LTM4611_15 Datasheet, PDF (16/30 Pages) Linear Technology – Ultralow VIN, 15A DC/DC Module Regulator

LTM4611
Applications Information
Decreasing the PLLFLTR/f_SET RC time constant can be
accomplished, for example, by driving the PLLFLTR/f_SET
pin with an external, lower impedance resistor divider
network from INTV_CC and GND to PLLFLTR/f_SET—in the
simplest of implementations, by shorting PLLFLTR/f_SET
to INTV_CC (thereby programming the switching frequency
to 780kHz, nominal), or by driving the PLLFLTR/f_SET pin
from a low impedance voltage source.
When, in addition to needing faster turn-on time, one is also
synchronizing to an external clock signal, one need bear in
mind: the PLL’s sink and source current is recommended
for not more than ±8µA loading, and the PLL will need
to successfully drive any external PLLFLTR/f_SET network
impedance to achieve phase lock; and lastly, some phase
shift in clock synchronization will occur as external loading
on PLLFLTR/f_SET becomes heavier.
To be clear, using a C_SS value of 10nF (or higher) eliminates
the need for any of the above special considerations or
provisions.
Ratiometric tracking can be achieved by a few simple
calculations and the slew rate value applied to the master’s
TRACK/SS pin. As mentioned above, the TRACK/SS pin has
a control range from 0V to 0.8V. 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
•
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 to 60.4k. R_TA is derived from equation:
R_TA =
V_FB
0.8V
+ ^VFB – ^VTRACK
60.4k R_FB2 R_TB
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 its final value before the master output.
For example, MR = 1.5V/ms, and SR = 1.2V/ms. Then R_TB
= 75k. Solve for R_TA to equal to 87k.
Beware that without any kind of soft-start ramp up, it is
important to provide thorough input filter capacitance to
handle input surge currents at start-up, so as to avoid
excessive input line sag and power supply motor boating.
Leaving provision for at least a soft-start capacitor in one’s
application is strongly recommended.
Overcurrent and Overvoltage Protection
The LTM4611 has overcurrent protection (OCP) in a
short circuit. The internal current comparator threshold
folds back during a short to reduce the output current.
An overvoltage condition (OVP) above 7.5% of the regu-
lated output voltage will force the top MOSFET off and
the bottom MOSFET on until the condition is cleared. An
input electronic circuit breaker or fuse can be sized to be
tripped or cleared when the bottom MOSFET is turned on
to protect against the overvoltage. Foldback current limiting
is disabled during soft-start or tracking start-up.
Run Enable
The RUN pin is used to enable the power module or
sequence the power module. The threshold is 1.22V. The
RUN pin must be used as an undervoltage lockout (UVLO)
function by connecting a resistor divider from the input
supply to the RUN pin:
R2 =
R1
V_{UVLO – 1}
1.22V
where V_FB is the feedback voltage reference of the regula-
tor, and V_TRACK is 0.8V. 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_FB2 with
V_FB = V_TRACK. Therefore R_TB = 60.4k, and R_TA = 121k in
Figure 4.
To achieve the lowest possible UVLO, 1.22V, leave R2
unpopulated. R1 can be 10k, or if R2 is unpopulated, R1
may be replaced with a hardwired connection from V_IN
to RUN.
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