AD2428W datasheet(30/38 Pages) AD

30 / 38 page

Rev. B

Page 30 of 38

January 2020

AD2420(W)/AD2426(W)/AD2427(W)/AD2428(W)/AD2429(W)

CURRENT AT VIN (IVIN)

The current at the VIN pin (IVIN) of the transceiver is the sum of

currents IVOUT1 and IVOUT2 and the quiescent current, shown in

Figure 34 and in the following equation:

IVIN = IVOUT1 + IVOUT2 + IVINQ

The A side node current is the line bias current from an earlier

node. In a bus powered node, it is also the power supply current

and a portion of this current supplies the next in line nodes.

IA = IVIN + IB + IVREGPERI

where:

IB = B side current to the next node (= IVSSN return current and

IA of the next in line node).

IVREGPERI = peripheral current supplied from IA by extra voltage

regulator, external to the transceiver (not illustrated in Figure 35

and Figure 36).

POWER DISSIPATION

The power dissipation of the transceiver is calculated using the

following equation:

Power = 

IVIN × VVIN + (IVSSN)

2 × RVSSN – IVEXT1 × VVOUT1 – IVEXT2 × VVOUT2

where:

IVIN = current at VIN pin.

VVIN = voltage at VIN pin.

IVSSN = B side current IB to the next node and return current

from the next node. The next node is the node connected to the

B terminal of the current node. See Figure 35.

RVSSN = internal VSSN on resistance (see Table 16).

IVEXT1 = peripheral supply current from VVOUT1.

IVEXT2 = peripheral supply current from VVOUT2.

VVOUT1 = output voltage from VREG1.

VVOUT2 = output voltage from VREG2.

RESISTANCE BETWEEN NODES

Figure 35 shows the dc model of a system with a combination of

local and bus powered A2B slaves.

A voltage drop of the dc bias is observed between the A2B nodes,

due to resistance and current consumption. Table 16 lists the

causes of the dc resistance between nodes (RBETWEEN) with exam-

ple resistance values.

Both bias supply and return currents are subject to resistance.

Therefore, some resistance values must be doubled (for exam-

ple, wire length resistance).

Table 16. Breakdown/Budget of Typical DC Resistance

Between Nodes

Resistance

Each

Qty

Total

Unit

Inductor DC Resistance

0.26

1.04

Short Circuit Protection Resistor

1.05

Positive Bias PMOS Switch



On-Resistance

0.11

Negative Bias Switch



On-Resistance RVSSN

1.2

Resistance of Connections

0.01

0.04

Total RSUM

N/A1

1 N/A means not applicable.

N/A1

2.39

Wire Length Resistance of Cable

0.121

0.242

Ω/m

Figure 35. A2B DC Power Model for a System with Local and Bus Powered Slaves

Master Node

IVIN

IVSSN

PMOS

Connections

Cable

VREG

ISUM

<VVIN max

ISUM

VNODE0

VNODEM

LOCAL POWER

Connections

Cable

Slave Node 0

PMOS

Connections

Cable

VREG

<VVIN min

Connections

Cable

Slave Node n

AD2426(W)/

AD2427(W)/

AD2428(W)

VSS

VIN

VSSN

VNODEn

BUS POWER

AD2428(W)

VSS

VSSN

VSS

VSSN

VIN

AD2427(W)/

AD2428(W)

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