BL1117
BL1117-ADJ provides a 1.25V reference voltage. Any output voltage between 1.25V~12V can be achievable by
choosing two external resistors (schematic is shown below), R1 and R2
BL1117
LC1117-ADJ
Vout
10uF
Vin
Vin
Vout
R1
10uF
ADJ
R2
Application Circuit of BL1117-ADJ
The output voltage of adjustable version follows the equation: Vout=1.25(1+R2/R1)+IAdjR2. We can ignore
+IAdj because IAdj (about 50uA) is much less than the current of R1 (about 2~10mA).
1) To meet the minimum load current (>10mA) requirement, R1 is recommended to be 125ohm or lower. As BL1117-ADJ
can keep itself stable at load current about 2mA, R1 is not allowed to be higher than 625ohm.
2) Using a bypass capacitor (CADJ) between the ADJ pin and ground can improve ripple rejection. This bypass capacitor
prevents ripple from being amplified as the output voltage is increased. The impedance of CADJ should be less than R1
to prevent ripple from being amplified. As R1 is normally in the range of 100Ω~500Ω, the value of CADJ should satisfy
this equation: 1/(2frippleCADJ)<R1.
THERMAL CONSIDERATIONS
We have to take heat dissipation into great consideration when output current or differential voltage of input and
output voltage is large. Because in such cases, the power dissipation consumed by BL1117 is very large. BL1117 series
uses SOT-223 package type and its thermal resistance is about 20°C /W. And the copper area of application board can
affect the total thermal resistance. If copper area is 5cm*5cm (two sides), the resistance is about 30°C /W. So the total
thermal resistance is about 20°C /W + 30°C /W. We can decrease total thermal resistance by increasing copper area in
application board. When there is no good heat dissipation copper are in PCB, the total thermal resistance will be as
high as 120°C /W, then the power dissipation of BL1117 could allow on itself is less than 1W. And furthermore, BL1117
will work at junction temperature higher than 125°C under such condition and no lifetime is guaranteed.
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