MC3392

7

MOTOROLA ANALOG IC DEVICE DATA

Over Temperature Shutdown

Internal Thermal Shutdown Circuitry is provided to protect

the MC3392 in the event the Operating Junction Temperature

(TJ) exceeds 150°C. Typically, Thermal Shutdown will occur

at 160

° to 170°C. The thermal shutdown sense element is

embedded within the output PNP (Q4) in order to afford very

fast thermal coupling of Q4 to the sense element. Any rise in

temperature due to the ambient is translated directly to Q4

and the sense element. If the junction temperature rises

excessively above 150

°C, the Thermal Shutdown circuit will

turn ON, quickly pulling the gate of Q2 to ground, which pulls

the base of Q4 to ground, turning it OFF. In addition, the

Thermal Shutdown circuit simultaneously turns Q5 ON and

with a suitable pull–up resistor at the Fault pin reports the

presence of a fault (logic low). The output PNP will remain

OFF until the junction temperature decreases to within the

operating range at which time Thermal Shutdown turns OFF,

ceasing to hold the gate of Q2 low, turning Q4 back ON. This

process will repeat as long as the thermal over load exists.

This mode of operation is a nondestructive safety feature of

the device and will correct itself real time when the cause of

over temperature is removed. A continued over temperature

condition will thermally Pulse Width Modulate (PWM) the

output and Fault and may be incorrectly interpreted as an

oscillating load if one does not consider the simultaneous

performance of the Fault pin.

Current Limit

The MC3392 protects itself against Vout to +VS hard

shorts as well as any over current conditions by reducing the

magnitude of output current (IO) to that of the short circuit

current limit value (ISC). When the output current monitored

by Q3 tries to exceed ISC, the Current Limit circuit lowers the

gate voltage of Q2, lowering the base of Q4, causing the load

current through Q4 to diminish. Simultaneously, when the

load current exceeds ISC, Q5 will turn ON reporting a fault

condition. If the output current is allowed to remain

excessively high for the degree of heatsinking incorporated,

and the junction temperature of the device is allowed to heat

beyond 150

°C, the Thermal Shutdown circuit will activate and

the output will thermally PWM. Again, these modes of

operation are safety features of the MC3392 and are not

destructive.

Overvoltage Detect

This circuitry protects the MC3392 from Vout voltages in

excess of 16 V by lowering the output current to a

nondestructive value. With increasing Vout voltage (16 V <

Vout < 45 V) the load current is reduced to below that of ISC

and produces a fold back current effect. As Vout increases in

excess of 16 V, the output current decreases linearly until

Vout exceeds 45 V. With an infinite heatsink and Vout > 45 V,

IO will be less than 100 mA. For the other extreme, no

heatsink and Vout > 45 V, IO can be expected to be less than

about 400 mA. This behavior of IO in relation to Vout is shown

in Figure 13.

For the infinite heatsink case, the output current initially

increases with increased voltage until Vout exceeds 16 V,

thereafter the behavior is expressed as,

IO = ISC [1–(Vout –16 V) / 30 V]

Beyond 45 V, IO is limited to less than 100 mA. Anytime the

Overvoltage Detect circuit is activated, the gate of Q5 is

pulled low causing Q5 to turn ON to report the fault at the

Fault pin.

Inductive Load Clamp

The MC3392 has an internal inductive load clamp for

protection against flyback voltages imposed on the output pin

in excess of 70 V. The incorporated zener clamp can quickly

dissipate up to 60 milli–Joules of inductive flyback energy.

Figure 14 shows the maximum inductive load versus load

current that the clamp can handle safely. As an example

(using Figure 14), if operating the MC3392 to drive a 0.33 H

inductor, the maximum load current should be adjusted to

600 mA or less. If the load current is too high for the inductor

used, some series resistance can be added to the load to limit

the current. If this is not possible, an external clamp must be

used to facilitate handling the higher energy. When using an

external clamp, the external clamp voltage must be less than

60 V so as to override the internal clamp. The output clamp

offers protection for the output when the MC3392 is in the

OFF state. During the ON state, other protection features

(Overvoltage, Current, and Temperature) are available to

protect the output.

Fault Logic

The Fault is comprised of an internal open drain FET

requiring an external pull–up resistor. Typically, a 5.0 k

pull–up resistor to a +5.0 V supply is satisfactory. The Fault

pin is afforded a minimum of 2000 V ESD protection (Human

Body Model) by virtue of the 7.2 V zener diode. The Fault will

report a fault (logic low state) whenever the MC3392

experiences a fault condition. Conditions producing a fault

are: IO > 1.3 A (over current/shorted load); TJ > 150°C (over

temperature); and Vout > 16 V (overvoltage).

If the device goes into Thermal Shutdown, caused by

environmental overheating (not resulting from another fault

condition), the Fault and Vout will thermally PWM as the

MC3392 repeatedly heats to shut off, cools, and again turns

on. If a current limit fault causes the device to go into Thermal

Shutdown, the output will oscillate while the Fault remains

pulled low. There is no thermal hysteresis designed in to

control the PWM effect and this fault mode of operation is not

destructive.

Fast Turn–Off

This circuitry enhances the MC3392 turn–off performance.

Whenever Vin goes to a logic low state, Vout is held in an OFF

state for approximately 15

µs. During fast turn–off, less than

30 mA of current is allowed to flow producing an abrupt

turn–off. This turn–off characteristic can be seen in Figure 12,

a photograph of the typical turn–off waveform.