LTC3558
28
3558f
Table 4. Recommended Inductors for the Buck-Boost Switching Regulator.
INDUCTOR TYPE
L
(μH)
MAX I
DC
(A)
MAX DCR
(m
Ω
)
SIZE IN mm
(L
×
W
×
H)
MANUFACTURER
DB3018C
D312C
DE2812C
DE2812C
2.4
2.2
2
2.7
1.31
1.14
1.4
1.2
80
140
81
87
3.8
×
3.8
×
1.4
3.6
×
3.6
×
1.2
3
×
3.2
×
1.2
3
×
3.2
×
1.2
Toko
www.toko.com
CDRH3D16
2.2
1.2
72
4
×
4
×
1.8
Sumida
www.sumida.com
SD12
2.2
1.8
74
5.2
×
5.2
×
1.2
Cooper
www.cooperet.com
*Typical DCR
APPLICATIONS INFORMATION
Input Current Limit
The input current limit comparator will shut the input PMOS
switch off once current exceeds 700mA typical. Before the
switch current limit, the average current limit amp (620mA
typical) will source current into the feedback pin to drop
the output voltage. The input current limit also protects
against a short-circuit condition at the V
OUT2
pin.
Reverse Current Limit
The reverse current limit comparator will shut the output
PMOS switch off once current returning from the output
exceeds 450mA typical.
Output Overvoltage Protection
If the feedback node were inadvertently shorted to ground,
then the output would increase indefi nitely with the maxi-
mum current that could be sourced from the input supply.
The buck-boost regulator protects against this by shutting
off the input PMOS if the output voltage exceeds a 5.75V
maximum.
Buck-Boost Regulator Soft-Start Operation
Soft-start is accomplished by gradually increasing the
reference voltage over a 500μs typical period. A soft-
start cycle occurs whenever the buck-boost is enabled,
or after a fault condition has occurred (thermal shutdown
or UVLO). A soft-start cycle is not triggered by changing
operating modes. This allows seamless output operation
when transitioning between Burst Mode operation and
PWM mode operation.
Buck-Boost Switching Regulator Inductor Selection
The buck-boost switching regulator is designed to work
with inductors in the range of 1μH to 5μH. For most
applications, a 2.2μH inductor will suffi ce. Larger value
inductors reduce ripple current which improves output
ripple voltage. Lower value inductors result in higher
ripple current and improved transient response time.
To maximize effi ciency, choose an inductor with a low
DC resistance and a DC current rating at least 1.5 times
larger than the maximum load current to ensure that the
inductor does not saturate during normal operation. If
output short-circuit is a possible condition, the inductor
current should be rated to handle up to the peak current
specifi ed for the buck-boost regulator.
The inductor value also affects Burst Mode operation.
Lower inductor values will cause Burst Mode switching
frequencies to increase.
Different core materials and shapes will change the size/cur-
rent and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials
are small and do not radiate much energy, but cost more
than powdered iron core inductors with similar electrical
characteristics. Inductors that are very thin or have a very
small volume typically have much higher core and DCR
losses and will not give the best effi ciency.
Table 4 shows some inductors that work well with the
buck-boost regulator. These inductors offer a good com-
promise in current rating, DCR and physical size. Consult
each manufacturer for detailed information on their entire
selection of inductors.
