33198 datasheet(13/17 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

33198

FUNCTIONAL DESCRIPTION

INTRODUCTION

the switch off time can be long, compared to the on-switching

time response. This is due to the 110mA gate discharge

current. To improve this parameter, a resistor can be added

in parallel with the gate of the MOSFET. See Figures 16 and

17.

Figure 16. Schematic with RGATE Resistor

Figure 17. RGATE Signal Comparison

This resistor will reduce (in some way) the charge pump

output voltage available for the MOSFET, but the device will

still provide enough Gate-to-Source voltage to maintain the

MOSFET “on” in good conditions. The resistor will mainly act

as an additional discharge current, which will reduce the

switch off time of the overall application. See the Table 6,

Switching Off Characteristics with MOSFET Additional Gate

Resistor and Figure 15, which show the pin 4 voltage

depending on the additional gate resistor and the off

switching time due to this resistor.

If a very low switching time is needed, the resistor has to

be an extremely low value, resulting in low gate voltage not

high enough to ensure proper MOSFET operation. In this

case, a logic level MOSFET can be used. Logic levels will

operate with VGS of 5.0V, with the same performance as a

standard MOSFET having a 12V VGS. Care should be taken

regarding the maximum gate to source voltage of a logic level

MOSFET. An additional zener might be necessary to prevent

gate oxide damage.

REVERSE BATTERY

The device does not sustain reverse battery operation for

VCC voltages greater than - 0.6V in magnitude. In application,

pin 5 should be protected from reverse battery by connecting

a diode in series with the VBAT line.

Figure 18. 33198 Reverse Battery

Vbat

LOAD

INPUT SIGNAL PIN7

Vcc + 15V typ

Vgate WITHOUT Rgate

Vgate WITH Rgate

Toff

Table 6. Switching Off Characteristics with MOSFET

Additional Gate Resistor

RGATE (RG)

VCC (V)

VGATE (V)

TOFF (µsec)

No R

7.0

450

700

750

780

68 k

Ω

7.0

160

230

220

39 k

Ω

7.0

100

160

150

15 k

Ω

7.0

17.5

28.5

Notes

Time from negative edge of input signal (Pin 7) to negative

edge of gate voltage (Pin 4) measured at 5V threshold.

Gate discharge time, not LOAD switching OFF time.L

TMOS used is Freescale MTP50N06, load 10

Ω resistor.

7 INPUT

6 STATUS

Vbat

LOAD

Rdrn

GATE

1 K

DRN

SOURCE 1

TIMER

GND

VCC

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