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LSN2 Series

Non-isolated, DOSA-SIP, 6/10/16A

Selectable-Output DC/DC Converters

MDC_LSN2.B06 Page 9 of 15

through forward biased ESD diodes. Conventional DC/DC’s 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 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

resistor (±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:

21070

:) = _____________ – 5110

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

Open

80.021 41.973 23.077 15.004

6.947

3.16

D12 Models Resistor Trim Equation:

10500

:) = _____________ – 1000

1.0V

1.2V

1.5V

1.8V

2.5V

3.3V

5V

Open

41.424 22.46 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

:. 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.

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:

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

LSN2 Power Sequencing

Whereas in the old days, one master switch simultaneously turned on the

power for all parts of a system, many modern systems require multiple supply

voltages for different on-board sections. Typically the CPU or microcontroller

needs 1.8 Volts or lower. Memory (particularly DDR) may use 1.8 to 2.5 Volts.

Interface “glue” and “chipset” logic might use +3.3Vdc power while Input/

Output subsystems may need +5V. Finally, peripherals use 5V and/or 12V.

Figure 6. Trim Connections