lm5118mhx National Semiconductor Corporation, lm5118mhx Datasheet - Page 17

• Manufacturer Part Number: lm5118mhx
• Description: Wide Voltage Range Buck-boost Controller
• Manufacturer: National Semiconductor Corporation

INDUCTOR SELECTION
L1
The inductor value is determined based upon the operating
frequency, load current, ripple current and the input and out-
put voltages. Refer to Figure 12 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 mini-
mum 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 de-
sirable 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. Ad-
ditionally, large value inductors present buck-boost mode
loop compensation challenges which will be discussed in er-
ror amplifier configuration section. For the design example,
the inductor values in both modes are calculated as:
Where:
V
V
f is the switching frequency
I
A selected for this example)
V
The resulting inductor values are:
L1 = 28 µH, Buck Mode
L1 = 9.8 µH Buck-Boost mode
A 10 µH inductor was selected which is a compromise be-
tween these values, while favoring the buck-boost mode. As
will be illustrated in the compensation section below, the in-
ductor value should be as low as possible to move the buck-
boost right-half-plane zero to a higher frequency. The ripple
current is then rechecked with the selected inductor value us-
ing the equations above,
I
I
RIPPLE
RIPPLE(BUCK)
RIPPLE(BUCK-BOOST)
OUT
IN1
IN2
is the maximum input voltage
is the minimum input voltage
is the output voltage
is the selected inductor peak to peak ripple current (1.2
FIGURE 12. Inductor Current Waveform
= 3.36A
= 1.17A
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17
Because the inductor selected is lower than calculated for the
Buck mode, the minimum load current for CCM in buck mode
is 1.68 A at maximum VIN.
With a 10 µH inductor, the worst case peak inductor currents
can be estimated for each case, assuming a 20% inductor
value tolerance.
For this example, the two equations yield:
I
I
An acceptable current limit setting would be 6.7A for buck
mode since the LM5118 automatically doubles the current
limit threshold in buck-boost mode. The selected inductor
must have a saturation current rating at least as high as the
buck-boost mode cycle-by-cycle current limit threshold, in this
case at least 13.5A. A 10 µH 15 amp inductor was chosen for
this application.
R13 = R
To select the current sense resistor value, begin by calculat-
ing the value of R
R13
For the buck-boost mode, R
R13
A R
guarantee the required maximum output current in the buck-
boost mode. A value of 15 mΩ was selected for component
tolerances and is a standard value.
R13 = 15 mΩ
C15 = C
With the inductor value selected, the value of C3 necessary
for the emulation ramp circuit is:
With the inductance value (L1) selected as 10 µH, the calcu-
lated value for C
was selected.
C9 - C12 = OUTPUT CAPACITORS
In buck-boost mode, the output capacitors C9 - C12 must
supply the entire output current during the switch on-time. For
this reason, the output capacitors are chosen for operation in
buck-boost mode, the demands being much less in buck op-
eration. Both bulk capacitance and ESR must be considered
1(PEAK)
2(PEAK)
SENSE
(BUCK)
(BUCK-BOOST)
= 5.43A
= 13.34A
SENSE
RAMP
= 23 mΩ
value of no more than 18.7 mΩ must be used to
= 18.7 mΩ
RAMP
SENSE
is 333 pF. A standard value of 330 pF
for both modes of operation.
SENSE
is given by:
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