Datasheet

R
SET
@
24k
I
W
LIM
- 5kW
I 20000 x
LIM
@
1.2V
5000 + R
SET
I 20,000
LIM
I
SET
@ ´
R
SET
OPA564
5kW
I
LIM
@
1.2V
R + 5kW
CL
( )
´ 20k
V-
I
SET
1.2V
Bandgap
I I
OUT LIM
£
1nF
(optional, for noisy
environments)
(1)
OPA564
www.ti.com SBOS372E –OCTOBER 2008– REVISED JANUARY 2011
Setting the Current Limit
ADJUSTABLE CURRENT LIMIT
Leaving the I
SET
pin unconnected damages the
The OPA564 provides over-current protection to the
device. Connecting I
SET
directly to V– is not
load through its accurate, user-adjustable current limit
recommended because it programs the current limit
(I
SET
pin). The current limit value, I
LIM
, can be set from
far beyond the 1.5A capability of the device and
0.4A to 1.5A by controlling the current through the
causes excess power dissipation. The minimum
I
SET
pin. Setting the current limit does not require
recommended value for R
SET
is 7.5kΩ, which
special power resistors. The output current does not
programs the maximum current limit to approximately
flow through the I
SET
pin.
1.9A. The maximum value for R
SET
is 55kΩ, which
A simple resistor to the negative rail is sufficient for a
programs the minimum current limit to approximately
general, coarse limit of the output current. Figure 30
0.4A. The simplest method for adjusting the current
exhibits the percent of error in the transfer function
limit (I
LIM
) uses a resistor or potentiometer connected
between I
SET
and I
OUT
versus the current limit set
between the I
SET
pin and V–, according to Equation 1.
resistor, R
SET
; Figure 31 and Figure 32 show how this
If I
LIM
has been defined, R
SET
can be solved by
error translates to variation in I
OUT
versus R
SET
. The
rearranging Equation 1 into Equation 3:
dotted line represents the ideal output current setting
which is determined by the following equation:
(3)
(1)
R
SET
in combination with a 5kΩ internal resistor
determines the magnitude of a small current that sets
The mismatch errors between the current limit set
the desired output current limit.
mirror and the output stage are primarily a result of
variations in the ~1.2V bandgap reference, an internal
Figure 37 shows a simplified schematic of the
5kΩ resistor, the mismatch between the current limit
OPA564 current limit architecture.
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:
(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
(1) At power-on, this capacitor is not charged. Therefore, the
the output stage; therefore, the user should exercise
OPA564 is programmed for maximum output current. Capacitor
caution when attempting to control the output current
values > 1nF are not recommended.
by dynamically toggling the current limit setting.
Figure 37. Adjustable Current Limit
Predictable performance is better achieved by
controlling the output voltage through the feedback
loop of the OPA564.
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