LTM4614 查看數據表（PDF） - Linear Technology
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LTM4614 Datasheet PDF : 20 Pages
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LTM4614
Applications Information
Fault Conditions: Current Limit and Overcurrent
Foldback
The LTM4614 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 foldback current limiting in the event of an
overload condition, the LTM4614 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 LTM4614. While each enable
pin is below 0.6V, the LTM4614 will be in a low quiescent
current state. At least a 0.9V level applied to the enable
pins will turn on the LTM4614 regulators. The voltage on
the RUN/SS pins clamp the control loop’s current com-
parator threshold. A RUN/SS pin voltage of 2.375V upon
completion of soft-start guarantees the regulator can deliver
full output current. These pins can be used to sequence
the regulator channels. Soft-start control is provided by
a 1M pull-up resistor (RSS) and a 1000pF capacitor (CSS)
as shown in the Block Diagram for each channel. Option-
ally, an external capacitor (CSSEXT) can be applied to the
RUN/SS pin to increase soft-start time. A typical value is
0.01µF. Soft-start time is approximately given by:
( ) tSOFTSTART
= In
VIN
VIN
– 1.8V
•RSS
•
CSS + CSSEXT
where RSS and CSS are shown in the Block Diagram of
Figure 1, and 1.8V is the 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. To turn off the module while VIN remains active, the
RUN/SS pin should be pulled low with a falling edge ≤ 1µs
to ensure the device does not transition slowly through
the internal undervoltage lockout threshold.
Output Voltage 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 LTM4614 uses a
very accurate 4.99k resistor for the internal top feedback
resistor. Figure 2 shows an example of coincident tracking.
Equations:
TRACK1=
RFB1
4.99k + RFB1
•
Master
Slave
=
1+
4.99k
RFB1
•
TRACK1
VIN 3V TO 5.5V
C1
C2
22µF
22µF
6.3V
6.3V
PGOOD1
R3
10k
VIN1
PGOOD1
VIN2
PGOOD2
1.2V
4A
VOUT1
VOUT2
C3
100µF
6.3V
C4
RTB
22µF
4.99k
6.3V 1.5V
RFB1
10k
RTA
10k
FB1
COMP1
LTM4614
FB2
COMP2
TRACK1
TRACK2
RUN/SS1
RUN/SS2
GND1 GND2
PGOOD2
R4
10k
VIN OR
CONTROL
RAMP
CSSEXT1
RFB2
5.76k
C7
100µF
6.3V
1.5V
4A
C9
22µF
6.3V
4614 F02
Figure 2. Dual Outputs (1.5V and 1.2V) with Tracking
4614fb
9
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