M63155FP

Rev.1.0, Sep.16.2003, page 19 of 40

9. Current Decay Method (DS)

The current decay method is determined by the input into the DS terminal. In slow-decay mode, only the high side

MOS FET is switched open during a PWM OFF (Low) cycle. The fast-decay mode switches both the high and low

side MOS FETs.

Table 1. gives the DS selection truth table.

TABLE 1.

DS Selection Truth Table

DS

Function Mode

High

Slow-Decay

Low

Fast-Decay

The output MOS FETs are controlled by PWM signal as follows.

I1

I2

D2

Q1

Q2

Q3

Q4

Q5

Q6

VM

RS

(1)Condition: Q1 is ON and Q4 is ON.

(PWM ON period)

The motor current I1 goes to RS through the

transistors Q1 and Q4.

(2)Condition: Q4 is ON and Q2, Q1 are OFF.

(PWM ON -> OFF switching period)

The discharge current I2 goes through the

diode D2. This diode is a parasitic diode of the

output power FET.

(3)Condition: Q4 is ON and Q2, Q1 are OFF.

(PWM OFF period)

The discharge current I2 keeps going through

the diode D2. Q2 keeps being OFF.

(4)Condition: Q4 is ON and Q2, Q1 are OFF.

(PWM OFF -> ON switching period)

Likewise state (3), the discharge current I2

keeps going through the diode D2. Q2 keeps

being OFF.

a) Slow-Decay Function

b) Fast-Decay Function

(1)Condition: Q1 is ON and Q4 is ON

(PWM ON period)

The motor current I1 goes to RS through the

transistors Q1 and Q4.

(2)Condition: Q1, Q4, Q2 and Q3 are OFF.

(PWM ON -> OFF switching period)

The discharge current I2 goes through the

diode D2 and D3. This diode is a parasitic diode of

the output power FET.

(3)Condition: Q1, Q4, Q2 and Q3 are OFF.

(PWM OFF period)

The discharge current I2 keeps going through

the diode D2 and D3.

(4)Condition: Q1, Q4, Q2 and Q3 are OFF.

(PWM OFF -> ON switching period)

Likewise state (2), the discharge current I2 goes

through the diode D2 and D3.

* When all the output power FETs are OFF, for example as

the phase change, the discharge current goes to VM through

these parasitic diodes.

I1

Q1

Q2

Q3

Q4

Q5

Q6

VM

RS

I2

D2

D3

Fig. 9 Current Decay Method at the MOS FETs Control with PWM Signal