FAN1581 Datasheet, PDF (7/13 Pages) Fairchild Semiconductor – 5A Adjustable/Fixed Ultra Low Dropout Linear Regulator

PRODUCT SPECIFICATION
FAN1581
The current out of the adjust pin adds to the current from R1
and is typically 50µA. Its output voltage contribution is
small and only needs consideration when a very precise out-
put voltage setting is required.
maximum junction temperature for both the control circuitry
and the power transistor. Calculate the maximum junction
temperature for both sections to ensure that both thermal
limits are met.
V_cntl
V_IN
+
C1
10µF
Vcntl Vsense
FAN1581
Vin Adj Vout
V_REF
I_ADJ
50µA
+
R1
VOUT
C2
22µF
R2
VOUT = V_REF (1+R2/R1) + I_ADJ (R2)
Figure 2. Basic Regulator Circuit
Load Regulation
The FAN1581 family provides true remote sensing, eliminat-
ing output voltage errors due to trace resistance. To utilize
remote sensing, connect the VSENSE pin directly to the
load, rather than at the VOUT pin. If the load is more than 1"
away from the FAN1581, it may be necessary to increase the
load capacitance to ensure stability.
Thermal Considerations
The FAN1581 series protect themselves under overload con-
ditions with internal power and thermal limiting circuitry.
However, for normal continuous load conditions, do not exceed
maximum junction temperature ratings. It is important to
consider all sources of thermal resistance from junction-to-
ambient. These sources include the junction-to-case resistance,
the case-to-heat sink interface resistance, and the heat sink
resistance. Thermal resistance specifications have been
developed to more accurately reflect device temperature and
ensure safe operating temperatures. The electrical character-
istics section provides a separate thermal resistance and
For example, look at using an FAN1581M-1.5 to generate
5A @ 1.5V ± 2% from a 3.3V source (3.2V to 3.6V).
Assumptions:
• V_in = 3.6V worst case
• V_OUT = 1.47V worst case
• I_OUT = 5A continuous
• T_A = 40°C
• Θ _{Case-to-Ambient} = 5°C/W (assuming both a heatsink and a
thermally conductive material)
The power dissipation in this application is:
P_D = (V_IN - V_OUT) * (I_OUT) = (3.6-1.47) * (5) = 10.65W
From the specification table,
T_J = T_A + (P_D) * (Θ Case-to-Ambient + Θ_JC)
= 40 + (10.65) * (5 + 3) = 125°C
The junction temperature is within the maximum rating.
Junction-to-case thermal resistance is specified from the IC
junction to the bottom of the case directly below the die. This
is the lowest resistance path for heat flow. Proper mounting
ensures the best thermal flow from this area of the package to
the heat sink. Use of a thermally conductive material at the
case-to-heat sink interface is recommended. Use a thermally
conductive spacer if the case of the device must be electri-
cally isolated and include its contribution to the total thermal
resistance.
REV. 1.2.0 11/6/03
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