3935

THREE-PHASE POWER

MOSFET CONTROLLER

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

10

Functional Description

Motor Lead Protection. A fault detection circuit monitors

the voltage across the drain to source of the external MOSFETs.

A fault is asserted “low” on the output terminal, FAULT, if the

drain-to-source voltage of any MOSFET that is instructed to turn

on is greater than the voltage applied to the V

When a high-side switch is turned on, the voltage from V

the appropriate motor phase output, V

motor lead is shorted to ground before the high side is turned on,

the measured voltage will exceed the threshold and the FAULT

terminal will go “low”. Similarly, when a low-side MOSFET is

turned on, the differential voltage between the motor phase

(drain) and the LSS terminal (source) is monitored. V

by a resistor divider to V

The V

side, drain-source monitor circuit. Voltage drops across the

power bus are eliminated by connecting an isolated PCB trace

from the V

This allows improved accuracy in setting the V

voltage. The low-side, drain-source monitor uses the LSS

terminal, rather than V

The A3935 merely reports these motor faults.

Fault Outputs. Transient faults on any of the fault outputs are

to be expected during switching and will not disable the gate

drive outputs. External circuitry or controller logic must

determine if the faults represent a hazardous condition.

FAULT. This terminal will go active “low” when any of the

following conditions occur:

V

V

V

Motor lead short-to-ground,

Motor lead short-to-supply (or battery),

Bridge (or V

V

Thermal shutdown.

OVFLT. Asserts “high” when a V

and resets “low” after a recovery hysteresis. It has a high-

impedance state when a thermal shutdown or V

occurs. The voltage at the OVSET terminal, V

the V

V

where K

V

variables must be in range and below maximum operating

specification.

UVFLT. Asserts “high” when a V

and resets “low” after a recovery hysteresis. It has a high-

impedance state when a thermal shutdown or V

occurs. OVFLT and UVFLT are mutually exclusive by defini-

tion.

Current Sensing. A current-sense amplifier is provided to

allow system monitoring of the load current. The differential

amplifier inputs are intended to be Kelvin connected across a

low-value sense resistor or current shunt. The output voltage is

represented by:

V

where V

and A

Shutdown. If a fault occurs because of excessive junction

temperature or undervoltage on V

outputs are driven “low” until the fault condition is removed. In

addition, the boost supply switch and the VREG are turned “off”

until those undervoltages and junction temperatures recover.

Boost Supply. V

control loop, and by an outer voltage-feedback loop. The

current-control loop turns “off” the boost switch for 5 µs

whenever the voltage across the boost current-sense resistor

exceeds 500 mV. A diode reverse-recovery current flows

through the sense resistor whenever the boost switch turns “on”,

which could turn it “off” again if not for the “blanking time”

circuit. Adjustment of this external sense resistor determines the

maximum current in the inductor. Whenever V

predefined threshold, nominally 16 V, the boost switch is

inhibited.