LTM4612
15
4612fc
For more information
Figure 6. Coincident Tracking
Figure 7. Coincident Output Tracking
OUTPUT
VOLTAGE
TIME
4612 F07
MASTER OUTPUT
SLAVE OUTPUT
APPLICATIONS INFORMATION
Fault Conditions: Current Limit and
Overcurrent Foldback
LTM4612 has a current mode controller, which inherently
limits the cycle-by-cycle inductor current not only in steady
state operation, but also in transient.
To further limit current in the event of an overload condi-
tion, the LTM4612 provides foldback current limiting. If the
output voltage falls by more than 50%, then the maximum
output current is progressively lowered to about one sixth
of its full current limit value.
Soft-Start and Tracking
The TRACK/SS pin provides a means to either soft-start
the regulator or track it to a different power supply. A
capacitor on this pin will program the ramp rate of the
output voltage. A 1.5µA current source will charge up the
external soft-start capacitor to 80% of the 0.6V internal
voltage reference plus or minus any margin delta. This will
control the ramp of the internal reference and the output
voltage. The total soft-start time can be calculated as:
t
SOFTSTART
≅
0.8 • 0.6
±
0.6 • V
OUT
Margin %
(
)
•
C
SS
1.5µA
If the RUN pin falls below 2.5V, then the soft-start pin
is reset to allow for the proper soft-start again. Current
foldback and force continuous mode are disabled during
the soft-start process. The soft-start function can also
be used to control the output ramp rising time, so that
another regulator can be easily tracked.
Output Voltage Tracking
Output voltage tracking can be programmed externally
using the TRACK/SS pin. The output can be tracked up and
down with another regulator. Figure 6 shows an example
of coincident tracking where the master regulator’s output
is divided down with an external resistor divider that is the
same as the slave regulator’s feedback divider. Ratiometric
modes of tracking can be achieved by selecting different
resistor values to change the output tracking ratio. The
master output must be greater than the slave output for
the tracking to work. Figure 7 shows the coincident output
tracking.
Tracking can be achieved by a few simple calculations
and the slew rate value applied to the master’s TRACK
pin. 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
• 100k
=
R2
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 R2 is
equal the 100k. R1 is derived from equation:
R1
=
0.6V
V
FB
100k
+
V
FB
R
FB
−
V
TRACK
R2
PGOOD
RUN
COMP
INTV
CC
DRV
CC
TRACK/SS
f
SET
V
OUT
V
FB
FCB
MARG0
MARG1
MPGM
TRACK
CONTROL
PLLIN
LTM4612
R
FB
5.23k
100k
10µF
MASTER
OUTPUT
R2
100k
C
OUT
SLAVE
OUTPUT
4612 F06
C
IN
V
IN
V
D
PGND
SGND
V
IN
R1
5.23k
