Datasheet

L
MIN
=
(V
IN
- V
OUT
) x D
'i
L
x f
SW
D =
V
OUT
V
IN
C
IN1
VIN
SW
AGND
FB
PGOOD
V
OUT
R
FB1
R
FB2
C
OUT
EN
SS/TRK
VCC
C
VCC
C
C1
SYNC
R
C1
V
IN
LM20333
BOOT
GND
L
C
BOOT
D1
(Optional)
C
SS
R
PG
V
PULLUP
C
IN2
COMP
LM20333
SNVS558D –MAY 2008–REVISED APRIL 2013 www.ti.com
Design Guide
This section walks the designer through the steps necessary to select the external components to build a fully
functional power supply. As with any DC-DC converter numerous trade-offs are possible to optimize the design
for efficiency, size, or performance. These will be taken into account and highlighted throughout this discussion.
To facilitate component selection discussions the circuit shown in Figure 25 below may be used as a reference.
Unless otherwise indicated all formulas assume units of amps (A) for current, farads (F) for capacitance, henries
(H) for inductance and volts (V) for voltages.
Figure 25. Typical Application Circuit
The first equation to calculate for any buck converter is duty-cycle. Ignoring conduction losses associated with
the FETs and parasitic resistances it can be approximated by:
(1)
INDUCTOR SELECTION (L)
The inductor value is determined based on the operating frequency, load current, ripple current and duty cycle.
The inductor selected should have a saturation current rating greater than the peak current limit of the device.
Keep in mind the specified current limit does not account for delay of the current limit comparator, therefore the
current limit in the application may be higher than the specified value. To optimize the performance and prevent
the device from entering current limit at maximum load, the inductance is typically selected such that the ripple
current, Δi
L
, is not greater than 30% of the rated output current. Figure 26 illustrates the switch and inductor
ripple current waveforms. Once the input voltage, output voltage, operating frequency and desired ripple current
are known, the minimum value for the inductor can be calculated by the formula shown below:
(2)
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