Texas Instruments | BTL Power Dissipation Calculation | Application notes | Texas Instruments BTL Power Dissipation Calculation Application notes

Texas Instruments BTL Power Dissipation Calculation Application notes
BTL Power Dissipation Calculation
Literature Number: SNLA162
National Semiconductor
Application Note 839
Joel Martinez,
Stephen Kempainen
July 1992
INTRODUCTION
Futurebus+ systems designed today have bus widths of 32
or 64. To support higher bandwidths in the future, Futurebus+ provides a data width extension of up to 256 bits. BTL
(Backplane Transceiver Logic) is the electrical signaling environment for Futurebus+. The number of BTL transceivers
in a system will increase as the bus width is extended to accommodate higher bandwidths. A total of 16 transceivers is
required to implement a 64-bit data bus. Four 6-bit Handshake Transceivers (DS3884A) are used for the handshake,
central arbitration, capability and serial bus lines. One 9-bit
Arbitration Transceiver (DS3885) is used for the arbitration
competition lines. Three 9-bit Data Transceivers (DS3883A
or DS3886A) are used for the command, status and tag
lines. A 64 wide bus with byte parity requires 8 data transceivers for the multiplexed address and data lines alone. A
256 wide bus requires 32 data transceivers. Including the
other lines, the total number of transceivers on single board
for a 256-bit data bus is 40. The power required and dissipated by these transceivers must be fully understood to design an efficient cooling and power supply system for the
backplane. Power calculations differ depending on the assumptions made concerning supply and output power. This
application note illustrates how to use graphs provided by
manufacturers to obtain accurate power calculations. Power
is calculated for these conditions; worst case, driver outputs
high, driver outputs low and outputs switching.
CALCULATING BTL POWER USING THE STANDARD
METHOD
The power equation may be divided into two parts. One part
is the supply power (PS) which is derived from the quiescent
current flowing into the power pins. The other is output
power (PO) derived from current flowing into or out of the input and output pins. The standard method to calculate PS
and PO is to use specifications available in the DC Electrical
Characteristics of datasheets. ICC(typ) and ICC(max) are
typical and maximum supply current which may be found directly within the DC Electrical Characteristics. Typical power
dissipation (PS__Typ) is equal to ICC(typ) x VCC(typ). Maximum power dissipation (PS__max) is equal to ICC(max) x VCC(max). Output power is different when the output is low and
when it is high. With the output high (the device is in high impedance) essentially zero current flows in or out of the device and PO equals zero. With the output low, PO is equal to
VOL(max) x IOL(max). PO is worst case when the output is
low or asserted. Typical and Maximum power calculations
are shown in Table 2 for DS3886A. VOL and IOL will differ depending on the termination resistor and voltage used on the
backplane which is not taken into account. Actual PO for Futurebus+ is less and an accurate calculation of power dissipation is presented later.
BTL Power Dissipation Calculation
BTL Power Dissipation
Calculation
POWER CALCULATION EQUATIONS AND
DEFINITIONS
Table 1 summarizes the equations and terms used in following discussions.
TABLE 1. Power Calculation Equations and Terms
Parameter
Equation
Description
VCC
4.5V < VCC < 5.5V
Supply Voltage
ICC
See DC Electrical Characteristics, Typical ICC vs
Temperature Curves or Typical ICC vs Frequency
Supply Current
VOL
See Typical IOL vs VOL Curves
Output Low Voltage
For the BTL Driver VOL will Depend on the
Termination Resistance Used
IOL
See Typical IOL vs VOL Curves
Output Low Current
For the BTL Driver IOL will Depend on the
Termination Resistance Used
Vt
2.1V
Termination Voltage
PS
ICC x VCC
Supply Power
Power Dissipated Due to Quiescent Current Flowing
into Power Pins
PO
VOL x IOL
Output Power
Power Dissipated per Channel at the Driver or
Receiver Output when the Output is Low
© 1998 National Semiconductor Corporation
Power Dissipated per Channel at the Driver or
Receiver Output when the Output is High; For BTL
IOH is Zero
AN011487
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AN-839
(VCC − VOH) x IOH
TABLE 2. Standard Power Calculations for the DS3886A
Typical
PS = 5.0V x 55 mA = 275 mW
PO = 1.0V x 65 mA = 65 mW
Max
PS = 5.5V x 62 mA = 341 mW
PO = 1.1V x 80 mA = 88 mW
Ptotal = 275 mW + (65 mW x 9) = 860 mW
Ptotal = 341 mW + (88 mW x 9) = 1.13W
REVIEW OF BTL — BACKPLANE TRANSCEIVER
LOGIC
A brief review of BTL is given to help the reader understand
how the driver operates. A schematic of the BTL output stage
is shown in Figure 1. The driver output is composed of a
Schottky diode — DS2 in series with the collector of transistor Q1. DS2 shields the capacitance of Q1 from the bus to reduce capacitive loading. When the driver is asserted Q1 is
“on”, node a is approximately 0.4V, and node b is approximately 1V. Current flows from the 2.1V termination voltage
through Rt/2, through DS2 and into the collector of Q1.
When the driver is released Q1 is “off”, a 12.5 kΩ resistor
pulls node a which is clamped to 3V by four diode clamps
(DC1 to DC4) in series. DS2 is reversed biased (node a is at
a higher potential than node b), with node b at 2.1V which is
equal to the termination voltage. Essentially zero current
flows into or out of the driver output.
AN011487-1
FIGURE 1. Futurebus+ BTL
Driver Output Schematic
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2
turebus+ requires 33Ω termination resistors in series with
2.1V at each end of the backplane as shown on Figure 2.
The Thevenin equivalent for the Futurebus+ termination of
33Ω is 16.5Ω in series with 2.1V which is shown in Figure 3.
IOL for a Futurebus+ termination will be less than 80 mA.
IOL for a Futurebus+ termination is equal to the intersection
point between the output VOL vs IOL curve and the resistor
load line as shown on Figure 4. The intersection point for a
Futurebus+ load line of 16.5Ω in series with 2.1V at 25˚C, is
65 mA and 1.025V. The intersection point for 12.5Ω in series
with 2.1V is 83 mA and 1.075V. The intersection point for 9Ω
in series with 2.1V is 110 mA and 1.125V. IOL and VOL may
be obtained for any termination resistor and voltage using
the load line intersection point.
CALCULATION OF OUTPUT POWER USING LOAD
LINES
IEEE 1194.1 BTL Electrical Characteristics define IOL and
VOL. BTL devices are required to sink 80 mA (IOL) within a
specified VOL range of 0.75V to 1.1V. This requirement was
established to maintain compatibility between vendors offering BTL devices. The BTL devices offered by National conform to this specification. The actual IOL flowing into the output is dependent on the termination resistor and voltage on
the backplane. National’s datasheets specify 12.5Ω in series
with 2.1V to test AC and DC requirements which conform to
an IOL of 80 mA. The equivalent representation of a 12.5Ω
load in a backplane environment is 25Ω at opposite ends.
The Thevenin equivalent of two 25Ω resistors is 12.5Ω. Fu-
AN011487-2
FIGURE 2. Futurebus+ Backplane Termination
AN011487-3
FIGURE 3. Thevenin Equivalent
of Futurebus+ Backplane
3
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AN011487-4
FIGURE 4. IOL vs VOL and Load Line
a 50% duty cycle. Power is calculated for a 50% duty cycle
in Table 3. To calculate power for other duty cycles the equation below may be used:
PO(k) = PO (asserted) x k
Output power (PO) per channel is equal to the product of IOL
vs VOL at the intersection point. For Futurebus+ with the
driver asserted, the point of intersection is 65 mA and
1.025V which yields 66.6 mW for output power. Output
power for a transceiver is directly proportional to the number
of bits. A 9-bit transceiver (like the DS3886A, DS3883A or
DS3885) with all outputs low will need to dissipate 599 mW,
which is 66 mW times 9 bits. In addition to PO, PS must also
be included for the total power. Calculations for PS will be
given later. Output power for different loads is shown on
Table 3. Output power almost doubles when the resistor is
reduced from 16.5Ω to 9Ω. When all drivers are released,
the outputs are in a high impedance state and pulled high by
the termination. At this state, essentially zero power is dissipated at the outputs.
Output power is also dependent on the duty cycle. The
power dissipated at the output will equal the average power
between the asserted and the released state. Assume that in
a normal operation the drivers are high half of the time and
low half of the time. This condition may be approximated to
where k is equal to the duty cycle.
For example, PO is calculated for a 9-bit device terminated
according to Futurebus+ specification with a 60% duty cycle.
PO(60%) = 600 mW x 0.60 = 360 mW
CALCULATION OF TOTAL BTL TRANSCEIVER POWER
Total Power is equal to the sum of PO and PS. Using PO from
Table 3 and PS derived from Figure 5, Figure 6, Figure 7,
Table 4 shows total typical power for different conditions; all
outputs high, all outputs low, 10 MHz and 20 MHz with 50%
duty cycles. Supply power is equal to ICC x VCC, where VCC
and ICC were taken from Figure 5, Figure 6, Figure 7. Table
5 shows Maximum power using ICC(max) x VCC(max) from
the datasheet.
TABLE 3. Calculation of BTL Output Power (T = 25˚C)
Termination
Termination
Output Low
Output Low
Output
Output
Output
Voltage
in Parallel
Current — IOL
Voltage — VOL
Power
Power 6-Bits
Power 9-Bits
(V)
(Ω)
(mA)
(V)
(mW)
(mW)
(mW)
600
BTL OUTPUT POWER PER CHANNEL WHEN DRIVERS ARE ASSERTED
2.1
16.5
65
1.025
66.6
400
2.1
12.5
83
1.050
87.2
523
784
2.1
9
110
1.125
123.8
743
1,114
0
0
0
66.6/2
200
300
BTL OUTPUT POWER PER CHANNEL WHEN DRIVERS ARE RELEASED
2.1
16.5, 12.5 or 9
0
2.1V
BTL OUTPUT POWER PER CHANNEL WHEN DRIVERS @ 50% DUTY CYCLE
2.1
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16.5
65
1.025
4
TABLE 3. Calculation of BTL Output Power (T = 25˚C) (Continued)
Termination
Termination
Output Low
Output Low
Output
Output
Output
Voltage
in Parallel
Current — IOL
Voltage — VOL
Power
Power 6-Bits
Power 9-Bits
(V)
(Ω)
(mA)
(V)
(mW)
(mW)
(mW)
BTL OUTPUT POWER PER CHANNEL WHEN DRIVERS @ 50% DUTY CYCLE
2.1
12.5
83
1.050
87.2/2
262
392
2.1
9
110
1.125
123.8/2
372
557
TABLE 4. DS3886 Typical Power with Futurebus+ Termination
Supply Power
Output Power
All Outputs Low (25˚C and 5V)
Parameter
210 mW
600 mW
Total Power
810 mW
All Outputs High (25˚C and 5V)
140 mW
0 mW
140 mW
Switching at 10 MHz (25˚C and 5V)
230 mW
300 mW
530 mW
Switching at 20 MHz (25˚C and 5V)
235 mW
300 mW
535 mW
TABLE 5. DS3886 Maximum Power with Futurebus+ Termination
Parameter
All Outputs Low
Supply Power
Output Power
Total Power
341 mW
600 mW
941 mW
AN011487-5
AN011487-6
FIGURE 5. DS3886A ICC vs Temperature
(All Bn Outputs Low)
FIGURE 6. DS3886A ICC vs Temperature
(All Bn Outputs High)
5
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AN011487-7
FIGURE 7. DS3886A ICC vs Switching Frequency (An to Bn)
(All Channels Switching at Room Temperature)
The maximum power dissipation result shown in Table 5 is
1.315W compared to 1.51W in Table 2 derived from the standard method. Power calculated using the standard method is
15% higher than actual maximum of 1.315W. IOL of 80 mA
and VOL of 1.1V was used in the standard method yielding
the higher PO. The results derived in Table 5 take into account the Futurebus+ termination specification in the calculation of PO. There are four typical power calculations in
Table 4 reflecting different conditions. Typical power varied
from 325 mW when all outputs are high to 990 mW when all
outputs are low. The typical power calculated in Table 2 is
1.08W which is 10% higher than that calculated in Table 4.
These different power calculations are important when determining the total power of a system. A Futurebus+ backplane
may contain several modules. Only one module may transmit data on the bus while others receive. It is important to
know how to calculate power dissipation for modules in the
transmit mode as well as modules in the receive mode to derive accurate system power.
65 mA x (89 + 32 + 4) = 8.13A
Calculation of typical system power is shown below. Two assumptions are made for the given equation. First, one module transmits while others receive. Second, the transmitting
module will have an average duty cycle equal to 50% and
running at 20 MHz. Power per bit allows easy calculation of
total power for different number of lines. Dividing the total
transceiver power by the number of bits on the transceiver
will yield power/bit. The total number of modules on the
backplane minus one gives the number of receiving modules. The receiving modules will have their outputs in a high
state.
(Power/bit with 50% Duty Cycle @ 20 MHz x # of Lines) +
(# Boards − 1) (Power/Bit with all Outputs High x # of Lines)
= Transceiver System Power
For a 32-bit Futurebus+ Interface with 14 boards the transceiver system power required will be;
(725 mW/9 x 89) + (14 − 1) (325 mW/9 x 89) = 49W
CALCULATION OF SYSTEM POWER REQUIREMENTS
In a backplane, one board will normally be active and driving
the bus while other modules are receiving or in standby. If all
lines were asserted, the termination voltage must be able to
supply enough current to all the lines. To determine the current requirement from the termination voltage, use this equation;
IOL x Number of Lines = Termination Current
CONCLUSION
There are several ways of calculating power dissipation. The
results of these calculations will greatly vary depending on
the assumptions. An error in calculating output power will be
multiplied when extending the results to determine overall
system power. A thorough understanding of how to calculate
power will yield accurate power calculations. A preferred
method of calculating output power is to use the load line intersection point. Typical power dissipation calculation should
use duty cycle approximation. Typical device curves are provided with datasheets. Refer to individual datasheets for
most up to date information. With this background, the designer will be able to make accurate thermal and power
analysis of the interface which may ultimately reduce cost.
In a 32-bit Futurebus+ backplane the total number of lines
will be 89 which includes all the required lines in addition to
the 32-bit address/data lines. The current requirement from
the termination supply will be;
65 mA x 89 Lines = 5.79A
A 64-bit Futurebus+ backplane requires;
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6
7
BTL Power Dissipation Calculation
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