L6238S datasheet(22/31 Pages) STMICROELECTRONICS

22 / 31 page

The high current flows from the grounded sub-

strate of the integrated circuit (p-type material),

through one or more epitaxial pockets (n-type ma-

terial) and out the center par pin.

This current can cause adverse operation of the

controllet due to substrate injection and might

possibility damage the internal metalization runs.

The normal current for this input is in the 200

µA

range.

Referring to figure 4-13, a simple protection

scheme consisting of a 1K resistor and a low cur-

rent Schottky diode should be added if the appli-

cation causes excessive current (i.e. >1mA) to

flow through the center tap pin.

5.0 PWM MOTOR CURRENT CONTROL

A unique feature of the L6238S in the optional

Pulse Width Modulation (PWM) control of motor

current.

Using Variable-frequency, Constant-OFF time

Current-Mode control, the L6238S can drive

higher power motors without the need for external

drivers, while minimizing internal power dissipa-

tion.

Additional benefits include reduced power supply

consumption (up to 50% savings) and lower watt-

age requirements for the current sensing resistor.

Constant-OFF time Current-Mode control, oper-

ates on the principle of monitoring the motor cur-

rent and comparison it to a reference or control

level.

When the motor current reaches this commanded

level, the output drivers turn OFF and remain

OFF for a Constant-OFF time. After this OFF time

the drivers turn back ON to repeat the cycle.

Figure 5.1 is a block diagram of the PWM control

circuitry. When using PWM as opposed to linear

control, two changes are made to the control

loop:

1.The slew rate control is disabled, allowing the

outputs to slew at a minimum rate of 10V/

µs.

This is accomplished by closing SW3 and

SW5.

2.The OTA amplifier is taken out of the control

loop via SW6. The lower drivers are now

driven into hard conduction by tying the gates

to the analog supply during the On time of the

PWM cycle.

The current in the motor windings is monitored via

the voltage dropped in the sensing resistor,

Rsense.

This voltage is multiplied by a factor of 4 in the

Current Sense Amplifier (CSA) and sent to nega-

tive input of the PWM Comparator (A2).

The control voltage, Vcontrol, is applied to the posi-

tive input of A2. When the output of the CSA

reaches a level that is equal to the commanded

level, the output of A2 switches low, toggling the

latch comprised of N1 and N2. This causes the

upper drivers to turn off and opens SW1. Q3 turns

OFF allowing the Constant-OFF time capacitors,

Islew

VPUMP

SW2

Cfet

UPPER A

PWM/SLEW

VPOWER

Islew

SW3

Cfet

OUTPUT

OUTA

SLEW RATE

REFERENCE

CURRENT

RSLW

RSENSE

CSA

VCTRL

LOWER A

D95IN318

VANALOG

3.1V

SW1

1.2V

SW5

SW4

PWM/LIN

CONTROL

FROM TRANS. LOOP

CSA

SW6

ANALOG

COFF

RSLEW

Figure 5-1

L6238S

22/31

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