Analog Integrated Circuit Device Data

12

Freescale Semiconductor

33198

FUNCTIONAL DESCRIPTION

INTRODUCTION

In addition, the pin 8 capacitor value should be calculated

to allow inrush current.

SOURCE (PIN1) AND DRAIN (PIN2)

FUNCTIONALITY

These two pins are used to sense the MOSFET and load

conditions. Pin 2 is connected to the internal pull down

current source of 80mA and to the C2 comparator. An

external resistor connected between pin 2 and VBAT creates

a voltage drop across this resistor. The voltage drop will be

the MOSFET maximum acceptable drain to source voltage

and the C2 comparator threshold.

Pin 1 is connected to the MOSFET source pin. Two

different cases should be considered, MOSFET on or off :

When the MOSFET is on and working under normal

developed at pin 2. So, the C2 comparator output is low and

the status is high. No current will flow through the pin 8

capacitor.

If the MOSFET encounters an overload or if the load is

shorted to Gnd, the voltage at the source will cross the pin 2

voltage and go below this pin 2 voltage, thereby turning on

the C2 comparator. The comparator will pull the status pin 6

low and will enable the charge of the pin 8 capacitor. When

the voltage at the capacitor reaches 5.5V, the C3 comparator

will switch off the MOSFET by disabling the charge pump and

the 110mA current source. The MOSFET gate will be

discharged only by the 10mA current source. The MOSFET

is latched off and can be turned back on again by switching

the input pin 7 to 0 and back to 1.

When the MOSFET is off, we have the same scheme.

Under normal conditions, the load should pull the source

voltage to Gnd, thus C2 comparator output is high and status

pin pulled low. If the load is shorted to VBAT for instance,

source pin will be higher than pin 2, The C2 output

comparator is low and the status pin is high. This is

summarized in Table 5, Status Functionality.

OPERATION WITH INDUCTIVE LOADS

The device can drive the MOSFET during inductive loads

switching applications. In this case, a 1.0kW resistor should

be connected between source of the MOSFET and device pin

1. The resistor will limit the current flowing into pin 1 and

prevent MC33198 from damage while switching the inductive

load off. The gate voltage is internally clamped at - Vbe (0.6V

prevent excessive power dissipation in the MOSFET. The

reasonable discharge current.

SWITCHING ON

The MOSFET switching on is ensured by the internal

charge pump. The charge pump response time versus the

MOSFET gate capacitance is shown in Table 4, Dynamic

Electrical Characteristics.

SWITCHING OFF

Here two cases have to be discussed: the MOSFET

normally switching off, and the switching off under a fault

condition. The normal switching off is done by internal pull

down current sources. The value is 110mA and is in fact

composed of two current sources in parallel: a 100mA and a

10mA source. The 10mA current is always connected to the

gate pin 4 as shown in the Internal Block Diagram on page 2.

The 100mA source can be disabled. This is the case when

the MOSFET is switched off under fault conditions. The

device will disable the 100mA current source and the

MOSFET gate will be discharged only by a 10mA current.

The time required to switch off the MOSFET will be much

longer in this case, and will result in a lower overvoltage at the

MOSFET, especially when the device drives high inductive

loads.

OFF STATE OPERATION WITHOUT VCC

CONNECTION

is present, the gate voltage is discharged by the 110mA

sustaining 5.0mA pull down current source is incorporated in

the device, to ensure that the MOSFET gate capacitor is

discharged and tied below 0.9V. In case of a VCC

disconnection, input pin 7 has no effect on the gate voltage,

which is maintained below 0.9V. In this case, the status pin is

high. Low leakage current at pin 2 (10mA max) allows the

operation with the MOSFET and MC33198 pin 2 permanently

switched off from the main battery line. See Figure 18.

PWM OPERATION

Since the MC33198 charge pump can deliver a high

current, the MOSFET gate can be charged fast enough to

allow for PWM operations. The maximum PWM frequency is

dependent on the MOSFET itself and mainly its gate to the

source capacitor value. Depending on the PWM frequency,

Table 5. Status Functionality

INPUT

PIN 7

CONDITION

LOAD

SOURCE

VOLTAGE

C2

OUTPUT

COMP

STATUS

PIN 6

TIMER

PIN 8

Low

Normal

High

Low

Low

Low

Short Circuit

Low

High

Charge by

10

µA

source

High

Normal

Low

High

Low

High

Short to GND

or Overload

High

Low

Charge by

10

µA

source