### FAIRCHILD AN-5019

```Fairchild Semiconductor
Application Note
July 2002
Revised July 2002
LVDS: Calculating Driver/Receiver Power
Introduction
(2)
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 application contributes to the total power dissipated by the system.
Calculated total device power dissipation can help determine a power source best suited for the specific application. 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.
This application note outlines an example of a power dissipation calculation for typical LVDS differential line drivers. It
provides designers a method for calculating power dissipation of individual LVDS components to assist in meeting
system power budgets.
PDOUTPUT(S)
= Power dissipated by the output(s)
IO
= Differential current per output
VCC
= Supply Voltage
VOD
= Differential Output
When dealing with LVDS products with multiple channels,
the formula to calculate the power dissipated by the output
is:
(3)
Total power dissipation can typically be divided into two
parts: a static and a dynamic component. The static component, 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.
The static power consumption of a device is the total DC
current that flows from VCC to GND with the inputs connected to VCC or GND with the outputs left open. To calculate the supply power, multiply the device supply current
(ICC) by the supply voltage (VCC). The maximum specifications are found in the DC electrical characteristics of the
datasheets.
(1)
PDDC(max)
= ICC(max)
*
VCC(max)
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 dissipated by the differential outputs:
(4)
Components of
Total Power Dissipation
PDOUTPUT(S) = [IO(VCC −VOD)]
Where,
PDTOTAL = PDDC + PDOUTPUT(S)
For an LVDS receiver, the supply power is calculated similarly to the approach used for the driver. The output power
of the receiver would be derived using the following equation 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 dissipation, with assigned values for illustrative purposes only. If
the exact application configuration is known, appropriate
adjustments can be made to the calculations.
Power Dissipation Calculation
Example
To illustrate the calculation for total power dissipation, this
example uses typical values for a Quad High-Speed Differential Line Driver (FI1031) with the following conditions:
Where,
PDDC
= Static DC Power
ICC
= Supply Current
VCC
= 3.6V (max)
VCC
= Supply Voltage
TA
= 25°C
VOD
= 350 mV (typical)
IOD
= 3.5 mA (typical)
ICC
= 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
output current (IO) being sourced and sunk. The formula to
calculate the output power dissipated by a single differential channel is:
(6)
Static DC Power
PDDC(max)
= ICC(max) * VCC(max)
= (4 mA) (3.6V)
= 14.4 mW
© 2002 Fairchild Semiconductor Corporation
AN500495
www.fairchildsemi.com
AN-5019 LVDS: Calculating Driver/Receiver Power
AN-5019
AN-5019 LVDS: Calculating Driver/Receiver Power
Power Dissipation Calculation Example
(7)
(Continued)
(9) Total Power
Dynamic Output Power
PDOUTPUTS
= (No. of channels) [IO(VCC − VOD)]
PDTOTAL
= PDDC + PDOUTPUT(S) + COUT (VCC)2(f)
COUT
= device output capacitive load
f
= switching frequency
= (4) [3.5 mA (3.6V − 350 mV)]
= 45.5 mW
(8) Total Power
PDTOTAL
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 exponentially, resulting in very low (almost flat) power consumption across frequency. Refer to Figure 1 for a relative
comparison.
= 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 switching frequency. Therefore, the equation would be as follows:
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 frequency 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.
LIFE SUPPORT POLICY
FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD
SEMICONDUCTOR CORPORATION. As used herein:
2. A critical component in any component of a life support
device or system whose failure to perform can be reasonably expected to cause the failure of the life support
device or system, or to affect its safety or effectiveness.
1. Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the
body, or (b) support or sustain life, and (c) whose failure
to perform when properly used in accordance with
instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the
user.
www.fairchildsemi.com
www.fairchildsemi.com
2
```