OPA564 Datasheet, PDF (15/35 Pages) Texas Instruments – 1.5A, 24V, 17MHz POWER OPERATIONAL AMPLIFIER

OPA564
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ADJUSTABLE CURRENT LIMIT
The OPA564 provides over-current protection to the
load through its accurate, user-adjustable current limit
(I_SET pin). The current limit value, I_LIM, can be set from
0.4A to 1.5A by controlling the current through the
I_SET pin. Setting the current limit does not require
special power resistors. The output current does not
flow through the I_SET pin.
A simple resistor to the negative rail is sufficient for a
general, coarse limit of the output current. Figure 30
exhibits the percent of error in the transfer function
between I_SET and I_OUT versus the current limit set
resistor, R_SET; Figure 31 and Figure 32 show how this
error translates to variation in I_OUT versus R_SET. The
dotted line represents the ideal output current setting
which is determined by the following equation:
1.2V
I_LIM @ 20000 x _{5000 + RSET}
(1)
The mismatch errors between the current limit set
mirror and the output stage are primarily a result of
variations in the ~1.2V bandgap reference, an internal
5kΩ resistor, the mismatch between the current limit
and the output stage mirror, and the tolerance and
temperature coefficient of the R_SET resistor
referenced to the negative rail. Additionally, an
increase in junction temperature can induce added
mismatch in accuracy between the I_SET and I_OUT
mirror. See Figure 53 for a method that can be used
to dynamically change the current limit setting using a
simple, zero drift current source. This approach
simplifies the current limit equation to the following:
I_LIM @ 20,000 ´ I_SET
(2)
The current into the I_SET pin is determined by the
NPN current source. Therefore, the errors contributed
by the internal 1.2V bandgap reference and the 5kΩ
resistor mismatch are eliminated, thus improving the
overall accuracy of the transfer function. In this case,
the primary source of error in I_SET is the R_SET resistor
tolerance and the beta of the NPN transistor.
It is important to note that the primary intent of the
current limit on the OPA564 is coarse protection of
the output stage; therefore, the user should exercise
caution when attempting to control the output current
by dynamically toggling the current limit setting.
Predictable performance is better achieved by
controlling the output voltage through the feedback
loop of the OPA564.
SBOS372E – OCTOBER 2008 – REVISED JANUARY 2011
Setting the Current Limit
Leaving the I_SET pin unconnected damages the
device. Connecting I_SET directly to V– is not
recommended because it programs the current limit
far beyond the 1.5A capability of the device and
causes excess power dissipation. The minimum
recommended value for R_SET is 7.5kΩ, which
programs the maximum current limit to approximately
1.9A. The maximum value for R_SET is 55kΩ, which
programs the minimum current limit to approximately
0.4A. The simplest method for adjusting the current
limit (I_LIM) uses a resistor or potentiometer connected
between the I_SET pin and V–, according to Equation 1.
If I_LIM has been defined, R_SET can be solved by
rearranging Equation 1 into Equation 3:
R_SET @
24kW
I
- 5kW
LIM
(3)
R_SET in combination with a 5kΩ internal resistor
determines the magnitude of a small current that sets
the desired output current limit.
Figure 37 shows a simplified schematic of the
OPA564 current limit architecture.
1.2V
Bandgap
I_OUT £ I_LIM
OPA564
5kW
I_SET
R_SET
( ) ^{ILIM @}
1.2V
R_CL + 5kW
´ 20k
1nF⁽¹⁾
(optional, for noisy
environments)
V-
(1) At power-on, this capacitor is not charged. Therefore, the
OPA564 is programmed for maximum output current. Capacitor
values > 1nF are not recommended.
Figure 37. Adjustable Current Limit
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