ltm4612 Linear Technology Corporation, ltm4612 Datasheet - Page 13

• Manufacturer Part Number: ltm4612
• Description: Ultralow Noise 36vin, 15vout, 5a, Dc/dc ?module
• Manufacturer: Linear Technology Corporation

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 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:
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.
APPLICATIONS INFORMATION
CONTROL
MASTER
OUTPUT
TRACK
t
SOFTSTART
R2
100k
R1
5.23k
V
C
IN
IN
≅
Figure 6. Coincident Tracking
0 8 0 6
. • .
100k
(
PGOOD
RUN
COMP
INTV
DRV
f
TRACK/SS
SET
V
CC
CC
D
V V
SGND
10μF
–
LTM4612
V
OUT MARGIN
IN
PGND
(
PLLIN
MARG0
MARG1
MPGM
V
FCB
OUT
V
FB
)
)
•
4612 F06
5.23k
1 5
C
.
SS
µA
C
SLAVE
OUTPUT
OUT
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. The master regulator’s
output is divided down with an external resistor divider
that is the same as the slave regulator’s feedback divider.
Figure 6 shows an example of coincident tracking. Ra-
tiometric 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.
RUN Enable
The RUN pin is used to enable the power module. The
pin has an internal 5.1V zener to ground. The pin can be
driven with 5V logic levels.
The RUN pin can also be used as an undervoltage lockout
(UVLO) function by connecting a resistor divider from
the input supply to the RUN pin. The equation for UVLO
threshold:
where R1 is the top resistor, and R2 is the bottom resistor.
Power Good
The PGOOD pin is an open-drain pin that can be used to
monitor valid output voltage regulation. This pin monitors
a ±10% window around the regulation point, and tracks
with margining.
V
UVLO
VOLTAGE
OUTPUT
=
R
Figure 7. Coincident Output Tracking
+ 1 2
R
2
R
• .
1 5
V
TIME
MASTER OUTPUT
SLAVE OUTPUT
LTM4612
4612 F07
13
4612f