®

LSN2 Series

Non-isolated, DOSA-SIP, 6/10/16A Selectable-Output DC/DC Converters

www.cd4power.com

Ω) = _____________ –5110

21070

Ω) = _____________ –1000

10500

The LSN2 series includes a pre-bias startup mode to prevent these initializa-

tion 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 synchro-

nous 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/DC’s 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 applica-

tions, 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 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 (pin 4)

or an external DC trim voltage applied between the Trim input and Power

Common. The output voltage range for W3 models is 0.75 to 3.3 Volts. For D12

models, the output range is 0.75 to 5 Volts.

IMPORTANT: On W3 models only, for outputs greater than 3 Volts up to 3.3

Volts maximum, the input supply must be 4.5 Volts minimum. To retain proper

regulation, do not exceed the 3.3V output.

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

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

aware that the output voltage accuracy is ±2% (typical) therefore you may

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

Two different trim equations are used for the W3 and D12 models.

W3 Models Resistor Trim Equation:

The W3 models fixed trim resistors to set the output voltage are:

0.7525V

1.0V

1.2V

1.5V

1.8V

2.5V

3.3V

Ω)

Open

80.021

41.973

23.077

15.004

6.947

3.16

D12 Models Resistor Trim Equation:

0.7525V

1.0V

1.2V

1.5V

1.8V

2.5V

3.3V

5V

13.05

9.024

5.009

3.122

1.472

Voltage Trim

The LSN2 Series may also be trimmed using an external voltage applied

between the Trim input and Output Common. Be aware that the internal “load”

impedance looking into trim pin is approximately 5k

Ω. Therefore, you may

have to compensate for this in the source resistance of your external voltage

reference.

Use a low noise DC reference and short leads. Mount the leads close to the

converter. Consider using a small bypass capacitor (0.1μF ceramic) between

trim and output common to avoid instability.

Two different trim equations are used for the W3 and D12 models.

W3 Models Voltage Trim Equation:

The LSN2 W3 fixed trim voltages to set the output voltage are:

1.0V

1.2V

1.5V

1.8V

2.5V

3.3V

Open

0.6928V

0.624V

0.5731V 0.5221V 0.4033V

0.267V

D12 Models Voltage Trim Equation:

The LSN2 D12 fixed trim voltages to set the output voltage are:

0.7525V

1.0V

1.2V

1.5V

1.8V

2.5V

3.3V

5V

Open

0.6835

0.670

0.650

0.630

0.583

0.530

0.4166

Figure 6. Trim Connections