LTM4603 LINER [Linear Technology], LTM4603 Datasheet - Page 11

• Manufacturer Part Number: LTM4603
• Description: 36VIN, 34VOUT High Efficiency Buck-Boost DC/DC
• Manufacturer: LINER [Linear Technology]

applications inForMation
This makes it advisable to properly derate the input capaci-
tor, or choose a capacitor rated at a higher temperature
than required. Always contact the capacitor manufacturer
for derating requirements.
Output Capacitors
In boost mode, the discontinuous current shifts from the
input to the output, so the output capacitor C
capable of reducing the output voltage ripple.
For boost and buck modes, the steady ripple due to charg-
ing and discharging the bulk capacitance is given by:
The steady ripple due to the voltage drop across the ESR
(effective series resistance) is given by:
The LTM4609 is designed for low output voltage ripple.
The bulk output capacitors defined as C
with low enough ESR to meet the output voltage ripple and
transient requirements. C
capacitor, the low ESR polymer capacitor or the ceramic
capacitor. Multiple capacitors can be placed in parallel to
meet the ESR and RMS current handling requirements. The
typical capacitance is 300µF . Additional output filtering may
be required by the system designer, if further reduction of
output ripple or dynamic transient spike is required. Table 3
shows a matrix of different output voltages and output
capacitors to minimize the voltage droop and overshoot
at a current transient.
Inductor Selection
The inductor is chiefly decided by the required ripple cur-
rent and the operating frequency. The inductor current
V
V
V
V
RIPPLE,BOOST
RIPPLE,BUCK
ESR,BUCK
ESR,BOOST
= ΔI
=I
=
L(MAX)
=
L(MAX)
8 • L • C
V
I
OUT(MAX)
OUT
• ESR
OUT
• V
• ESR
(
OUT
C
OUT
IN(MAX)
can be the low ESR tantalum
• V
• V
(
• V
IN(MAX)
OUT
OUT
− V
− V
• ƒ
OUT
OUT
• ƒ
IN(MIN)
)
2
OUT
are chosen
)
must be
ripple ΔI
inductor current. In the inductor design, the worst cases
in continuous mode are considered as follows:
where:
The inductor should have low DC resistance to reduce the
I
current without saturation. To minimize radiated noise,
use a toroid, pot core or shielded bobbin inductor. Please
refer to Table 3 for the recommended inductors for dif-
ferent cases.
R
R
Since the maximum inductor valley current at buck mode
is much lower than the inductor peak current at boost
mode, different sensing resistors are suggested to use
in buck and boost modes.
The current comparator threshold sets the peak of the
inductor current in boost mode and the maximum inductor
valley current in buck mode. In boost mode, the allowed
maximum average load current is:
where ΔI
2
SENSE
SENSE
R losses, and must be able to handle the peak inductor
ƒ is operating frequency, Hz
Ripple% is allowable inductor current ripple, %
V
V
V
I
I
L
L
OUT(MAX)
OUT(MAX,BOOST)
OUT(MAX)
IN(MAX)
OUT
BOOST
BUCK
is chosen based on the required inductor current.
Selection and Maximum Output Current
is output voltage, V
L
L
≥
is typically set to 20% to 40% of the maximum
is peak-to-peak inductor ripple current.
≥
is maximum input voltage, V
V
is maximum output load current, A
V
IN(MAX)
is maximum output voltage, V
2
OUT(MAX)
V
OUT
V
=
• ƒ •I
2



• V
IN
R
160mV
(
• V
SENSE
IN(MAX)
• ƒ •I
(
OUT(MAX)
OUT(MAX)
OUT(MAX)
−
− V
ΔI
2
L
• Ripple%
OUT

 •

− V
LTM4609
• Ripple%
V
)
IN
V
OUT
IN
)
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