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參數(shù)資料
| 型號： | MC33293 |
| 廠商： | Motorola, Inc. |
| 英文描述： | QUAD LOW SIDE SWITCH |
| 中文描述： | 四路，低邊開關 |
| 文件頁數(shù)： | 12/16頁 |
| 文件大小： | 236K |
| 代理商： | MC33293 |
MC33293A
12
MOTOROLA ANALOG IC DEVICE DATA
Output-On open-load Fault
The MC33293A always checks for an open-load on the
outputs whether the outputs are ON or OFF. An output-on
open-load Fault is detected if an open-load exists when the
output is ON (corresponding input at a logic high state). The
output-on open-load Fault detection occurs when the load
current is less than the minimum Output-On Open-Load
Detect current (IOon(th)), specified in this data sheet. The
value of IOon(th) is, typically, 75 mA at room temperature.
See Figure 3.
The minimum load resistance value that the MC33293A
will interpret as an output-on open-load (Ropen(on)) is a
function of; the Output-On Open-Load Detect current
(IOon(th)); the load supply voltage (Vload); and the resistance
of the output (RDS(on)), as shown below.
Ropen(on) = [Vload / IOon(th)] –
RDS(on)
≈
Vload / IOon(th)
Using Equation 1 for the steady state case,
when: Vload = 14 V
RDS(on) = 0.3
IOon(th) = 75 mA
an output-on open-load Fault will be detected and reported
whenever Rload
≥
187
.
Each output has an output-on open-load fault detect circuit
that performs real time load current monitoring. Load current
is monitored immediately after any output is turned ON.
Since it takes a finite amount of time for load current to begin,
the MC33293A detects an output-on open-load Fault from
the time the output is turned ON until the load current
exceeds the Output-On Open-Load Detect current (IOon(th)).
It is important to note that a fault will notbe reported at the
Fault Status output during this short period of time. This is
due to the built-in output-on open-load Fault Status Delay
Time (toson), see Figure 3. This delay time is incorporated in
the MC33293A to mask the reporting of a false output-on
open-load Fault at the Fault Status output. The delay is
typically 2.2 ms.
The purpose for the tos(on) delay is to prevent false fault
reporting, especially when driving inductive loads. The load
inductance causes a current lag when the load is turned ON.
The normal current lag of an inductive load could be
misinterpreted as an open-load if it weren’t for the built-in
delay. This delay or masking is accomplished internally with
a single timer which resets every time any input switches from
a low-to-high logic state. An output-on open-load Fault will be
reported by the Fault Status output as a result of turning ON
an output having an open-load Fault and the most recent
tos(on) is allowed to lapse after switching ON any input.
The time it takes the load current to reach IOon(th) is a
function of the load resistance (Rload); load inductance
(Lload); output on resistance (RDS(on)); load supply voltage
(Vload); and the turn-on time (ton) as shown below. The value
of ton is comprised of the low-to-high Vin propagation delay
time (ton(dly)), and the output voltage rise time (tr).
See Figure 2.
ton(false fault) = –
τ
ln [(IOon(th)– Iload)/
(– Iload)] + ton
(1)
where:
τ
= Lload / Rload = time constant
Iload = Vload / [Rload + RDS(on)]
ton = ton(dly) + tr
(2)
(3)
(4)
(5)
Using Equation
2 for the transient case,
when: Vload = 14 V
RDS(on) = 0.3
Lload = 10 mH
Rload = 14
IOon(th) = 75 mA
an output-on open-load Fault will be detected, but not
reported after initial turn ON for a duration of 57
μ
s + ton.
Output-Off open-load Fault
The MC33293A checks for open-loads on the outputs
regardless of an output being on or off. An output-off
open-load Fault is detected if an open-load exists when the
output is turned OFF (corresponding input at a logic low
state). When any one of the four outputs are turned OFF, an
independent internal current source tied to each output tries
to pull a small amount of zero input voltage drain current
(IDS(off), typically 23
μ
A), through the load. If, while this zero
input voltage drain current is being pulled through the load,
the output voltage is less than the output-off open-load
Detect Threshold Voltage (VOoff(th), typically 3.7 V), an
output-off open-load Fault will be detected.
The zero input voltage drain current could be provided by
a large external resistor connected from the output to ground.
However, if an external resistor were used to provide this
zero input voltage drain current, only “opens” resulting from
open-loads or output to ground shorts could be detected. The
external resistor could notguarantee detection of an open
resulting from an output wire bond failure internal to the
MC33293A. Because the current source is provided
internally, open loads, output to ground shorts, and loss of
output wire bonds will all be detected.
The value of load resistance that will be detected as an
output-off open-load (Ropen(off)), is a function of the zero input
voltage drain current (IDS(off)); the oad supply voltage (Vload);
and the output-off open-load Detect Threshold Voltage
(VOoff(th)), as shown next by:
[Vload – VOoff(th)]
IDS(off)
Using Equation 6 for the steady state case,
when: Vload = 14 V
IDS(off) = 23
μ
A
VOoff(th) = 3.7 V
an output-off open-load Fault will be detected and reported
whenever RL
≥
448 k
.
Each output has an output-off open-load fault detect circuit
that performs real time output voltage monitoring. Output
voltage is monitored immediately after any output is turned
off. A finite amount of time is required for output voltage to
rise. The MC33293A detects an output-off open-load Fault
from when an output is turned off until the output voltage
exceeds the output-off open-load Detect Threshold Voltage
(VOoff(th)). It is important to note a fault will notbe reported at
the Fault Status output during this rise time. This is due to the
built-in output-off open-load Fault Status Delay Time, tos(off),
see Figure 4. This delay time is incorporated in the
MC33293A to delay the reporting of an output-off open-load
Fault at the Fault Status Output. The delay is typically 19
μ
s.
(6)
Ropen(off) =
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