AD2428W datasheet(32/38 Pages) AD

32 / 38 page

Rev. B

Page 32 of 38

December 2019

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

For a system with all in-line bus powered nodes, IA is calculated

cumulatively from the last in-line node as

IA = IVIN + IB

where:

IA = current that a bus powered slave node draws from an earlier

node.

IVIN = current at VIN pin.

IB = B side return current from the next in line node. The next in

line node is the node connected to the B terminal of the current

node.

REDUCING POWER CONSUMPTION

The following sections describe three methods that reduce

power consumption.

Power-Down Mode

Any node in an A2B bus powered system can be shut down by

disconnecting the power supply in the previous slave node

(towards the master). Disconnecting a slave from the power

supply also powers down all of the following slaves (towards the

last slave).

Standby Mode

The A2B bus enters standby mode by setting the 

AD242X_DATCTL.STANDBY bit of the master. The SCF in

standby mode is 19 bits long, instead of 64 bits. In standby

mode, there is no upstream and no downstream traffic on the

A2B bus, and only a minimal SCF keeps all the slave nodes syn-

chronized. This keeps the A2B bus power in the lowest power

state while maintaining clock synchronization between nodes.

Using the equations in the Downstream/Upstream Activity

Level section, the bus activity level in standby mode is,

• Downstream activity level = 19 ÷ 1024 = 1.9%

• Upstream activity level = 0%

The digital transceiver current, including the LVDS TX and RX

current, are subject to the activity levels. The LVDS TX and RX

current also are subject to idle current during the bus idle time.

Control Mode

In control mode, there are no data channels in a superframe.

The superframe only has the 64-bit SCF and SRF in the frame

with the control data embedded in the header bits. Therefore,

the A2B bus power is less when compared to normal mode,

which has data channels in the superframe. Using the equations

in the Downstream/Upstream Activity Level section, the bus

activity level in control mode is,

• Bus activity level downstream = 64 ÷ 1024 = 6.3%

• Bus activity level upstream = 64 ÷ 1024 = 6.3%

The LVDS TX and RX current also are subject to idle current

during the bus idle time.

THERMAL POWER

When calculating power, system designers must consider ther-

mal power. Thermal power calculations are based on the

package thermal characteristics, shown in the following

equation:



JA = thermal resistance

(TJ – TA) ÷ power [°C/W] with airflow = 0 m/s

Table 17 provides the thermal power allowance example. These

values are derived from a JEDEC standard 2S2P test board. JA

values vary significantly and depend on system design and con-

ditions. For the example calculation in Table 17, the JA value

provided is determined using the JEDEC standard conditions.

In this example (Table 17), a slave node with 292 mW of power

dissipation is used to provide the maximum estimated power.

The margin is calculated by subtracting the maximum estimated

power from the thermal power allowance.

Table 17. Thermal Power Allowance Example

Parameter

Example Value Unit

105

°C

125

°C

Delta (TJ – TA)20

°C

Example



31.6

°C/W

Maximum Allowed Thermal Power

633

Maximum Estimated Power

292

Power Margin

341

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