CN-0537

Circuit Note

Rev. A | Page 6 of 8

To mitigate the effect of condensation, heating resistors are

placed around the optical module, which are meant to dissipate

enough heat to clear out dew formation as the need arises. When

selecting resistors for this purpose, a compromise is needed

between the desired temperature rise and the current drawn

from the power supply.

Note that this heating block significantly increases the total

system power required by the design. Using lower resistance

values generates more heat but draws more supply current.

For battery-powered systems, lower resistance values also

results in a shorter battery life and must be an additional

design consideration.

Figure 9. Infrared Image of the CN-0537 Heating Resistors

The CN-0537 reference design uses a parallel combination of

three 25 Ω resistors, resulting in a temperature increase of

10°C to 20°C. A transistor switch allows the microcontroller

board to activate the heating circuit block through a pulse

width modulated (PWM) output or a general-purpose

input/output (GPIO) pin.

Figure 10. Placement of Heating Resistors

System Power Management

Power to the CN-0537 is sourced from the controller board

through the Arduino form-factor connectors and the most

active devices in the circuit are directly powered by the 3.3 V

and 5 V supplies. The ADPD188BI, however, requires supply

voltage levels of 1.8 V, 3.3 V, and ≥5 V for proper operation.

To produce the 1.8 V required to power the ADPD188BI, the

3.3 V regulated power is fed into an ADP151. This CMOS linear

regulator has an input voltage range of 2.2 V to 5.5 V and a

maximum output current of 200 mA. The output voltage of

the ADP151 is fixed to 1.8 V during production of the device

and only requires input and output capacitors for operation,

greatly simplifying the circuit design.

Figure 11. ADP151 Basic Connections

Similarly, to produce the voltage required to power the

integrated blue LED of the ADPD188BI, the 5 V supply is fed

into an LT8410 ultra-low power boost converter, implemented as

shown in Figure 12.

Figure 12. LT8410 Basic Connections

The regulated output of the LT8410 is directly related to the

voltage level on its positive feedback pin (FBP). To produce a

desired level of VOUT, the necessary level of VFBP is calculated

using Equation 18.

VFBP = VOUT ÷ 31.85

(18)

where:

VFBP is the voltage across the FBP pin and GND in V.

VOUT is the desired output voltage in V.

For an output voltage of 6 V, Equation 18 results in a VFBP of

approximately 0.1884 V. To achieve this voltage level, a simple

voltage divider is used with the integrated 1.235 V reference

voltage of the LT8410, as shown in Figure 12. When selecting

resistors for the voltage divider, ensure that the series

resistance is greater than 200 kΩ to avoid loading down the

VREF pin.

As recommended by the LT8410 data sheet, the capacitors

used for the input and output pins have values of 2.2 µF and

1 µF, respectively and 0.1 µF capacitors are used for the CAP

and VREF pins.

6

VIN = 3.3V

VOUT = 1.8V

1µF

1µF

5

4

VIN

NC

ON

OFF

EN

VOUT

NC

GND

1

2

3

ADP151

SW

VIN = 5V

VOUT = 6V

CIN

2.2µF

L1

100µH

0.1µF

0.1µF

CAP

GND

FBP

SHDN

VREF

330kΩ

59kΩ

VCC

VOUT

LT8410

COUT

1µF