Output Overcurrent Detection

Overloading the power converter's output for an extended time will invariably

cause internal component temperatures to exceed their maximum ratings and

eventually lead to component failure. High-current-carrying components such

as inductors, FET's and diodes are at the highest risk. LSM D3 SMT Series

DC/DC converters incorporate an output overcurrent detection and shutdown

function that serves to protect both the power converter and its load.

If the output current exceeds it maximum rating by typically 70% (17 Amps) or

if the output voltage drops to less than 98% of it original value, the LSM D3's

internal overcurrent-detection circuitry immediately turns off the converter,

which then goes into a "hiccup" mode. While hiccupping, the converter will

continuously attempt to restart itself, go into overcurrent, and then shut down.

Under these conditions, the average output current will be approximately

600mA, and the average input current will be approximately 110mA. Once the

output short is removed, the converter will automatically restart itself.

Output Voltage Trimming

Allowable trim ranges for each model in the LSM D3 SMT Series are ±10%.

Trimming is accomplished with either a trimpot or a single fixed resistor. The

trimpot should be connected between +Output and Common with its wiper

connected to the Trim pin as shown in Figure 6 below.

A trimpot can be used to determine the value of a single fixed resistor

which can then be connected, as shown in Figure 7, between the Trim pin

and +Output to trim down the output voltage, or between the Trim pin and

Common to trim up the output voltage. Fixed resistors should have absolute

TCR’s less than 100ppm/ C to ensure stability.

The equations below can be starting points for selecting specific trim-resistor

values. Recall, untrimmed devices are guaranteed to be 1% accurate.

Adjustment beyond the specified ±10% adjustment range is not recommended.

When using trim in combination with Remote Sense, the maximum rated power

must not be exceeded (see Remote Sense).

On/Off Control

The On/Off Control pin may be used for remote on/off operation. LSM D3

Series DC/DC converters are designed so that they are enabled when the

control pin is left open (open collector) and disabled when the control pin is

pulled low (to less than +0.8V relative to Common). As shown in Figure 4, all

models have an internal 5k

Dynamic control of the on/off function is best accomplished with a mechanical

relay or open-collector/open-drain drive circuit (optically isolated if appropri-

ate). The drive circuit should be able to sink appropriate current when

activated and withstand appropriate voltage when deactivated.

Applying an external voltage to the On/Off Control pin when no input power is

applied to the converter can cause permanent damage to the converter. The

on/off control function, however, is designed such that the converter can be

disabled (control pin pulled low) while input voltage is ramping up and then

"released" once the input has stabilized (see also power-up sequencing).

+INPUT

COMMON

EXTERNAL

OPEN

COLLECTOR

INPUT

5k

10k

Figure 5. Driving the External Power-Up Open Collector

ON/OFF pin open:

Logic High = DC/DC converter On

ON/OFF pin <0.4V:

Logic Low = DC/DC converter Off

External Input Open:

On/Off pin Low = DC/DC converter Off

External Input Low:

On/Off pin High = DC/DC converter On

Power-up sequencing

If a controlled start-up of one or more LSM D3 Series DC/DC converters

is required, or if several output voltages need to be powered-up in a given

sequence, the On/Off control pin can be driven with an external open collector

device as per Figure 5.

Leaving the input of the external circuit open during power-up will have the

output of the DC/DC converter disabled. When the input to the external open

collector is pulled low, the DC/DC converters output will be enabled.

Output Overvoltage Protection

LSM D3 SMT Series DC/DC converters do not incorporate output overvolt-

age protection. In the extremely rare situation in which the device’s feedback

all possible overvoltage situations, voltage limiting circuitry must be provided

external to the power converter.

Figure 4. Driving the On/Off Control Pin with an Open-Collector Drive Circuit

LSM-10A D3 Models

10 Amp DC/DC’s in SMT Packages

www.murata-ps.com

MDC_LSM-10A D3.B01 Page 5 of 13