LTM8001
16
8001fd
For more information www.linear.com/LTM8001
The RUN pin has an absolute maximum voltage of 6V.
To accommodate the largest range of applications, there
is an internal Zener diode that clamps this pin, so that it
can be pulled up to a voltage higher than 6V through a
resistor that limits the current to less than 100µA. For
applications where the supply range is greater than 4:1,
size R2 greater than 375k.
Input Precautions
The LTM8001 contains a step-down switching regulator
that operates at a user-selectable frequency in forced
continuous mode. Step-down switching regulators that
operate in forced continuous mode are capable of both
sinking and sourcing current to maintain output voltage
regulation
When the LTM8001 is sinking current, it maintains its
output voltage regulation by power conversion, not power
dissipation. This means that the energy provided to the
LTM8001 is in turn delivered to its input power bus.
There must be something on this power bus to accept or
use the energy
,
or the LTM8001’s input voltage will rise.
Left unchecked, the energy can raise the input voltage
above the absolute maximum voltage rating and damage
the LTM8001.
In many cases, the system load on the LTM8001 input
bus will be sufficient to absorb the energy delivered by the
μModule regulator. The power required by other devices
will consume more than enough to make up for what
the LTM8001 delivers. In cases where the LTM8001 is
the largest or only power converter, this may not be true
and some means may need to be devised to prevent the
LTM8001’s input from rising too high. Figure 5a shows a
passive crowbar circuit that will dissipate energy during
momentary input overvoltage conditions. The breakdown
voltage of the zener diode is chosen in conjunction with
the resistor R to set the circuit’s trip point. The trip point
is typically set well above the maximum V
IN
voltage under
normal operating conditions. This circuit does not have a
precision threshold, and is subject to both part-to-part and
temperature variations, so it is not suitable for applications
where high accuracy is required or large voltage margins
are not available.
The circuit in Figure 5b also dissipates energy during mo
-
mentary overvoltage conditions, but is more precise than
that in
Figure 5a. It uses an inexpensive comparator and
the V
REF
output of the LTM8001 to establish a reference
voltage. The optional hysteresis resistor in the comparator
circuit avoids MOSFET chatter. Figure 5c shows a circuit
that latches on and crowbars the input in an overvoltage
applicaTions inForMaTion
Load Sharing
The V
OUT0
step-down switching converter operates in
fixed frequency forced continuous mode, so it is able to
source and sink current. It is therefore not suitable for
load current sharing.
The linear regulators connected to V
OUT1
-V
OUT5
are inter-
nally ballasted and may be paralleled. To do this, simply
tie the V
OUTx
and SETx terminals together. When the SET
pins of the regulators are tied together, the R
SET
resistor
is determined by the equation:
R
SET
=
OUT
n•10µA
where n is the number of linear regulator outputs tied
together.
All paralleled LDOs must be active in order for this equa
-
tion to be true, as it is assumed that all paralleled LDOs
are contributing 10µA to a single voltage set resistor. If
any LDO is off or inactive, it will be unable to contribution
its share of the set current and the output voltage will be
lower than expected.
When paralleling LDOs, tie all of the V
OUTx
and all of the
SETx pins together. Examples are shown in the Typical
Applications section.
LTM8001
V
IN
R2
V
R1
RUN
Figure 4. UVLO Configuration
