Tyco MX4428 technical manual Quiescent Current, Heat Loss

Document: LT0273	MX4428 MXP Engineering /Technical Manual
	MXP Current Consumption

It is of interest to recalculate the current consumption assuming for example the supply voltage is only 17.0V (the minimum operating voltage of the MXP). In this case the consumption is increased to 721mA. It can be seen that if the responder loop power wiring has too much resistance, the voltage to the responders is reduced by their current consumption, which results in them requiring even more current and compounding the problem.

5.1.2QUIESCENT CURRENT

The quiescent current of all responders can be calculated and used to ensure there is enough battery capacity and supply current at the MX4428.

The quiescent current for the MX loop is calculated as in section 3.2.2, but using the quiescent current instead of the alarm current. Then the MXP supply current can be calculated as described in section 5.1. Once again iterative calculations may be required to adjust for the responder loop voltage drop.

5.1.3HEAT LOSS

The heat loss from the MXP PCB can be calculated as follows –

W = ITOT(mA) * 40V * ( 1 - PCE) / PCE + IQ(mA) * 24V

Where
W	=	Heat loss in milliwatts
PCE	=	Power converter efficiency = 0.80
IQ	=	MXP quiescent current = 50mA at 24V.
ITOT	=	Total current sourced into the AL and AR terminals, which can
		be calculated as shown in Section 3.2.2

Using the above figures, the equation simplifies to

W(mW) = ITOT(mA) * 10V + 1200mW

This can be calculated separately for quiescent and alarm conditions, depending on whether quiescent or alarm figures are used to calculate IQ.

The maximum possible heat loss is 5.2 watts.

Issue 1.5 24 March 2006 Page 5-3