LTM4649_15 Datasheet, PDF (16/30 Pages) Linear Technology – 10A Step-Down DC/DC Module Regulator

LTM4649
Applications Information
MASTER OUTPUT
SLAVE OUTPUT
TIME
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Figure 6. Output Coincident Tracking Waveform
TRACK/SS pin. As mentioned above, the TRACK/SS pin
has a control range from 0V to 0.6V. The master’s TRACK/
SS pin slew rate is directly equal to the master’s output
slew rate in Volts/Time. The equation:
MR
SR
•
10k
=
R_TB
where MR is the master’s output slew rate and SR is the
slave’s output slew rate in Volts/Time. When coincident
tracking is desired, then MR and SR are equal, thus R_TB
is equal the 10k. R_TA is derived from equation:
R_TA
=
V_FB
+
0.6V
V_{FB −} V_TRACK
10k R_FB R_TB
where V_FB is the feedback voltage reference of the regula-
tor, and V_TRACK is 0.6V. Since R_TB is equal to the 10k top
feedback resistor of the slave regulator in equal slew rate
or coincident tracking, then R_TA is equal to R_FB with V_FB
= V_TRACK. Therefore R_TB = 10k, and R_TA = 10k in Figure 5.
In ratiometric tracking, a different slew rate maybe desired
for the slave regulator. R_TB can be solved for when SR is
slower than MR. Make sure that the slave supply slew
rate is chosen to be fast enough so that the slave output
voltage will reach it final value before the master output.
Each of the TRACK/SS pins will have the 1.3µA current
source on when a resistive divider is used to implement
tracking on that specific channel. This will impose an offset
on the TRACK/SS pin input. Smaller values resistors with
the same ratios as the resistor values calculated from the
above equation can be used. For example, where the 10k
is used then a 1.0k can be used to reduce the TRACK/SS
pin offset to a negligible value.
Power Good
The PGOOD pins are open-drain pins that can be used to
monitor valid output voltage regulation. This pin monitors
a ±7.5% window around the regulation point. A resistor
can be pulled up to a particular supply voltage no greater
than 6V maximum for monitoring.
Stability Compensation
The module has already been internally compensated
for all output voltages. Table 3 is provided for most ap-
plication requirements. The Linear Technology µModule
Power Design Tool will be provided for other control loop
optimization.
Run Enable
The RUN pin has an enable threshold of 1.35V maximum,
typically 1.22V with 80mV of hysteresis. It controls the
turn-on of the µModule. The RUN pin can be pulled up to
V_IN for 5V operation, or a 5V Zener diode can be placed
on the pin and a 10k to 100k resistor can be placed up to
higher than 5V input for enabling the µModule. The RUN
pin can also be used for output voltage sequencing.
In parallel operation the RUN pins can be tied together and
controlled from a single control. See the Typical Applica-
tion circuits in Figures 19 and 20. The RUN pin can also
be left floating. The RUN pin has a 1µA pull-up current
source that increases to 4.5µA during ramp-up.
Differential Remote Sense Amplifier
An accurate differential remote sense amplifier is provided
in the LTM4649 to sense low output voltages accurately
at the remote load points. This is especially true for high
current loads. It is very important that the DIFFP and
DIFFN are connected properly at the output, and DIFFOUT
is connected to V_{OUT_LCL}. Review the parallel schematics
in Figures 19 and 20.
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For more information www.linear.com/LTM4649