LTM4613 datasheet(16/28 Pages) LINER

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LTM8001

8001f

For more information www.linear.com/8001

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 VIN voltage under

normal operating conditions. This circuit does not have a

precisionthreshold,andissubjecttobothpart-to-partand

temperaturevariations,soitisnotsuitableforapplications

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 VREF 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 VOUT0 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 VOUT1-VOUT5 are inter-

nally ballasted and may be paralleled. To do this, simply

tie the VOUTx and SETx terminals together. When the SET

pins of the regulators are tied together, the RSET resistor

is determined by the equation:

RSET =

VOUT

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 VOUTx and all of the

SETx pins together. Examples are shown in the Typical

Applications section.

LTM8001

VIN

8001 F04

RUN

Figure 4. UVLO Configuration

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