LMH6321 Datasheet, PDF (16/21 Pages) National Semiconductor (TI) – 300 mA High Speed Buffer with Adjustable Current Limit

Application Hints (Continued)
OUTPUT CURRENT AND SHORT CIRCUIT
PROTECTION
The LMH6321 is designed to deliver a maximum continuous
output current of 300 mA. However, the maximum available
current, set by internal circuitry, is about 700 mA at room
temperature. The output current is programmable up to 300
mA by a single external resistor and voltage source.
The LMH6321 is not designed to safely output 700 mA
continuously and should not be used this way. However, the
available maximum continuous current will likely be limited
by the particular application and by the package type cho-
sen, which together set the thermal conditions for the buffer
(see Thermal Management section) and could require less
than 300 mA.
The programming of both the sourcing and sinking currents
into the load is accomplished with a single resistor. Figure 3
shows a simplified diagram of the V to I converter and I_SC
protection circuitry that, together, perform this task.
Referring to Figure 3, the two simplified functional blocks,
labeled V/I Converter and Short Circuit Protection, comprise
the circuitry of the Current Limit Control.
The V/I converter consists of error amplifier A1 driving two
PNP transistors in a Darlington configuration. The two input
connections to this amplifier are V_CL (inverting input) and
GND (non-inverting input). If GND is connected to zero volts,
then the high open loop gain of A1, as well as the feedback
through the Darlington, will force C_L, and thus one end R_EXT
to be at zero volts also. Therefore, a voltage applied to the
other end of R_EXT will force a current
I_EXT = V_PROG/R_EXT
(3)
into this pin. Via this pin, I_OUT is programmable from 10 mA
to 300 mA by setting I_EXT from 25 µA to 750 µA by means of
a fixed R_EXT of 10 kΩ and making V_CL variable from 0.25V to
7.5V. Thus, an input voltage V_CL is converted to a current
I_EXT. This current is the output from the V/I converter. It is
gained up by a factor of two and sent to the Short Circuit
Protection block as I_PROG. I_PROG sets a voltage drop across
R_SC which is applied to the non-inverting input of error amp
A2. The other input is across R_SENSE. The current through
R_SENSE, and hence the voltage drop across it, is proportional
to the load current, via the current sense transistor Q_SENSE.
The output of A2 controls the drive (I_DRIVE) to the base of the
NPN output transistor, Q3 which is, proportional to the
amount and polarity of the voltage differential (V_DIFF ) be-
tween AMP2 inputs, that is, how much the voltage across
R_SENSE is greater than or less than the voltage across R_SC.
This loop gains I_EXT up by another 200, thus
I_SC = 2 x 200 (I_EXT) = 400 I_EXT
(4)
Therefore, combining Equations (3) and (4), and solving for
R_EXT , we get
R_EXT = 400 V_PROG/I_SC
(5)
If the V_CL pin is left open, the output short circuit current will
default to about 700 mA. At elevated temperatures this cur-
rent will decrease.
Only the NPN output I_SC protection is shown. Depending on the polarity of V_DIFF, AMP2 will turn I_DRIVE either on or off.
FIGURE 3. Simplified Diagram of Current Limit Control
20138629
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