ML4425

PRODUCT SPECIFICATION

12

REV. 1.0.2 7/2/01

Cross Conduction Comparator

When the ML4425 goes from align mode into ramp mode,

there is a possibility of cross conduction in phase 3 of the

bridge power stage. This cross conduction can happen when

HC is on in the align mode shown as state R in Table 1, and

the controller transitions to state A in ramp mode where HC

is turned off and LC is turned on. Cross conduction can

appear due to the differences in turn on and turn off times of

the power devices. To solve this problem, the LC output

shown in Figure 10.

Braking

When the BRAKE pin is pulled below 1.4V, the low side

output drivers LA, LB, and LC are turned on and the high

side output drivers HA, HB, HC are turned off. Braking

causes rapid deceleration of the motor and current limiting is

de-activated, and care should be taken when using the

BRAKE pin. BRAKE is has an internal 4k

Ω pull-up as

shown in Figure 10, and can be driven by a switch to ground,

an open collector or drain logic signal, or a TTL logic signal.

Undervoltage Lockout

Undervoltage lockout is used to protect the 3-phase bridge

output on the UV FAULT pin. Undervoltage lockout also

turns off all output drivers (LA, LB, LC, HA, HB, and HC).

The comparator that triggers undervoltage lockout has

150mV of hystresis.

Design Considerations

Interfacing to a 3-Phase Bridge Power Stage

The ML4425 output drivers are configured to drive a 3 phase

bridge power stage. For applications with buss voltages from

12V up to 80V, level shifting circuitry can be used to drive

higher voltage P-channel MOSFETS for the high side

switches as shown in Figure 11.

The most flexible configuration is to use high side drivers to

control N-Channel MOSFETs (or IGBTs) which allows

applications from less than 12V up to 600V. Figure 12 shows

the interface between the ML4425 and IR2118 high side

drivers from International Rectifier. This configuration is

capable of driving motors from busses of up to 320V. The

BRAKE pin can be pulsed prior to startup with an RC

circuit. This charges the bootstrap capacitors (C19, C20, and

C21) for the three high side drivers, allowing the reset phase

to operate normally. These capacitors must be sized so that

they stay sufficiently charged during the align mode. Refer to

AN-43 for additional applications information on the

ML4425.

Figure 10. Cross Conduction, Brake, and UVLO Circuits

2

4

3

9

10

HA

HB

HC

LA

LB

GATING

LOGIC

&

OUTPUT

DRIVERS

–

+

1.4V

9.5V

25

BRAKE

VDD

4k

Ω

14

VDD

28

GND

27

RREF

11

LC

REFERENCE

18

UV FAULT

7

VREF

FROM

COMMUTATION

STATE MACHINE

FROM

SPEED CONTROL LOOP

& CURRENT LIMIT

+

–