lt3015edd Linear Technology Corporation, lt3015edd Datasheet - Page 14

• Manufacturer Part Number: lt3015edd
• Description: 1.5a, Low Noise, Negative Linear Regulator With Precision Current Limit
• Manufacturer: Linear Technology Corporation

LT3015
APPLICATIONS INFORMATION
The problem occurs with a heavy output load when input
voltage is high and output voltage is low. Such situations
occur easily after the removal of a short-circuit or if the
shutdown pin is pulled high after the input voltage has
already been turned on. The load line for such a load
intersects the output current curve at two points. If this
happens, the regulator has two stable output operating
points. With this double intersection, the input power
supply may need to be cycled down to zero and brought
up again to make the output recover.
Shutdown/UVLO
The SHDN pin is used to put the LT3015 into a micro power
shutdown state. The LT3015 has an accurate –1.20V
threshold (during turn-on) on the SHDN pin. This threshold
can be used in conjunction with a resistor divider from the
system input supply to define an accurate undervoltage
lockout (UVLO) threshold for the regulator. The SHDN pin
current (at the threshold) needs to be considered when
determining the resistor divider network.
Thermal Considerations
The LT3015’s maximum rated junction temperature of
125°C limits its power handling capability. Two components
comprise the power dissipated by the device:
1. Output current multiplied by the input-to-output dif-
2. GND pin current multiplied by the input voltage:
Determine GND pin current using the GND Pin Current
curves in the Typical Performance Characteristics sec-
tion. Total power dissipation is the sum of the above two
components.
14
ferential voltage: I
I
GND
• V
IN
OUT
• (V
IN
- V
OUT
) and
The LT3015 regulator incorporates a thermal shutdown
circuit designed to protect the device during overload
conditions. The typical thermal shutdown temperature is
165°C and the circuit incorporates about 8°C of hyster-
esis. For continuous normal conditions, do not exceed the
maximum junction temperature rating of 125°C. Carefully
consider all sources of thermal resistance from junction
to ambient, including other heat sources mounted in close
proximity to the LT3015.
The undersides of the DFN and MSOP packages have ex-
posed metal from the lead frame to the die attachment.
Both packages allow heat to directly transfer from the
die junction to the printed circuit board metal to control
maximum operating junction temperature. The dual-in-line
pin arrangement allows metal to extend beyond the ends
of the package on the topside (component side) of the
PCB. Connect this metal to IN on the PCB. The multiple
IN and OUT pins of the LT3015 also assist in spreading
heat to the PCB.
For surface mount devices, heat sinking is accomplished
by using the heat spreading capabilities of the PC board
and its copper traces. Copper board stiffeners and plated
through-holes can also be used to spread the heat gener-
ated by power devices.
Tables 2-4 list thermal resistance as a function of copper
area in a fixed board size. All measurements were taken
in still air on a 4 layer FR-4 board with 1oz solid internal
planes and 2oz top/bottom external trace planes with a total
board thickness of 1.6mm. The four layers were electrically
isolated with no thermal vias present. PCB layers, copper
weight, board layout and thermal vias will affect the resul-
tant thermal resistance. For more information on thermal
resistance and high thermal conductivity test boards,
refer to JEDEC standard JESD51, notably JESD51-12 and
JESD51-7. Achieving low thermal resistance necessitates
attention to detail and careful PCB layout.
3015f