ltm4628 Linear Technology Corporation, ltm4628 Datasheet - Page 17

• Manufacturer Part Number: ltm4628
• Description: Ltm4628 - Dual 8a Or Single 16a Dc/dc ?module Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
when the RUN pin is below 1.2V. The total soft-start time
can be calculated as:
Regardless of the mode selected by the MODE/PLLIN pin,
the regulator channels will always start in pulse-skipping
mode up to TRACK = 0.5V. Between TRACK = 0.5V and
0.54V, it will operate in forced continuous mode and revert
to the selected mode once TRACK > 0.54V. In order to track
with another channel once in steady state operation, the
LTM4628 is forced into continuous mode operation as
soon as V
the MODE/PLLIN pin.
Ratiometric tracking can be achieved by a few simple
calculations and the slew rate value applied to the master’s
TRACK pin. As mentioned above, the TRACK pin has a
control range from 0 to 0.6V. The master’s TRACK pin
slew rate is directly equal to the master’s output slew rate
in Volts/Time. The equation:
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
is equal the 60.4k. R
where V
tor, and V
top feedback resistor of the slave regulator in equal slew
rate or coincident tracking, then R
V
Figure 6.
In ratiometric tracking, a different slew rate maybe desired
for the slave regulator. R
slower than MR. Make sure that the slave supply slew rate
FB
R
t
MR
SR
SOFT-START
= V
TA
• 60.4k = R
TRACK
=
FB
TRACK
FB
60.4k
is the feedback voltage reference of the regula-
V
FB
is below 0.54V regardless of the setting on
. Therefore R
=
is 0.6V. Since R
+
⎛
⎜
⎝
R
TB
V
1.3μA
0.6V
FB
C
FB
TA
SS
−
is derived from equation:
TB
V
⎞
⎟ • 0.6
⎠
TB
TRACK
R
can be solved for when SR is
TB
= 60.4k, and R
TB
TA
is equal to the 60.4k
is equal to R
TA
= 60.4k in
FB
with
TB
is chosen to be fast enough so that the slave output voltage
will reach it final value before the master output.
For example, MR = 1.5V/1ms, and SR = 1.2V/1ms. Then
R
Each of the TRACK 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 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 60.4k is
used then a 6.04k can be used to reduce the TRACK 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 4 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 pins have an enable threshold of 1.35V maximum,
typically 1.22V with 80mV of hysteresis. They control the
turn-on of each of the channels. These pins can be pulled
up to V
placed on the pins and a 10k to 100k resistor can be placed
up to higher than 5V input for enabling the channels. The
RUN pins can also be used for output voltage sequencing.
In parallel operation the RUN pins can be tie together and
controlled from a single control. See the Typical Applica-
tion circuits in Figure 28. 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.
TB
= 76.8k. Solve for R
IN
for 5V operation, or a 5V Zener diode can be
TA
to equal to 49.9k.
LTM4628
17
4628f