LM3754 datasheet(21/47 Pages) TI

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LM3754

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SNVS789B – JANUARY 2012 – REVISED APRIL 2013

On the master controller, the differential amplifier is used to provide Kelvin sensing of the output voltage at the

load. This provides the most accurate sampling for load regulation.

On the slave controllers, the differential amplifier is used to sense the COMP signal of the master controller with

respect to its signal ground and drive the COMP pin of that slave controller relative to its local signal ground. This

allows the master controller to accurately provide the target duty cycle of the slave controllers.

The differential amplifier has a low output impedance to allow it to drive the COMP pins of the Slave controllers.

This is necessary because the current sense signal is internally added to COMP to provide the duty cycle

adjustment for phase-to-phase current sharing.

APPLICATION INFORMATION

Number of Phases

The number of phases can be calculated by dividing the maximum output load current by 25A. Therefore a 120A

load requirement will need at least 5 phases, or 3 controllers. It may be better to use 6 phases which will still

require 3 controllers, but will reduce the maximum current/phase to 20A. Increasing the number of phases will

also reduce the output voltage ripple and the input capacitor requirements. Note that the 25A/phase is dictated

by external components and not by the LM3754. After the number of phases has been chosen, the PH pin on

each controller should be programmed as discussed in the Functional Description under Phase Number

Selection. The same number of phases must be selected for each controller.

Powering Options

The power connections will be determined by the VIN range and the availability of an external 5V rail. This is

discussed in detail in the Functional Description under Power Connections. For 12V input systems, the use of an

external 5V rail to power the VDD bus can improve overall system efficiency.

Multi-Controller Systems

For systems with more than 2 phases, there will be one controller configured as the Master and from 1 to 5

controllers configured as Slave.

The Master controller uses the differential amplifier to sense the output voltage at the load point. It also provides

the common COMP signal used by all controllers, provides the loop compensation and synchronizes the system

clock to an external clock if one is provided.

The SYNCOUT of the Master is connected to the SYNC input of the first Slave controller.

The Slave controllers are configured by tying the FB input to the VCC pin of that controller. Each Slave uses the

differential amplifier to sense the COMP signal of the Master controller and drive its own COMP input. The

SYNCOUT of each Slave controller is connected to the SYNC input of the next Slave controller.

All controllers have the same parallel RC components connected from the FREQ pin to local ground

corresponding to the desired system clock even if synchronizing to an external clock.

Common connections for all controllers:

1) IAVE (each controller will have a parallel RC filter to local ground).

2) FAULT

3) EN

4) SS

5) PGOOD

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