AN-5019 [FAIRCHILD]
Calculating Driver/Receiver Power; 计算驱动器/接收器电源
| 型号： | AN-5019 |
| 厂家： | 
FAIRCHILD SEMICONDUCTOR |
| 描述： | Calculating Driver/Receiver Power |
| 文件： | 总2页 (文件大小：105K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
Fairchild Semiconductor
Application Note
July 2002
AN-5019
Revised July 2002
LVDS: Calculating Driver/Receiver Power
(2) PDOUTPUT(S) = [IO(VCC−VOD)]
Where,
PDOUTPUT(S)
Introduction
To insure system functionality and reliability many board
and system level designs must employ power budgets. The
cumulative power dissipated by each device in the applica-
tion contributes to the total power dissipated by the system.
Calculated total device power dissipation can help deter-
mine a power source best suited for the specific applica-
tion. It can also provide an understanding of the system’s
(or board’s) operating conditions that might have an impact
on system reliability or cause damage to on board ICs.
= Power dissipated by the output(s)
= Differential current per output
= Supply Voltage
IO
VCC
VOD
= Differential Output
When dealing with LVDS products with multiple channels,
the formula to calculate the power dissipated by the output
is:
This application note outlines an example of a power dissi-
pation calculation for typical LVDS differential line drivers. It
provides designers a method for calculating power dissipa-
tion of individual LVDS components to assist in meeting
system power budgets.
(3) PDOUTPUT(S) = (# of channels) [IO(VCC−VOD)]
The approximate total power dissipated by the differential
driver is the sum of the supply power and the power dissi-
pated by the differential outputs:
(4) PDTOTAL = PDDC + PDOUTPUT(S)
Components of
Total Power Dissipation
Total power dissipation can typically be divided into two
parts: a static and a dynamic component. The static com-
ponent, or supply power, is derived from current flowing
into the power pins. The dynamic component is the output
power derived from current into or out of the output pins.
For an LVDS receiver, the supply power is calculated simi-
larly to the approach used for the driver. The output power
of the receiver would be derived using the following equa-
tion and inserting the values from the datasheet electricals:
(5) PDOUTPUT = VOL * IOL + [(VCC − VOH) * IOH
]
The device switching frequency component of the total
power varies from application to application. The following
example demonstrates how to calculate total power dissi-
pation, with assigned values for illustrative purposes only. If
the exact application configuration is known, appropriate
adjustments can be made to the calculations.
The static power consumption of a device is the total DC
current that flows from VCC to GND with the inputs con-
nected to VCC or GND with the outputs left open. To calcu-
late the supply power, multiply the device supply current
(ICC) by the supply voltage (VCC). The maximum specifica-
Power Dissipation Calculation
Example
To illustrate the calculation for total power dissipation, this
example uses typical values for a Quad High-Speed Differ-
ential Line Driver (FI1031) with the following conditions:
tions are found in the DC electrical characteristics of the
datasheets.
(1) PDDC(max) = ICC(max)
Where,
PDDC
*
VCC(max)
= Static DC Power
= Supply Current
= Supply Voltage
VCC
TA
= 3.6V (max)
= 25°C
ICC
VCC
VOD
IOD
ICC
= 350 mV (typical)
= 3.5 mA (typical)
= 4 mA (max)
The current sinking and sourcing capability of the driver’s
output structure, along with the load being driven, dictates
the amount of power being consumed.
To calculate the dynamic power dissipated by the device
outputs, use the differential output voltage (VOD) and the
(6) Static DC Power
output current (IO) being sourced and sunk. The formula to
calculate the output power dissipated by a single differen-
tial channel is:
PDDC(max)
= ICC(max) * VCC(max)
= (4 mA) (3.6V)
= 14.4 mW
© 2002 Fairchild Semiconductor Corporation
AN500495
www.fairchildsemi.com
Power Dissipation Calculation Example (Continued)
(7) Dynamic Output Power
(9) Total Power
PDOUTPUTS
= (No. of channels) [IO(VCC − VOD)]
PDTOTAL
= PDDC + PDOUTPUT(S) + COUT (VCC)2(f)
= (4) [3.5 mA (3.6V − 350 mV)]
= 45.5 mW
COUT
f
= device output capacitive load
= switching frequency
(8) Total Power
For most differential line drivers the magnitude of the CV2f
term on total device power dissipation is negligibly small.
The significant advantage of LVDS technology is the low
power requirement because of the constant current source
driver rather than a voltage mode driver. With minimal
switching spikes in the driver, ICC does not increase expo-
PDTOTAL
= PDDC + PDOUTPUT(S)
= 14.4 mW + 45.5 mW
= 59.9 mW
A more comprehensive total power dissipation calculation
would include power dissipation from the device’s switch-
ing frequency. Therefore, the equation would be as follows:
nentially, resulting in very low (almost flat) power consump-
tion across frequency. Refer to Figure 1 for a relative
comparison.
FIGURE 1. ICC vs. Frequency
Summary
An advantage of LVDS is its low power at high data rates.
With a current draw of 3.5 mA per output, an LVDS output
at 3.3V dissipates about 11 mW, a constant with the fre-
quency of operation. A method for calculating the total
power dissipated by an LVDS TIA/EIA-644 compliant driver
and receiver was presented. This approach can be applied
to similar LVDS devices designed to meet the TIA/EIA-644
requirements and specifications.
Fairchild does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and
Fairchild reserves the right at any time without notice to change said circuitry and specifications.
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