The simplest way to calculate the resistor values is to as-

sume a value for R

R

To assure best output voltage accuracy, the value selected

for R

be used.

As the ohmic value of R

divider inside the LM3460 will cause the output voltage to

deviate from the value predicted by the formula shown.

App Circuit Technical Information

Figure 1 and Figure 2 highlight two applications of the

LM3460. This section provides details of circuit function.

1.5V/7A TYPICAL APPLICATION

Figure 1 shows the schematic of a wide-bandwidth linear

regulator which provides a regulated 1.5V output at up to 7A

of load current from a 3V-3.6V input.

The pass element of the regulator (which supplies the load

current) is made up of a three-transistor complimentary Dar-

lington composed of Q2, Q3, and Q4. The bias current flow-

ing through R1 will drive the pass element ON, until such

time as Q1 pulls down and takes the drive away from the

base of Q2.

The circuit regulates the output to 1.5V using the LM3460

precision controller, which sources current from its output

whenever the voltage at the IN pin reaches 1.5V.

When the LM3460 sources current from its output, it turns on

Q1 (stealing the base drive for Q2) which reduces the cur-

rent from the 1.5V regulated output. In this way, a negative

feedback loop is established which locks the output at 1.5V.

C1 and C2 are used for compensation, and should be ce-

ramic capacitors.

C4 is required for regulator stability, and both C3 and C4 af-

fect transient response. Circuit performance should be care-

fully evaluated if substitutions are made for these two com-

ponents.

PERFORMANCE DATA

All data taken at 20˚C ambient:

LOAD/LINE REGULATION: The output voltage changed

voltage was varied from 3.0V-3.6V.

DROPOUT VOLTAGE: The dropout voltage (which is de-

fined as the minimum input-output voltage differential re-

quired to maintain a regulated output) was measured at 7A

and found to be 1.4V. This means that a minimum input volt-

age of 2.9V is required to keep the 1.5V output in regulation.

TRANSIENT RESPONSE: Transient response was tested

using a 0.2

Ω power resistor connected to the output using a

mechanical contact to provide a 0-7A load current step.

When the load was applied, the change in output voltage

µs (see

Figure 5).

HEATSINKING/COMPONENT SELECTION

HEATSINKING: As with

any linear regulator, the power dissi-

pated in the pass transistor (Q4) is approximately:

P = (V

Q4 must be provided with adequate heatsinking so that the

junction temperature never exceeds 150˚C.

Figure 6 shows the maximum allowable values of thermal

resistance (from heatsink-to-ambient) that must be provided

for various values of the load current.

These values are calculated assuming a maximum ambient

of 50˚C, 3.3V input, and a TO-220 power transistor mounted

using thermal grease and a mica insulator.

DS012603-10

FOR BEST RESULTS: SELECT R

FIGURE 4. Setting the Output Voltage

DS012603-11

FIGURE 5. Output Transient Response

DS012603-12

FIGURE 6. Q4 Heatsink Requirements for Circuit

Shown in

Figure 1

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