FB1
OUT FB2
R
V 0.598 V 1 R
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LM21305
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SNVS639G –DECEMBER 2009–REVISED DECEMBER 2015
Typical Application (continued)
9.2.1 Design Requirements
Table 1 shows the Bill of Materials for an LM21305 converter.
Table 1. Bill of Materials (FSW = 500 kHz)
VOUT 1.2 V 1.8 V 2.5 V 3.3 V 5 V PACKAGE
CIN1 TANT, 47 µF, 25 V TANT, 47 µF, 25 V TANT, 47 µF, 25 V TANT, 47 µF, 25 V TANT, 47 µF, 25 V CASE D
CIN2 10 µF, 25 V, X5R 10 µF, 25 V, X5R 10 µF, 25 V, X5R 10 µF, 25 V, X5R 22 µF, 25 V, X5R 1210
CIN3 0.1 µF, 25 V, X7R 0.1 µF, 25 V, X7R 0.1 µF, 25 V, X7R 0.1 µF, 25 V, X7R 0.1 µF, 25 V, X7R 1206
CF1.0 µF, 25 V, X7R 1.0 µF, 25 V, X7R 1.0 µF, 25 V, X7R 1.0 µF, 25 V, X7R 1.0 µF, 25 V, X7R 0603
C2V5, CBOOT 0.1 µF, 16 V, X7R 0.1 µF, 16 V, X7R 0.1 µF, 16 V, X7R 0.1 µF, 16 V, X7R 0.1 µF, 16 V, X7R 0603
C5V0 1.0 µF, 16 V, X7R 1.0 µF, 16 V, X7R 1.0 µF, 16 V, X7R 1.0 µF, 16 V, X7R 1.0 µF, 16 V, X7R 0603
CFRQ 100 pF, 25 V, X7R 100 pF, 25 V, X7R 100 pF, 25 V, X7R 100 pF, 25 V, X7R 100 pF, 25 V, X7R 0603
CC1 3.3 nF, 16 V, X7R 3.3 nF, 16 V, X7R 3.3 nF, 16 V, X7R 3.3 nF, 16 V, X7R 3.3 nF, 16 V, X7R 0603
COUT1, COUT2 47 µF, 6.3 V, X5R 47 µF, 6.3 V, X5R 47 µF, 6.3 V, X5R 47 µF, 6.3 V, X5R 47 µF, 10 V, X5R 1206
L 1.5 µH, 10 A 2.2 µH, 10 A 2.2 µH, 10 A 3.3 µH, 10 A 3.3 µH, 10 A SMD
RF1Ω, 5% 1 Ω, 5% 1 Ω, 5% 1 Ω, 5% 1 Ω, 5% 0603
RFRQ, RPG 100 kΩ, 1% 100 kΩ, 1% 100 kΩ, 1% 100 kΩ, 1% 100 kΩ, 1% 0603
RFB2, REN 10 kΩ, 1% 10 kΩ, 1% 10 kΩ, 1% 10 kΩ, 1% 10 kΩ, 1% 0603
RC3.32 kΩ, 1% 4.22 kΩ, 1% 5.10 kΩ, 1% 7.15 kΩ, 1% 8.2 kΩ, 1% 0603
RFB1 10 kΩ, 1% 20 kΩ, 1% 31.6 kΩ, 1% 45.3 kΩ, 1% 73.2 kΩ, 1% 0603
9.2.2 Detailed Design Procedure
9.2.2.1 Setting the Output Voltage
Connect the FB pin of the LM21305 directly to VOUT or through a feedback resistor divider network to scale up
from the 0.598-V feedback voltage to the desired output voltage. Figure 24 shows the resistor divider connection
and the FB pin.
Figure 24. Setting the Output Voltage by Resistor Divider
The output voltage is found by Equation 7:
(7)
For example, if the desired output voltage is 1.8 V, RFB1 = 20 kΩand RFB2 = 10 kΩcan be used.
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