LSN2-T/30-D12 Series

DOSA-SIP, 30A POL DC/DC Converters

www.murata-ps.com

MDC_LSN2-T/30-D12 Series.B15 Page 11 of 17

Pre-Biased Startup

Newer systems with multiple power voltages have an additional problem

besides startup sequencing. Some sections have power already partially

applied (possibly because of earlier power sequencing) or have leakage power

present so that the DC/DC converter must power up into an existing voltage.

This power may either be stored in an external bypass capacitor or supplied by

an active source.

This “pre-biased” condition can also occur with some types of program-

mable logic or because of blocking diode leakage or small currents passed

through forward biased ESD diodes. Conventional DC/DCs may fail to start up

correctly if there is output voltage already present. And some external circuits

are adversely affected when the low side MOSFET in a synchronous rectifier

converter sinks current at start up.

The LSN2-T/30-D12 series includes a pre-bias startup mode to prevent

these initialization problems. Essentially, the converter acts as a simple buck

converter until the output reaches its set point voltage at which time it converts

to a synchronous rectifier design. This feature is variously called “monotonic”

because the voltage does not decay (from low side MOSFET shorting) or

produce a negative transient once the input power is applied and the startup

sequence begins.

Don’t Use Pre-Biasing and Sequencing Together

Normally, you would use startup sequencing on multiple DC/DCs to solve the

Pre-Bias problem. By causing all power sources to ramp up together, no one

source can dominate and force the others to fail to start. For most applications,

do not use startup sequencing in a Pre-Bias application, especially with an

external active power source.

If you have active source pre-biasing, leave the Sequence input open so

that the output will step up quickly and safely. A symptom of this condition is

repeated failed starts. You can further verify this by removing the existing load

and testing it with a separate passive resistive load which does not exceed full

current. If the resistive load starts successfully, you may be trying to drive an

external pre-biased active source.

It may also be possible to use pre-bias and sequencing together if the Pre-

Bias source is in fact only a small external bypass capacitor slowly charged by

leakage currents. Test your application to be sure.

Output Adjustments

The LSN2-T/30-D12 series includes a special output voltage trimming feature

which is fully compatible with competitive units. The output voltage may be

varied using a single trim resistor from the Trim input to Power Common.

As with other trim adjustments, be sure to use a precision low-tempco resis-

tor (±100ppm/°C.) mounted close to the converter with short leads. Also be

aware that the output voltage accuracy is ±1.5% (typical) therefore you may

need to vary this resistance slightly to achieve your desired output setting.

Use short leads. Mount the leads close to the converter.

Resistor Trim Equation

1200

− 100

Figure 14.Trim Connections

TRIM

COMMON

TRIM

COMMON

+

–