ltm4601ahv Linear Technology Corporation, ltm4601ahv Datasheet - Page 13

• Manufacturer Part Number: ltm4601ahv
• Description: 12a, 28vin Dc/dc ?module With Pll, Output Tracking And Margining
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
f is frequency and m is the number of parallel phases.
This calculation process can be easily fulfi lled using our
Excel tool (Refer to the Linear Technology μModule Power
Design Tool).
Fault Conditions: Current Limit and Overcurrent
Foldback
LTM4601AHV has a current mode controller, which inher-
ently 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 condition,
the LTM4601AHV 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:
When the RUN pin falls below 1.5V, then the SS pin is reset
to allow for proper soft-start control when the regulator
is enabled 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 up time, so that another regulator can be easily
tracked to it.
Output Voltage Tracking
Output voltage tracking can be programmed externally
using the TRACK/SS pin. The output can be tracked up and
t
SOFTSTART
=
0 8 0 6
. • .
(
V V
–
OUT MARGIN
(
)
)
•
1 5
C
.
SS
µA
V
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 5
shows an example of coincident tracking. 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 6 shows the coincident output
tracking characteristics.
C
IN
IN
VOLTAGE
OUTPUT
100k
PGOOD
MPGM
RUN
COMP
INTV
DRV
CC
CC
SGND
V
IN
LTM4601AHV
PLLIN
PGND
Figure 5
Figure 6
TRACK/SS
TIME
f
SET
V
DIFFV
LTM4601AHV
TRACK CONTROL
OUT_LCL
MARG0
MARG1
V
V
MASTER OUTPUT
OSNS
OSNS
SLAVE OUTPUT
V
OUT
OUT
V
FB
+
–
60.4k FROM
V
INTERNAL
OUT
4601AHV F06
TO V
4601AHV F05
R
40.2k
SET
FB
MASTER
OUTPUT
SLAVE OUTPUT
C
OUT
R2
60.4k
R1
40.2k
13
4601ahvf