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參數資料
| 型號： | MC3392 |
| 廠商： | Motorola, Inc. |
| 英文描述： | LOW SIDE PROTECTED SWITCH |
| 中文描述： | 低邊保護開關 |
| 文件頁數： | 7/12頁 |
| 文件大小： | 198K |
| 代理商： | MC3392 |
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
μ
s. 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.
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