AN-5019 Datasheet, PDF (1/2 Pages) Fairchild Semiconductor – Calculating Driver/Receiver Power

AN-5019
Fairchild Semiconductor
Application Note
July 2002
Revised July 2002
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:
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)
(6) Static DC Power
PD_DC(max)
= I_CC(max) * V_CC(max)
= (4 mA) (3.6V)
= 14.4 mW
© 2002 Fairchild Semiconductor Corporation AN500495
www.fairchildsemi.com