Circuit Note

CN-0537

Rev. A | Page 7 of 8

When selecting an inductor, the LT8410 data sheet recommends

an inductor with at least 45 µH and a saturation current rating

higher than the peak inductor current. Use Equation 19 to

calculate the maximum peak inductor current.

6

150 10

IN

PK

LIMIT

V

II

L

−

××

=

+

(19)

where:

IPK is the peak inductor current in mA.

ILIMIT is the switch current limit in mA.

VIN is the input voltage in V.

L is the inductance in H.

A 100 µH inductor was used in this design and substituting

this value in the equation results in a peak inductor current of

37.5 mA for an input voltage of 5 V and a maximum switch

current limit of 30 mA.

Use Equation 20 to Equation 23 to determine the maximum

output current of the LT8410 circuit.

6

(

1

) 200 10

OUT

IN

RIPPLE

VV

I

L

−

+−

×

×

=

(20)

()

2

RIPPLE

IN AVG

PK

I

II

=

−

(21)

()

()

0.7

IN AVG

IN

OUT NOM

OUT

I

V

I

V

×

×

=

(22)

IOUT = IOUT(NOM) × 0.8

(23)

where:

IRIPPLE is the inductor ripple current in mA.

IIN(AVG) is the average LT8410 input current in mA.

IOUT(NOM) is the nominal output current in mA.

IOUT is the maximum output current in mA.

Substituting the inductance of 100 µH and a peak inductor

current of 37.5 mA into these equations results in a maximum

output current of 14.23 mA for an input voltage of 5 V and a

typical switch current limit of 25 mA. Simulating this LT8410

circuit in LTspice® across a range of load currents results in

the efficiency plot shown in Figure 13.

Figure 13. LT8410 Efficiency vs. Load Current Plot

Use Equation 24 to calculate the amount of supply current for

each LED required by the ADPD188BI from the ADP151 and

LT8410 circuits.

ILED_AVE_x = SLOTx_LED_WIDTH × ILED1_PK × DR ×

PULSE_COUNT

(24)

where:

ILED_AVE_x is the average LED supply current in mA for Time

Slot A or Time Slot B.

SLOTx_LED_WIDTH is the blue LED pulse width in seconds

for Time Slot A or Time Slot B.

ILEDx_PK is the peak current in mA (calculated using Equation 3

to Equation 6).

DR is the output data rate in Hz.

PULSE_COUNT is the number of LED pulses in Time Slot A

or Time Slot B.

COMMON VARIATIONS

If humidity sensing is not needed for the smoke detection

algorithm, the ADT7302 digital temperature sensors can be

used in place of the ADPD188BI. The ADT7302 has a small

footprint that can be placed under the chamber, an accuracy

of 2°C with a resolution of 0.03125°C, but comes at a fraction

of the cost of the default sensor (ADPD188BI).

CIRCUIT EVALUATION AND TEST

The following section outlines the general setup for evaluating

the CN-0537. For complete setup and other detailed instructions

see the EVAL-CN0537-ARDZ Hardware User Guide.

Equipment Needed

The following equipment is needed:

EVAL-CN0537-ARDZ

EVAL-ADICUP3029

Micro SD card

Micro USB to USB Type A cable

PC or laptop with a USB port

Serial terminal application

CN0537.hex file

Setup and Test

For system evaluation, the CN-0537 includes demonstration

application software that allows the user to communicate to

the on-board ADPD188BI using the EVAL-ADICUP3029

development platform. Take the following steps to use this

software:

1. Download the latest version of the CN-0537

demonstration application software (*.hex) file from the

EVAL-CN0537-ARDZ Hardware User Guide.

100

40

50

70

90

60

80

0.01

100

10

1

0.1

LOAD CURRENT (mA)