Datasheet

LTM4601/LTM4601-1
14
4601fd
For more information www.linear.com/LTM4601
Therefore, the output ripple voltage can be calculated with
the known effective output ripple current. The equation:
ΔV
OUT(P-P)
≈ (ΔI
L
/(8 • f • m • C
OUT
) + ESR • ΔI
L
), where f
is frequency and m is the number of parallel phases. This
calculation process can be easily accomplished by using
LTpowerCAD™.
Fault Conditions: Current Limit and Overcurrent
Foldback
LTM4601 has a current mode controller, which inher
-
ently limits the cycle-by-cycle inductor current not only in
steady-state operation, but also in response to transients.
To further limit current in the event of an overload condi
-
tion, the LTM4601 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 T
racking
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 • 0.6V ± V
OUT(MARGIN)
( )
•
C
SS
1.5µA
When the RUN pin falls below 1.5V, then the TRACK/SS
pin is reset to allow for proper soft-start control when the
regulator is enabled again. Current foldback and forced
continuous mode are disabled during the soft-start pro
-
cess. 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
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.
Figure 5. Coincident Tracking Schematic
Figure 6. Coincident Output Tracking Characteristics
OUTPUT
VOLTAGE
TIME
4601 F06
MASTER OUTPUT
SLAVE OUTPUT
applicaTions inForMaTion
V
OUT
V
FB
MARG0
MARG1
V
OUT_LCL
DIFFV
OUT
V
OSNS
+
V
OSNS
–
PGOOD
MPGM
RUN
COMP
INTV
CC
DRV
CC
TRACK/SS
TRACK CONTROL
PLLIN
LTM4601
R
SET
40.2k
100k
R1
40.2k
MASTER
OUTPUT
R2
60.4k
C
OUT
SLAVE OUTPUT
4601 F05
C
IN
V
IN
f
SETPGNDSGND
V
IN