Datasheet
L =
V
IN
x D
F x 'I
L
I
L
=
I
OUT
1 - D
D =
lV
OUT
l + V
D
V
IN
+ lV
OUT
l + V
D
- V
Q
,
VOUT
(negative voltage)
L1
C4 D2
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Component Selection
Figure 2. Optional Protection Diode D2
5 Component Selection
This section details the component calculation and selection for polarity-inverting converter applications.
The calculations are for continuous current conduction mode (CCM) operation.
6 Inductor Selection
Duty-cycle is calculated as:
(1)
where, V
D
is the D1 diode voltage drop and V
Q
is the voltage drop across the LM22670 internal power N-
FET. The R
DS(ON)
of the FET is specified in the LM22670/LM22670Q 42V, 3A SIMPLE SWITCHER®, Step-
Down Voltage Regulator with Features Data Sheet (SNVS584) to calculate V
Q
according to the FET
current.
V
Q
= I
PEAK
x R
DS(ON)
, (2)
where, I
PEAK
is the peak switch current of the application. The average inductor current, I
L
, in reference to
the application load current, I
OUT
, is defined as:
(3)
There are multiple ways to calculate the required inductance for a switching application. The
recommended calculation is to choose the inductor ripple current, ΔI
L
, of approximately 30% of the
average inductor current I
L
. This makes the regulator operate in continuous current conduction mode
(CCM) and the application circuit has a small load transient response with an acceptable output voltage
ripple. Therefore the peak-to-peak inductor ripple current, ΔI
L
, is selected as:
ΔI
L
≊ 0.3 x I
L
(4)
This makes the required inductance:
(5)
where, F is the switching frequency of the application. The LM22670 switches at 500 kHz typical if the
RT/SYNC pin is floating. The inductor should have a RMS current rating equal to or greater than the
maximum current limit, I
CL
, in order to avoid inductor saturation. The values for maximum current limit, I
CL
,
can be found in the Electrical Characteristics section of the LM22670/LM22670Q 42V, 3A SIMPLE
SWITCHER®, Step-Down Voltage Regulator with Features Data Sheet (SNVS584).
3
SNVA363E–September 2008–Revised April 2013 AN-1888 LM22670 Evaluation Board Inverting Topology
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