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LTM8056

8056fa

For more information www.linear.com/LTM8056

APPLICATIONS INFORMATION

LTM8056 if the output is overloaded or short circuited.

A frequency that is too low can result in a final design

that has too much output ripple or too large of an output

capacitor or is even unstable.

Parallel Operation

Two or more LTM8056s may be combined to provide

increased output current by configuring them as a mas-

ter and a slave, as shown in Figure 1. Each LTM8056 is

equipped with an IOUTMON and a CTL pin. The IOUTMON

pin’s0Vto1.2Vsignalreflectsthecurrentpassingthrough

the output sense resistor, while a voltage less than 1.2V

appliedtotheCTLpinwilllimitthecurrentpassingthrough

the output sense resistor. By applying the voltage of the

master’s IOUTMON pin to the slave’s CTL pin, the two units

will source the same current to the load, assuming each

LTM8056 output current sense resistor is the same value.

2. Apply a FB resistor network to the individual slaves

so that the resulting output is higher than the desired

output voltage.

3. Apply the appropriate output current sense resistors

between VOUT and IOUT. If the same value is used for the

master and slave units, they will share current equally.

4. Connect the master IOUTMON to the slaves’ CTL pin

through a unity gain buffer. The unity gain buffer is

requiredtoisolatetheoutputimpedanceoftheLTM8056

from the integrated pull-up on the CTL pins.

5. Tie the outputs together.

Note that this configuration does not require the inputs to

be tied together, making it simple to power a single heavy

load from multiple input sources. Ensure that each input

power source has sufficient voltage and current sourcing

capability to provide the necessary power. Please refer

to the Maximum Output Current vs VIN and Input Current

vs Output Current curves in the Typical Performance

Characteristics section for guidance.

ParalleledLTM8056sshouldnormallybeallowedtoswitch

in discontinuous mode enabled to prevent current from

flowing from the output of one unit into another; that is,

the MODE pin should be tied to LL. In some cases, operat-

ing the master in forced continuous (MODE open) and the

slaves in discontinuous mode (MODE = LL) is desirable.

If so, current from the output can flow into the master’s

input. Please refer to Input Precaution in this section for

a discussion of this behavior.

Minimum Input Voltage and RUN

The LTM8056 needs a minimum of 5V for proper opera-

tion, but system parameters may dictate that the device

operate only above some higher input voltage. For ex-

ample, a LTM8056 may be used to produce 12VOUT, but

the input power source may not be budgeted to provide

enough current if the input supply voltage is below 8V.

The RUN pin has a typical falling voltage threshold of

1.2V and a typical hysteresis of 25mV. In addition, the

pin sinks 3µA below the RUN threshold. Based upon the

Figure 1. Two or More LTM8056s May Be Connected in a

Master/Slave Configuration for Increased Output Current

IOUT

MASTER

IOUTMON

OUTPUT CURRENT

SENSE RESISTOR

TO LOAD

VOUT

IOUT

CTL

SLAVE

VOUT

UNITY GAIN

BUFFER

8056 F01

OUTPUT CURRENT

SENSE RESISTOR

The design of a master-slave configuration is straight-

forward:

1. Apply the FB resistor network to the master, choosing

the proper values for the desired output voltage. Sug-

gested values for popular output voltages are provided

in Table 1.

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