Datasheet
Iripple
Ipk+
Io Buck
Ipk-
Iripple
1/Fs
L1 Current
Io/(1-D) B-B
R
T
=
6.4 x 10
9
f
- 3.02 x 10
3
LM5118, LM5118-Q1
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SNVS566G –APRIL 2008–REVISED FEBRUARY 2013
APPLICATION INFORMATION
The procedure for calculating the external components is illustrated with the following design example. The
designations used in the design example correlate to the final schematic shown in Figure 26. The design
specifications are:
• VOUT = 12V
• VIN = 5V to 75V
• F = 300 kHz
• Minimum load current (CCM operation) = 600 mA
• Maximum load current = 3A
R7 = R
T
RT sets the oscillator switching frequency. Generally speaking, higher operating frequency applications will use
smaller components, but have higher switching losses. An operating frequency of 300 kHz was selected for this
example as a reasonable compromise for both component size and efficiency. The value of R
T
can be calculated
as follows:
(9)
therefore, R7 = 18.3 kΩ
Figure 20. Inductor Current Waveform
INDUCTOR SELECTION
L1
The inductor value is determined based upon the operating frequency, load current, ripple current and the input
and output voltages. Refer to Figure 20 for details.
To keep the circuit in continuous conduction mode (CCM), the maximum ripple current IRIPPLE should be less
than twice the minimum load current. For the specified minimum load of 0.6A, The maximum ripple current is
1.2A p-p. Also, the minimum value of L must be calculated both for a buck and buck-boost configurations. The
final value of inductance will generally be a compromise between the two modes. It is desirable to have a larger
value inductor for buck mode, but the saturation current rating for the inductor must be large for buck-boost
mode, resulting in a physically large inductor. Additionally, large value inductors present buck-boost mode loop
compensation challenges which will be discussed in the Error Amplifier Configuration section. For the design
example, the inductor values in both modes are calculated as:
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