12

ML4425

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 driver is gated off until the HC is

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

W 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

indicated by a TTL low 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.

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

+

–

Figure 10. Cross Conduction, Brake, and UVLO Circuits