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BA6870

3-phase motor driver

ROHM Semiconductor 

Motor driver ICs

BA6870S

(6) Series regulator

The BA6870S has a series regulator output pin. The IC

outputs a sink current according to the HIGH level output

voltage detected.

As shown in Fig. 7 the regulator circuit reduces the power

consumed by the IC by reducing the collector-to-emitter

(C-E) voltage of the driver transistors.

Nearly all the power dissipated by the IC is dissipated be-

tween the collectors and emitters of the output transis-

tors. More power is consumed as the C-E voltage in-

creases and as the output current increases.

The output transistor C-E voltage is equal to the differ-

ence between the supply voltage and the voltage applied

to the motor. Because the voltage across the motor de-

creases with decreasing drive current, the C-E voltage

must increase if the supply voltage is fixed.

Therefore, to improve the efficiency of the driver and to

prevent the power rating of the IC being exceeded, the

supply voltage must be varied in response to changes in

the output current. The supply voltage is decreased at

low current and increased at high current so that no ex-

cessive voltage is applied between the output transistor

collectors and emitters.

(7) Output-to-ground short-circuit detection

The motor output pins of the IC may be short-circuited to

the ground by some fault conditions. A short-circuited

output can destroy the output transistors because of ex-

cessive current, excessive voltage, or both. Even when

a short-circuit condition does not completely destroy the

device, it can still cause extreme overheating. To prevent

this, the BA6870S contains a short-circuit detection cir-

cuit that turns off the motor drive current if the output-to-

ground potential becomes abnormally low.

FOperation notes

The BA6870S has two thermal shutdown circuits (TSD1

and TSD2) to protect the IC. The typical shutdown tem-

peratures are 175_C for TSD1 and 215_C for TSD 2.

When the TSD1 is activated at an elevated chip tempera-

ture, the output pins (pins 1, 3, and 23) are set to the open

state. TSD1 is functional against excessive power dis-

sipation, output short-circuiting, and other irregularities in

the output current, but does not work against overheating

caused by high internal currents due to externally caused

IC damage or pin-to-pin short-circuiting.

When TSD2 is activated at a higher chip temperature, the

high- and low-side output transistors are turned on, and

the internal resistance between the motor power supply

pin (pin 24) and the output ground pin (pin 2) drops to less

than 3Ω. The motor power supply current (IM) is then giv-

en by

VM (V)

IM =

RM+R2P+3 (Ω)

Where

IM is the motor supply current

VM is the motor supply voltage,

RM is the motor power supply output resistance,

R2P is the pin-2 resistance.

In your application, make sure to connect between the

motor power supply and pin 24 a circuit breaker that op-

erates at currents less than IM.

670

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