LM3674_15 Datasheet, PDF (16/26 Pages) Texas Instruments – LM3674 2-MHz, 600-mA Step-Down DC-DC Converter in SOT-23

LM3674
SNVS405G – DECEMBER 2005 – REVISED APRIL 2015
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8.2.2.2 Detailed Design Procedure
8.2.2.2.1 Output Voltage Selection for Adjustable (LM3674-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 will be adjusted to make FB equal to 0.5 V. The resistor from FB to GND (R2) should
be 200 kΩ to keep the current drawn through this network small but large enough that it is not susceptible to
noise. If R₂ is 200 kΩ, and given the V_FB is 0.5 V, then the current through the resistor feedback network will be
2.5 µA. The output voltage formula is:
V_OUT =
R₁
V_FB * (
+ 1)
R₂
where:
• V_OUT = Output voltage (V)
• V_FB = Feedback voltage (0.5 V typical)
• R₁ = Resistor from V_OUT to FB (Ω)
• R₂ = Resistor from FB to GND (Ω)
(10)
For any output voltage greater than or equal to 1.0 V, a frequency zero must be added at 45 kHz for stability.
The formula is:
1
C₁ = _{2 x 5 x R1 x 45 kHz}
(11)
For output voltages greater than or equal to 2.5 V, a pole must also be placed at 45 kHz as well. If the pole and
zero are at the same frequency the formula for calculation of C2 is:
1
C₂ = _{2 x 5 x R2 x 45 kHz}
(12)
The formula for location of zero and pole frequency created by adding C1,C2 are given below. It can be seen
that by adding C1, a zero as well as a higher frequency pole is introduced.
1
1
Fz =
Fp =
(2 * 5 * R1 * C1) 2 * 5 * (R1 R2) * (C1+C2)
(13)
See Table 4.
VOUT (V)
1.0
1.1
1.2
1.5
1.6
1.7
1.8
1.875
2.5
2.8
3.3
Table 4. Adjustable LM3674 Configurations for Various V_OUT
R1 (kΩ)
200
R2 (kΩ)
200
C1 (pF)
18
C2 (pF)
None
L (µH)
2.2
C_IN (µF)
4.7
191
158
18
None
2.2
4.7
280
200
12
None
2.2
4.7
357
178
10
None
2.2
4.7
442
200
8.2
None
2.2
4.7
432
178
8.2
None
2.2
4.7
464
178
8.2
None
2.2
4.7
523
191
6.8
None
2.2
4.7
402
100
8.2
None
2.2
4.7
464
100
8.2
33
2.2
4.7
562
100
6.8
33
2.2
4.7
C_OUT (µF)
10
10
10
10
10
10
10
10
10
10
10
16
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