

mation is 5µs or less. The equation for finding the CIOS

capacitance value is as follows:

COS = t--O----F----F-2---×.--4---5V---0----µ----A--

(1)

Commutation Control

A 3-phase BLDC motor requires electronic commutation to

achieve rotational motion. Electronic commutation requires

the switching on and off of the power switches of a 3-phase

half bridge. For torque production to be achieved in one

direction, the commutation is dictated by the rotor position.

Electronic commutation in the ML4425 is achieved by turn-

ing on and off, in the proper sequence, one N output from

one phase and one P output from another phase. There are

six combinations of N and P outputs (six switching states)

that constitute a full commutation cycle. These combinations

are illustrated in Table 1 and Figure 4, and are labeled states

A through F. This sequence is programmed into the commu-

tation state machine. Clocking of the commutation state

machine is provided by a voltage controlled oscillator

(VCO).

FROM

RSENSE

VREF

ISENSE

×5

–

+

VREF

16kΩ

ILIMIT

2.9V

0V

8kΩ

PWM

ON/OFF

SQ

RQ

STOP

START

CIOS

30µA

Figure 2. Current Sense Circuitry

460mV

0V

(a)

(b)

Figure 3. Current Sense Resistor Waveforms

(a) Without Filtering, and (b) With Filtering

REV. 1.0.2 7/2/01

7

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