LTM8003 datasheet(19/28 Pages) AD

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LTM8051

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Load Sharing

The four LTM8051 channels may be paralleled to produce

higher currents. To do this on two or more LTM8051, tie

the VINn, VOUTn, FBn and SHAREn pins of all the paral-

leled channels/modules together (see Figure 7). To ensure

that paralleled channels start up together, the TRSS

n pins

may be tied together, as well. If it is inconvenient to tie

the TRSS

n pins together, make sure that the same value

soft-start capacitors are used for each µModule regulator.

When load sharing among n units and using a single RFB

resistor, the value of the resistor is:

RFB =

199.2

n(VOUT −0.8)

,where RFB is in kΩ

Examples of load sharing applications are given in Figure 6

through Figure 8.

Burst Mode Operation

To enhance efficiency at light loads, the LTM8051

automatically switches to Burst Mode operation which

keeps the output capacitor charged to the proper volt-

age while minimizing the input quiescent current. During

Burst Mode operation, the LTM8051 delivers single cycle

bursts of current to the output capacitor followed by sleep

periods where most of the internal circuitry is powered off

and energy is delivered to the load by the output capacitor.

During the sleep time, VINn and BIASn quiescent currents

are greatly reduced, so, as the load current decreases

towards a no load condition, the percentage of time that

the LTM8051 operates in sleep mode increases and the

average input current is greatly reduced, resulting in

higher light load efficiency.

Burst Mode operation is enabled by tying SYNC to GND.

Minimum Input Voltage

The LTM8051 is a step-down converter, so a minimum

amount of headroom is required to keep the output in

regulation. Keep the input above 3V to ensure proper

operation. Voltage transients or ripple valleys that cause

the input to fall below 3V may turn off the LTM8051.

VIN1 must be above 3V for channel 1 and channel 4 to

operate. If VIN1 is above 3V, channel 4 will operate as long

as VIN4 is above 2V.

VIN23 must be above 3V for channel 2 and channel 3 to

operate.

Output Voltage Tracking and Soft-Start

The LTM8051 allows the user to adjust its output voltage

ramp rate by means of the TRSS

n pin. An internal 2μA

pulls up the TRSS

n pin to about 2.4V. Putting an external

capacitor on TRSS

n enables soft starting the output to

reduce current surges on the input supply. During the

soft-start ramp the output voltage will proportionally track

the TRSS

n pin voltage. For output tracking applications,

TRSS

n can be externally driven by another voltage source.

From 0V to 0.8V, the TRSS

n voltage will override the

internal 0.8V reference input to the error amplifier, thus

regulating the FB

n pin voltage to that of the TRSSn pin.

When TRSS

n is above 0.8V, tracking is disabled and the

feedback voltage will regulate to the internal reference

voltage. The TRSS

n pin may be left floating if the function

is not needed.

An active pull-down circuit is connected to the TRSS

pin which will discharge the external soft-start capacitor

in the case of fault conditions and restart the ramp when

the faults are cleared. Fault conditions that clear the soft-

start capacitor are the RUN

n pin transitioning low, VINn

voltage falling too low, or thermal shutdown.

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