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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
conditions, the V DS should be less than the voltage
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 .
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
typical), and the V DS is limited to V BAT + V BE + V GSON to
prevent excessive power dissipation in the MOSFET. The
load voltage is limited to V BE + V GSON and allows a
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
When pin 7 is in the low state, the MOSFET is off. If V BAT
is present, the gate voltage is discharged by the 110mA
INPUT
PIN 7
LOAD C2
CONDITION SOURCE OUTPUT
VOLTAGE COMP
STATUS
PIN 6
TIMER
PIN 8
current source. In the case of a V BAT disconnection, a self
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
Low
Normal
<V PIN2
High
Low
Low
disconnection, input pin 7 has no effect on the gate voltage,
which is maintained below 0.9V. In this case, the status pin is
Low
Short Circuit
to V BAT
>V PIN2
Low
High
Charge by
10 μ A
high. Low leakage current at pin 2 (10mA max) allows the
operation with the MOSFET and MC33198 pin 2 permanently
source
connected to the battery. V CC and other functions can be
High
High
Normal
Short to GND
or Overload
>V PIN2
<V PIN2
Low
High
High
Low
Low
Charge by
10 μ A
switched off from the main battery line. See Figure 18 .
PWM OPERATION
Since the MC33198 charge pump can deliver a high
source
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,
33198
Analog Integrated Circuit Device Data
12
Freescale Semiconductor
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