LM1950T National Semiconductor, LM1950T Datasheet - Page 4

• Manufacturer Part Number: LM1950T
• Description: IC SWITCH 750MA HI SIDE TO220-5
• Manufacturer: National Semiconductor
• Type: High Side Switchr

Specifications of LM1950T

• Number Of Outputs: 1
• Internal Switch(s): Yes
• Current Limit: 750mA
• 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: *LM1950T

Application Hints
HIGH CURRENT OUTPUT
The 750 mA output is fault protected against overvoltage If
the supply voltage rises above approximately 30V the out-
put will automatically shut down This protects the internal
circuitry and enables the IC to survive higher voltage tran-
sients than would otherwise be expected The LM1950 will
survive transients and DC voltages up to 60V on the supply
The output remains off during this time independent of the
state of the input logic voltage This protects the load The
high current output is also protected against short circuits to
either ground or supply voltage Standard thermal shutdown
circuits are employed to protect the LM1950 from over heat-
ing
FLYBACK RESPONSE
Since the LM1950 is designed to drive inductive as well as
any other type of load inductive kickback can be expected
whenever the output changes state from ON to OFF (See
Waveform on Figure 1 ) The driver output was left un-
clamped since it is often desirable in many systems to
achieve a very rapid decay in the load current In applica-
tions where this is not true such as in Figure 2 a simple
external diode clamp will suffice In this application the inte-
grated current in the inductive load is controlled by varying
the duty cycle of the input to the drive IC This technique
achieves response characteristics that are desirable for cer-
tain automotive transmission solenoids for example
For applications requiring a rapid controlled decay in the
solenoid current such as fuel injector drivers an external
zener and diode can be used as in Figure 3 The voltage
rating of the zener should be such that it breaks down be-
fore the output of the LM1950 The minimum output break-
down voltage of the IC output is rated at
to the supply voltage
FIGURE 2 Diode Clamp
b
54V with respect
TL H 11237 – 5
FIGURE 1
4
The LM1950 can be used alone as a simple relay or sole-
noid driver where a rapid decay of the load current is de-
sired but the exact rate of decay is not critical to the sys-
tem If the output is unclamped as in Figure 1 and the load
is inductive enough the negative flyback transient will cause
the output of the IC to breakdown and behave similarly to a
zener clamp Relying upon the IC breakdown is practical
and will not damage or degrade the IC in any way There are
two considerations that must be accounted for when the
driver is operated in this mode The IC breakdown voltage is
process and lot dependent Output clamp voltages ranging
from
countered over time on different devices This is not at all
critical in most applications An important consideration
however is the additional heat dissipated in the IC as a
result This must be added to normal device dissipation
when considering junction temperatures and heat sinking
requirements Worst case for the additional dissipation can
be approximated as
Where I
For solenoids where the inductance is less than ten milli-
henries the additional power dissipation can be ignored
Overshoot undershoot and ringing can occur on certain
loads The simple solution is to lower the Q of the load by
the addition of a resistor in parallel or series with the load A
value that draws one tenth of the current or DC voltage of
the load is usually sufficient
For frequency stability of the switch a 0 1 F or larger out-
put bypass capacitor is required
b
40V to
L
f
e
e
e
FIGURE 3 Zener Clamp for Rapid
Peak Solenoid Current (Amps)
Maximum Frequency Input Signal (Hz)
Solenoid Inductance (Henries)
Additional P
b
Controlled Current Decay
120V (with V
D
e
CC
I
2
x L x f(Watts)
supply of 14V) will be en-
TL H 11237 – 4
TL H 11237 – 6