Datasheet

LTM4613
14
4613f
APPLICATIONS INFORMATION
to 12V design. The duty cycle is about 0.33. The 2-phase
curve shows a ratio of ~0.33 for a duty cycle of 0.33. This
0.33 ratio of output ripple current to the inductor ripple
current ∆I
L
at 5.0A equals 1.65A of the output ripple cur-
rent (∆I
O
).
The output voltage ripple has two components that are
related to the amount of bulk capacitance and effective
series resistance (ESR) of the output bulk capacitance.
The equation is:
∆V
OUT(P−P)
≈
∆I
O
8 • f • N • C
OUT






+
ESR • ∆I
O
N
Where f is the frequency and N is the number of paral-
leled phases.
Fault Conditions: Current Limit and
Overcurrent Foldback
LTM4613 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 LTM4613 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 5 shows an ex-
ample 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
Figure 5. Coincident Tracking
Figure 6. Coincident Output Tracking
PGOOD
RUN
COMP
INTV
CC
DRV
CC
TRACK/SS
f
SET
V
OUT
V
FB
FCB
MARG0
MARG1
MPGM
TRACK
CONTROL
PLLIN
LTM4613
R
FB
5.23k
51k
MASTER
OUTPUT
R2
100k
C
OUT
SLAVE
OUTPUT
4613 F05
C
IN
V
IN
V
D
PGNDSGND
V
IN
R1
5.23k
10µF
×3
OUTPUT
VOLTAGE
TIME
4613 F06
MASTER OUTPUT
SLAVE OUTPUT