LM1951T National Semiconductor, LM1951T Datasheet - Page 8

• Manufacturer Part Number: LM1951T
• Description: IC SWITCH SOLID ST 1A TO220-5
• Manufacturer: National Semiconductor
• Type: High Side Switchr

Specifications of LM1951T

• Number Of Outputs: 1
• Internal Switch(s): Yes
• Current Limit: 1A
• Voltage - Input: 4.75 ~ 26 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Through Hole
• Package / Case: TO-220-5
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Rds (on): -
• Other names: *LM1951T

Application Hints
When inductive loads are turned OFF they produce a nega-
tive voltage spike The LM1951 contains a voltage clamp
that limits these spikes to approximately
ternal clamp is not necessary in most applications
Loads with an inductance of greater than 1H driven to full
output current may damage the clamp simply by exceeding
the power capabilities of the LM1951 An LM1951 can dissi-
pate 25W continuous at 25 C ambient when mounted on a
large heatsink If the load current is limited to 800 mA the
sustained spike from an infinitely large inductance can be
handled Sustained spikes produced by higher currents and
high inductances will exceed the 25W limit
For inductances above 1H care should be taken to see that
the output current does not exceed a value that could dam-
age the clamp While 800 mA is acceptable for the device
running at 25 C ambient on a heatsink derate this current
for smaller heatsinks or higher ambient temperatures to limit
the junction temperature to 150 C Alternatively an external
clamp or resonating capacitor can be added to handle any
combination of load inductance load current and device
temperature This is especially important if the output cur-
rent is boosted such as the application shown in Figure 3 A
peak power of 750W could be developed in the internal
clamp if an inductive load is switched without external
clamping
Another case where the clamp’s power capability may be
exceeded is when driving a solenoid The inductance of a
solenoid is greatest when energized with the plunger pulled
in As the plunger is pulled out of the solenoid the induc-
tance goes down Under certain conditions of high solenoid
inductance and fast mechanical time constants the current
may actually increase when the solenoid is turned OFF
Since the energy stored in an inductor cannot change in-
stantaneously the current must increase to conserve ener-
gy when the inductance decreases This condition is traced
by observing the load current with a current probe and stor-
age oscilloscope
Load capacitances larger than 1 nF will slow rise and fall
times Inductive loads having a capacitive component larger
than 1 nF will also exhibit overshoot Furthermore ringing
b
30V thus an ex-
8
may be evident in a combination inductive capacitive load
or in an inductive load with supply decoupling capacitors in
the range of 100 nF to 1 F For fast rise and fall times and
minimum ringing with inductive loads a supply decoupling
capacitor of 10 nF and an output capacitor of 1 nF is recom-
mended These should be located as close to the IC pins as
possible
The error flag is an open collector output that pulls low un-
der certain fault conditions These errors include overvolt-
age (V
(I
circuit to supply and junction temperature greater than
150 C By connecting a 2 k
output to a 5V supply a logic output to a microprocessor is
provided
The error flag can give seemingly false indications in a num-
ber of situations Slewing large capacitive loads (
can drive the LM1951 into temporary current limit produc-
ing a momentary error indication Incandescent lamps and
DC motors require an inrush current that will also cause a
temporary current limit and error indication Large inductive
loads (
error flag The error flag pulses for about 1
load is turned ON since the output is initially at ground In
microprocessor systems these false indications are easily
ignored in software In discrete logic circuits utilizing a latch
at the error flag output some filtering may be required
An internal current sink (10
the input pin 5 If this pin is left open it is guaranteed to pull
low switching the LM1951 OFF This characteristic is im-
portant under certain fault conditions such as when the con-
trol line fails open cirucit
Although the input threshold has hysteresis the switch
points are derived from a very stable band-gap reference In
many applications such as Figures 5 and 7 the LM1951
input can replace an extenal reference and comparator
The input (pin 5) is clamped at
resistance of approximately 30 k
tive inputs of up to 1 mA without affecting the performance
of the chip
OUT k
CC l
l
50 mH) initially appear as open circuits falsing the
2 mA) output short circuit to ground output short
26V) overcurrent (I
A minimum) is connected to
b
resistor from the error flag
OUT l
0 7V and includes a series
This pin tolerates nega-
1 3A) undercurrent
s when any
l
100 nF)