LM3404_1 Datasheet, PDF (1/6 Pages) National Semiconductor (TI) – Thermal Performance of the LM3404/04HV in SO-8 and PSOP-8 Packages

Thermal Performance of the
LM3404/04HV in SO-8 and
PSOP-8 Packages
National Semiconductor
Application Note 1629
Chris Richardson
May 2007
Introduction
The LM3404/04HV is a buck-regulator designed for driving
high powered LEDs at forward currents of up to 1.0A. LED
drivers often experience thermal conditions that are extreme
even by the standards of switching converters. For example,
LED drivers are often placed on the same metal-core PCB
(MCPCB) as the LEDs themselves. At 1.0A, a single-die white
LED can dissipate more than 3W. The temperature of the
MCPCB can easily reach 60°C or more. Even when the driver
is placed on a separate PCB, the combination of high power
dissipation, small, enclosed spaces, and little-to-no air flow
create high ambient temperatures and even higher junction
temperatures.
Thermal conditions for integrated (MOSFET on-board) LED
drivers are made worse by the high duty cycles of LED drivers.
For applications that use multiple LEDs, as many LEDs as
possible are placed in series to match the current and voltage
limitations of the regulator regulator. The result is that output
voltage is just below the input voltage. A voltage regulator that
provides a 5V output from a 24V input has a duty cycle of 21%,
meaning that the internal MOSFET is on for 21% of the time.
In contrast, a 24V input is often used to drive five series-con-
nected white LEDs, and at 3.5V each this gives an output
voltage of 17.5V, forcing the MOSFET to conduct for 73% of
the time.
The first part of this application note will explore the perfor-
mance of the LM3404HV in a high current, high input voltage,
high duty cycle application typical of many LED drivers, using
lab-tested thermal performance results and simulations. The
industry standard SO-8 package and the pin-for-pin compat-
ible PSOP-8 package with an exposed thermal pad (also
called a die-attach paddle, or DAP) will be compared to help
the user estimate the die temperature under various condi-
tions, and determine which package is best for their applica-
tion.
The second part of this application note uses the power dis-
sipation calculated in the first section to estimate the
LM3404HV’s die temperature under two typical configura-
tions of the LEDs relative to the LED driver. The first case is
for the LM3404HV mounted on a separate PCB that is con-
nected to the LEDs by a wiring harness. This case assumes
an ambient temperature influenced by the heating of the
LEDs, but no direct heating of the PCB by the LEDs them-
selves. The second case assumes the LM3404HV is mounted
to the same MCPCB as the LEDs themselves. In this config-
uration the temperature of the MCPCB has a much stronger
influence over the LM3404HV die temperature than the am-
bient temperature, and the tests assume a fixed MCPCB
temperature instead of a fixed ambient temperature.
Test Circuit
The test circuit uses the LM3404HV to drive ten series-con-
nected 3W white LEDs from an input voltage of 48V ±5%. The
total forward current, I_F, is 1A ±5% at a typical forward drop
of 36V (in thermal equilibrium). Output current ripple is 70
mA_P-P or less. Switching frequency is 550 kHz ±10% and the
circuit is surge protected up to 60V. A complete BOM is listed
at the back of this document, and performance waveforms are
given in Application Note AN-1585. The schematic is shown
in Figure 1.
FIGURE 1.
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