Datasheet
Fp =
1
2 * S * (R1 R2) * (C1+C2)
Fz =
1
(2 * S * R1 * C1)
C2 =
1
(2 * S * R2 * 45 kHz)
C1 =
1
(2 * S * R1 * 45 kHz)
V
OUT
= V
FB
1 +
R1
R2
¹
·
©
§
LM3671
LM3671Q
www.ti.com
SNVS294Q –NOVEMBER 2004–REVISED NOVEMBER 2013
APPLICATION INFORMATION
Output Voltage Selection for LM3671-ADJ
The output voltage of the adjustable parts can be programmed through the resistor network connected from V
OUT
to FB, then to GND. V
OUT
is adjusted to make the voltage at FB equal to 0.5V. The resistor from FB to GND (R2)
should be 200 kΩ to keep the current drawn through this network well below the 16 µA quiescent current level
(PFM mode) but large enough that it is not susceptible to noise. If R2 is 200 kΩ, and V
FB
is 0.5V, the current
through the resistor feedback network will be 2.5 µA. The output voltage of the adjustable parts ranges from 1.1V
to 3.3V.
The formula for output voltage selection is:
where
• V
OUT
: output voltage (volts)
• V
FB
: feedback voltage = 0.5V
• R1: feedback resistor from V
OUT
to FB
• R2: feedback resistor from FB to GND (2)
For any output voltage greater than or equal to 1.1V, a zero must be added around 45 kHz for stability. The
formula for calculation of C1 is:
(3)
For output voltages higher than 2.5V, a pole must be placed at 45 kHz as well. If the pole and zero are at the
same frequency the formula for calculation of C2 is:
(4)
The formula for location of zero and pole frequency created by adding C1 and C2 is given below. By adding C1,
a zero as well as a higher frequency pole is introduced.
(5)
(6)
See the "LM3671-ADJ configurations for various V
OUT
" table.
Table 1. LM3671-ADJ Configurations For Various V
OUT
(Circuit of Figure 2)
(Refer to Note 11 for VIN requirements)
V
OUT
(V) R1(kΩ) R2 (kΩ) C1 (pF) C2 (pF) L (µH) C
IN
(µF) C
OUT
(µF)
0.90 160 200 22 none 2.2 4.7 10
1.1 240 200 15 none 2.2 4.7 10
1.2 280 200 12 none 2.2 4.7 10
1.3 320 200 12 none 2.2 4.7 10
1.5 357 178 10 none 2.2 4.7 10
1.6 442 200 8.2 none 2.2 4.7 10
1.7 432 178 8.2 none 2.2 4.7 10
1.8 464 178 8.2 none 2.2 4.7 10
1.875 523 191 6.8 none 2.2 4.7 10
2.5 402 100 8.2 none 2.2 4.7 10
2.8 464 100 8.2 33 2.2 4.7 10
3.3 562 100 6.8 33 2.2 4.7 10
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