Datasheet

LM21305
FB
R
FB2
V
OUT
R
FB1
VIN
LM21305
SW
COMP
FREQ
EN
FB
PGOOD
PGND
2V5
AVIN
AGND
5V0
CBOOT
PVIN
0.598V
C
IN3
L
R
F
C
5V0
C
2V5
C
F
C
c1
R
c
C
FRQ
R
FRQ
R
FB1
R
FB2
C
BOOT
C
OUT1
VOUT
R
EN
R
PG
C
OUT2
C
IN2
C
IN1
LM21305
www.ti.com SNVS639F –DECEMBER 2009–REVISED MARCH 2013
DESIGN GUIDE
This section walks the designer through the steps necessary to select the external components to build a fully
functional efficient step-down power supply. As with any DC-DC converter, numerous tradeoffs are possible to
optimize the design for efficiency, size, and performance. These will be taken into account and highlighted
throughout this discussion. To facilitate component selection discussions, the typical application circuit shown
below may be used as a reference. Unless otherwise indicated, all formulae assume units of Amps (A) for
current, Farads (F) for capacitance, Henries (H) for inductance, Volts (V) for voltages and Hertz (Hz) for
frequencies.
Figure 24. LM21305 Typical Application Circuit
SETTING THE OUTPUT VOLTAGE
The FB pin of the LM21305 can be connected to V
OUT
directly or through a resistor divider. With an external
resistor divider, the output voltage can be scaled up from the 0.598V feedback voltage. Figure 25 shows the
connection of the divider and the FB pin.
Figure 25. Setting the Output Voltage by Resistor Divider
The output voltage can be found by:
(7)
For example, if the desired output voltage is 1.2V, R
FB1
= 10 kΩ and R
FB2
= 10 kΩ can be used.
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