LM3677_0810 Datasheet, PDF (18/24 Pages) National Semiconductor (TI) – 3MHz, 600mA Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits

OUTPUT VOLTAGE SELECTION FOR LM3677-ADJ
The output voltage of the adjustable parts can be pro-
grammed through the resistor network connected from V_OUT
to FB to GND. The resistor and FB to GND (R₂) should be 200
kΩ to keep the current drawn through this network well below
16 µA quiescent current level (PFM mode) but large enough
that it is not susceptible to noise. If R₂ is 200 kΩ, and given
the V_FB is 0.5V, then the current through the resistor feedback
network will be 2.5 µA. The output voltage of the adjustable
parts ranges from 1.2V and 3.3V. The output voltage formula
is:
R₁: feedback resistor from V_OUT to FB (Ω)
R₂: feedback resistor from to FB to GND (Ω)
For the fixed output voltage parts the feedback resistors are
internal and R₁ is 0Ω.
The bypass capacitors C₁ and C₂ (labeled C3 and C4 on
Evaluation Board) in parallel with the feedback resistors are
chosen for increased stability. Below are the formulas for C₁
and C₂:
30008404
30008406
V_OUT: output voltage (V)
V_FB: feedback voltage (0.5V typical)
30008405
TABLE 3. LM3677–ADJ Configurations for Various V_OUT (Circuit of Figure 2)
V_OUT (V)
1.2
1.3
1.5
1.6
1.8
2.5
2.8
3.3
R₁(kΩ )
280
320
357
442
464
402
464
562
R₂(kΩ )
200
200
178
200
178
100
100
100
C₁(pF)
8.2
8.2
6.8
5.6
5.6
6.0
5.6
5.6
C₂(pF)
none
none
none
none
none
none
24
24
L (µH)
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
C_IN (µF)
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
C_OUT (µF)
10
10
10
10
10
10
10
10
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