LSN2-T30-D12 Datasheet, PDF (11/17 Pages) Murata Manufacturing Co., Ltd. – DOSA-SIP, 30A POL DC/DC Converters

LSN2-T/30-D12 Series
DOSA-SIP, 30A POL DC/DC Converters
[10] Any external capacitance added to the converter’s output may affect ramp current. If the resistive load starts successfully, you may be trying to drive an
up/down times and ramp tracking accuracy.
external pre-biased active source.
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.
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.
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.
Resistor Trim Equation
RTRIM (Ω) = ¹²⁰⁰ − 100
VO − 0.80
Where VO is the desired output voltage.
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
6/54
42)-
#/--/.
242)-
Figure 14. Trim Connections
www.murata-ps.com
Technical enquiries email: [e-mail], tel: [phone]
MDC_LSN2-T/30-D12 Series.B17 Page 11 of 17