? 2002 fairchild semiconductor corporation an500495 www.fairchildsemi.com fairchild semiconductor application note july 2002 revised july 2002 AN-5019 lvds: calculating driver/receiver power AN-5019 lvds: calculating driver/receiver power 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. 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. 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. the static power consumption of a device is the total dc current that flows from v cc to gnd with the inputs con- nected to v cc or gnd with the outputs left open. to calcu- late the supply power, multiply the device supply current (i cc ) by the supply voltage (v cc ). the maximum specifica- tions are found in the dc electrical characteristics of the datasheets. (1) pd dc(max) = i cc(max) *v cc(max) where, pd dc = static dc power i cc = supply current v cc = supply voltage 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 (v od ) and the output current (i o ) being sourced and sunk. the formula to calculate the output power dissipated by a single differen- tial channel is: (2) pd output(s) = [i o (v cc ? v od )] where, pd output(s) = power dissipated by the output(s) i o = differential current per output v cc = supply voltage v od = differential output when dealing with lvds products with multiple channels, the formula to calculate the power dissipated by the output is: (3) pd output(s) = (# of channels) [i o (v cc ? v od )] 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) pd total = pd dc + pd output(s) 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) pd output = v ol * i ol + [(v cc ? v oh ) * i oh ] 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. 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: (6) static dc power v cc = 3.6v (max) t a = 25 c v od = 350 mv (typical) i od = 3.5 ma (typical) i cc = 4 ma (max) pd dc(max) = i cc(max) * v cc(max) = (4 ma) (3.6v) = 14.4 mw
www.fairchildsemi.com 2 AN-5019 lvds: calculating driver/receiver power power dissipation calculation example (continued) (7) dynamic output power (8) total power 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: (9) total power for most differential line drivers the magnitude of the cv 2 f 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, i cc does not increase expo- nentially, resulting in very low (almost flat) power consump- tion across frequency. refer to figure 1 for a relative comparison. figure 1. i cc 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. pd outputs = (no. of channels) [i o (v cc ? v od )] = (4) [3.5 ma (3.6v ? 350 mv)] = 45.5 mw pd total = pd dc + pd output(s) = 14.4 mw + 45.5 mw = 59.9 mw pd total = pd dc + pd output(s) + c out (v cc ) 2 (f) c out = device output capacitive load f = switching frequency 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: 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 rea- sonably expected to result in a significant injury to the user. 2. a critical component in any component of a life support device or system whose failure to perform can be rea- sonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. www.fairchildsemi.com
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