LMH6321 datasheet(16/21 Pages) NSC

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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

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 REXT

to be at zero volts also. Therefore, a voltage applied to the

other end of R

EXT will force a current

EXT =VPROG/REXT

(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 k

Ω and making V

CL variable from 0.25V to

7.5V. Thus, an input voltage V

CL is converted to a current

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.IPROG sets a voltage drop across

SC which is applied to the non-inverting input of error amp

A2. The other input is across R

SENSE. The current through

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

SENSE is greater than or less than the voltage across RSC.

This loop gains I

EXT up by another 200, thus

SC = 2 x 200 (IEXT) = 400 IEXT

(4)

Therefore, combining Equations (3) and (4), and solving for

EXT ,weget

EXT = 400 VPROG/ISC

(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.

20138629

Only the NPN output ISC protection is shown. Depending on the polarity of VDIFF, AMP2 will turn IDRIVE either on or off.

FIGURE 3. Simplified Diagram of Current Limit Control

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