LM3753 Datasheet, PDF (19/38 Pages) National Semiconductor (TI) – Scalable 2-Phase Synchronous Buck Controllers with Integrated FET Drivers and Linear Regulator Controller

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 max-
imum 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 con-
trollers, 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 LM3753/54. After the number of phases has been chosen,
the PH pin on each controller should be programmed as dis-
cussed 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 CONNEC-
TIONS. 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 con-
troller 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 com-
mon COMP signal used by all controllers, provides the loop
compensation and synchronizes the system clock to an ex-
ternal 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 con-
nected 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 (LM3754), TRACK (LM3753)
5) PGOOD
TRACKING (LM3753)
The LM3753 will track the output of an external power supply
by connecting a resistor divider to the TRACK pin as shown
in Figure 8. This allows the output voltage slew rate to be
controlled for loads that require precise sequencing.
A value of 10 kΩ 1% is recommended for R_T1 as a good com-
promise between high precision and low quiescent current
through the divider. Note that the TRACK pin must finish at
least 100 mV higher than the 0.6V reference to achieve the
full accuracy of the LM3753 regulation. To meet this require-
ment the tracking voltage is offset by 150 mV. The output
voltage will reach its final value at 80% of the external supply
voltage. The tracking resistors are determined by:
30091929
FIGURE 8. Tracking an External Supply
30091930
FIGURE 9. Tracking an External Supply
For equal slew rates, the relationship for the tracking divider
is set by:
30091932
FIGURE 10. Tracking an External Supply with Equal Slew
Rates
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