AND8391-D Datasheet, PDF (1/8 Pages) ON Semiconductor – Thermal Considerations for the ON Semiconductor

AND8391/D
Thermal Considerations for
the ON Semiconductor
Family of Discrete Constant
Current Regulators (CCR)
for Driving LEDs in
Automotive Applications
http://onsemi.com
APPLICATION NOTE
Prepared by: Mike Sweador (AE),
David Helzer (PE)
ON Semiconductor
Introduction
The ON Semiconductor Constant Current Regulator
(CCR) family of devices offer outstanding regulation for
LEDs and other current based loads, such as battery
charging circuits. The CCR reduces the complexity of
resistor biased designs for sensitive loads, such as LED
strings connected in series. The CCR can also be connected
in parallel for higher load current applications. The
two−terminal CCR requires no external components to
regulate at the specified current. These devices can be used
wherever a constant current is needed to maintain
luminosity under varying voltage conditions.
See application note AND8349/D for basic circuit
considerations.
The purpose of this paper is to explore the temperature and
power boundaries for devices in the SOD−123 and
SOT−223 packages operating from typical currents of
20 mA to 30 mA in automotive applications. The SOD−123
devices available are rated at 20 mA, 25 mA, and 30 mA.
The SOT−223 devices are rated at 25 mA and 30 mA. See
Appendix A for device list.
Reference to Datasheet
The datasheet describes the devices and defines the
following terms that will be used throughout this note:
Vak = Voltage applied between the Anode and Cathode of
the device.
V_overhead = V_IN − V_LEDs
I_reg(SS) = The current through the device supplied to the
LEDs under steady−state operating conditions (device on
w10 sec)
I_reg(P) = The current through the device supplied to the LEDs
under pulse test conditions (v 300 msec).
V_R = Reverse Voltage
P_D = Device power dissipation, typically in mW.
T_A = Ambient Temperature in °C
T_J = Device Junction Temperature in °C
The SOD−123 and SOT−223 Datasheet Thermal
Characteristics table lists the thermal performance of each
device as related to the heat spreader area and thickness.
These datasheet tables and curves show thermal
specifications and limits with the device junction
temperature (T_J) operating at 150°C, the maximum
allowable continuous junction temperature.
Operating at T_{J max} continuously is not recommended for
long term reliability.
Figure 1 shows power dissipation over changes in
ambient temperature for the SOD−123 package. Figure 2
shows q_JA (°C/W) and P_D (W) for various Cu areas and
thicknesses. These tables and graphs illustrate the effect of
Cu area, thickness and ambient temperature (T_A) over the
range of −40°C to 85°C, which encompasses the area of
interest for automotive LED operation. LED data sheets
show an extreme reduction in luminosity above 85°C T_A.
© Semiconductor Components Industries, LLC, 2009
1
August, 2009 − Rev. 2
Publication Order Number:
AND8391/D