Datasheet

LTM4615
12
4615fb
For more information www.linear.com/LTM4615
applicaTions inForMaTion
Fault Conditions: Current Limit and Overtemperature
Protection
The LTM4615 has current mode control, which inher
-
ently limits the cycle-by-cycle inductor current not only
in steady-state operation, but also in transient.
Along with current limiting in the event of an overload
condition, the LTM4615 has overtemperature shutdown
protection that inhibits switching operation around 150°C
for each channel.
Run Enable and Soft-Start
The RUN/SS pins provide a dual function of enable and
soft-start control for each channel. The RUN/SS pins are
used to control turn on of the LTM4615. While each enable
pin is below 0.5V, the LTM4615 will be in a low quiescent
current state. At least a 0.8V level applied to the enable
pins will turn on the LTM4615 regulators. This pin can be
used to sequence the regulator channels. The soft-start
control is provided by a 1M pull-up resistor (R
SS
) and a
1000pF capacitor (C
SS
) as drawn in the block diagram
for each channel. An external capacitor can be applied to
the RUN/SS pin to increase the soft-start time. A typical
value is 0.01µF. The approximate equation for soft-start:
t
SOFTSTART
=In
V
IN
V
IN
– 1.8V
⎛
⎝
⎜
⎞
⎠
⎟
•R
SS
• C
SS
where R
SS
and C
SS
are shown in the block diagram of
Figure 1, and the 1.8V is soft-start upper range. 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 V
oltage Tracking
Output voltage tracking can be programmed externally
using the TRACK pins. Either output can be tracked up
or 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
to implement coincident tracking. The LTM4615 uses a
very accurate 4.99k resistor for the internal top feedback
resistor. Figure 2 shows an example of coincident tracking.
Equations:
TRACK1=
R
FB1
4.99k +R
FB1
⎛
⎝
⎜
⎞
⎠
⎟
•Master
Slave = 1+
4.99k
R
FB1
⎛
⎝
⎜
⎞
⎠
⎟
• TRACK1
PGOOD1
PGOOD1
FB1
COMP1
TRACK1
RUN/SS1
LDO_IN
EN3
BOOST3
GND1
GND2
LTM4615
GND3
V
IN1
V
IN2
V
OUT1
1.5V
1.2V
V
IN
3V TO 5.5V
R
FB1
10k
R3
10k
C4
22µF
6.3V
C5
22µF
6.3V
C7
22µF
6.3V
C6
22µF
6.3V
C8
22µF
6.3V
C11
10µF
6.3V
C13
C
SSEXT
C9
22µF
6.3V
C3
22µF
6.3V
C12
10µF
C1
10µF
6.3V
C2
10µF
6.3V
C10
22µF
6.3V
L1 0.2µH*
SLAVE
1.2V
4A
MASTER
1.5V
4A
PGOOD2
PGOOD2
FB2
COMP2
TRACK2 V
IN
OR A CONTROL RAMP
RUN/SS2
LDO_OUT
FB3
PGOOD3 PGOOD3
V
OUT2
R
TA
10k
*FAIR-RITE 0805 2508056007Y6
OPTIONAL FILTER
R
TB
4.99k
R
FB2
5.76k
R6
10k
4615 F02
1V
LOW NOISE
1V LOW NOISE AT 1A
R5
3.32k
R4
10k
Figure 2. Dual Outputs (1.5V and 1.2V) with Tracking