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 chosen, which together set the thermal conditions for the buffer (see Section 6.8) 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 6-3 shows a simplified diagram of the V to I converter and I_SC protection circuitry that, together, perform this task.

Referring to Figure 6-3, 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, as shown in Equation 3 a voltage applied to the other end of R_EXT will force a current into this pin.

Through the VCL 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_PROG variable from 0.25 V to 7.5 V. Thus, an input voltage V_PROG 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, through 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) between 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

Therefore, combining Equation 3 and Equation 4, and solving for R_EXT, we get

If the V_CL pin is left open, the output short circuit current will default to about 700 mA. At elevated temperatures this current 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 6-3 Simplified Diagram of Current Limit Control