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

of 36V (in thermal equilibrium). Output current ripple is 70

mA

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.

30019201

FIGURE 1.

© 2007 National Semiconductor Corporation

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