LT1528
8
1528fb
The LT1528 is specifi ed with the SENSE pin tied to
the OUTPUT pin. This sets the output voltage to 3.3V.
Specifi cations for output voltage greater than 3.3V will
be proportional to the ratio of the desired output voltage
to 3.3V (V
OUT
/3.3V). For example, load regulation for an
output current change of 1mA to 1.5A is – 5mV (typical) at
V
OUT
= 3.3V. At V
OUT
= 12V, load regulation would be:
(12V/3.3V) • (–5mV) = (–18mV)
Thermal Considerations
The power handling capability of the device will be limited
by the maximum rated junction temperature (125°C). The
power dissipated by the device will be made up of two
components:
1. Output current multiplied by the input/output voltage
differential, I
OUT
• (V
IN
– V
OUT
), and
2. GND pin current multiplied by the input voltage,
I
GND
• V
IN.
The GND pin current can be found by examining the GND
Pin Current curves in the Typical Performance Character-
istics. Power dissipation will be equal to the sum of the
two components listed above.
The LT1528 has internal thermal limiting designed to pro-
tect the device during overload conditions. For continuous
normal load conditions the maximum junction temperature
rating of 125°C must not be exceeded. It is important to
give careful consideration to all sources of thermal resis-
tance from junction-to-ambient. Additional heat sources
mounted nearby must also be considered.
For surface mount devices heat sinking is accomplished
by using the heat spreading capabilities of the PC board
and its copper traces. Experiments have shown that the
heat spreading copper layer does not have to be electri-
cally connected to the tab of the device. The PC material
can be very effective at transmitting heat between the
pad area, attached to the tab of the device, and a ground
or power plane either inside or on the opposite side of
the board. Although the actual thermal resistance of the
PC material is high, the length/area ratio of the thermal
resistor between layers is small. Copper board stiffeners
and plated through holes can also be used to spread the
heat generated by power devices.
Table 1a lists thermal resistance for the DD package.
For the TO-220 package (Table 1b) thermal resistance is
given for junction-to-case only since this package is usu-
ally mounted to a heat sink. Measured values of thermal
resistance for several different copper areas are listed for
the DD package. All measurements were taken in still air
on 3/32" FR-4 board with one ounce copper. This data
can be used as a rough guideline in estimating thermal
resistance. The thermal resistance for each application will
be affected by thermal interactions with other components
as well as board size and shape. Some experimentation
will be necessary to determine the actual value.
Table 1a. Q-Package, 5-Lead DD
COPPER AREA
BOARD AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
TOPSIDE* BACKSIDE
2500 sq mm 2500 sq mm 2500 sq mm 23°C/W
1000 sq mm 2500 sq mm 2500 sq mm 25°C/W
125 sq mm 2500 sq mm 2500 sq mm 33°C/W
*Device is mounted on topside.
Table 1b. T Package, 5-Lead TO-220
Thermal Resistance (Junction-to-Case) 2.5°C/W
Calculating Junction Temperature
Example: Given an output voltage of 3.3V, an input voltage
range of 4.5V to 5.5V, an output current range of 0mA to
500mA and a maximum ambient temperature of 50°C,
what will the maximum junction temperature be?
The power dissipated by the device will be equal to:
I
OUT(MAX)
• (V
IN(MAX)
– V
OUT
) + [I
GND
• V
IN(MAX)
]
where,
I
OUT(MAX)
= 500mA
V
IN(MAX)
= 5.5V
I
GND
at (I
OUT
= 500mA, V
IN
= 5.5V) = 4mA
so,
P = 500mA • (5.5V – 3.3V) + (4mA • 5.5V) = 1.12W
If we use a DD package, the thermal resistance will be in
the range of 23°C/W to 33°C/W depending on the copper
area. So the junction temperature rise above ambient will
be approximately equal to:
1.12W • 28°C/W = 31.4°C
APPLICATIONS INFORMATION
