Datasheet

LTM4628
17
4628fe
For more information www.linear.com/LTM4628
APPLICATIONS INFORMATION
when the RUN pin is below 1.2V. The total soft-start time
can be calculated as:
t
SOFT-START
=
C
SS
1.3µA






• 0.6V
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
FB
is below 0.54V regardless of the setting
on the MODE_PLLIN pin.
Ratiometric tracking can be achieved by a few simple cal
-
culations 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:
MR
SR
• 60.4k = 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 60.4k. R
TA
is derived from equation:
R
TA
=
0.6V
V
FB
60.4k
+
V
FB
R
FB
−
V
TRACK
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 60.4k
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
= 60.4k, and R
TA
= 60.4k in
Figure 6.
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.
For example, MR = 1.5V/1ms, and SR = 1.2V/1ms. Then
R
TB
= 76.8k. Solve for R
TA
to equal to 49.9k.
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 application
requirements. LTpowerCAD is available for other control
loop optimization.
Run Enable
The RUN pins have an enable threshold of 1.4V maximum,
typically 1.25V with 150mV of hysteresis. They control
the turn-on of each of the channels. These pins can be
pulled up to V
IN
for 5V operation, or a 5V Zener diode can
be placed on the pins and a 10k to 100k resistor can be
placed up to higher than 5V input for enabling the chan
-
nels. 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 Application circuits in Figure 28. The RUN pin can
also be left floating. The RUN pin has a 1µA pull-
up cur-
rent sour
ce that increases by an additional 4.5µA during
ramp-up once above the on/off threshold.