®
LSN2 Series
Non-isolated, DOSA-SIP, 6/10/16A Selectable-Output DC/DC Converters
D12 Models Resistor Trim Equation:
10500
R
TRIM
(Ω) = _____________
–1000
V
O
– 0.7525
V
OUT
R
TRIM
(kΩ)
0.7525V
Open
1.0V
41.424
1.2V
22.46
1.5V
13.05
1.8V
9.024
2.5V
5.009
3.3V
3.122
5V
1.472
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:
21070
R
TRIM
(Ω) = _____________
–5110
V
O
– 0.7525
The W3 models fixed trim resistors to set the output voltage are:
V
OUT
(Typ.)
R
TRIM
(kΩ)
0.7525V
Open
1.0V
80.021
1.2V
41.973
1.5V
23.077
1.8V
15.004
2.5V
6.947
3.3V
3.16
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 5kΩ. 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:
V
TRIM
(in Volts) = 0.7 –(0.1698 x (V
O
– 0.7525))
The LSN2 W3 fixed trim voltages to set the output voltage are:
V
OUT
(Typ.)
0.7525V
V
TRIM
Open
1.0V
0.6928V
1.2V
0.624V
1.5V
1.8V
2.5V
3.3V
0.267V
0.5731V 0.5221V 0.4033V
D12 Models Voltage Trim Equation:
V
TRIM
(in Volts) = 0.7 –(0.0667 x (V
O
– 0.7525))
The LSN2 D12 fixed trim voltages to set the output voltage are:
V
OUT
V
TRIM
(V)
0.7525V
Open
1.0V
0.6835
1.2V
0.670
1.5V
0.650
1.8V
0.630
2.5V
0.583
3.3V
0.530
5V
0.4166
Figure 6. Trim Connections
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LSN2 Series
Page 8 of 14
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