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LTM4609
APPLICATIONS INFORMATION
I OUT(MAX) ? ( V OUT – V IN(MIN) )
C OUT ? V OUT ? ?
V 2IN ? ( V OUT(MAX) – V IN )
V 2OUT(MAX) ? ? ? I OUT(MAX) ? Ripple%
V OUT ? ( V IN(MAX) – V OUT )
V IN(MAX) ? ? ? I OUT(MAX) ? Ripple%
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 OUT must be
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:
V RIPPLE,BOOST =
ripple Δ I L is typically set to 20% to 40% of the maximum
inductor current. In the inductor design, the worst cases
in continuous mode are considered as follows:
L BOOST ≥
L BUCK ≥
where:
? is operating frequency, Hz
Ripple% is allowable inductor current ripple, %
V RIPPLE,BUCK =
V OUT ? ( V IN(MAX) – V OUT )
8 ? L ? C OUT ? V IN(MAX) ? ? 2
V OUT(MAX) is maximum output voltage, V
V IN(MAX) is maximum input voltage, V
V OUT is output voltage, V
I OUT(MAX,BOOST) = ?
160mV ?I L ? V IN
? R SENSE
The steady ripple due to the voltage drop across the ESR
(effective series resistance) is given by:
V ESR,BUCK = ?I L(MAX) ? ESR
V ESR,BOOST =I L(MAX) ? ESR
The LTM4609 is designed for low output voltage ripple.
The bulk output capacitors defined as C OUT are chosen
with low enough ESR to meet the output voltage ripple and
transient requirements. C OUT can be the low ESR tantalum
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
I OUT(MAX) is maximum output load current, A
The inductor should have low DC resistance to reduce the
I 2 R losses, and must be able to handle the peak inductor
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 SENSE Selection and Maximum Output Current
R SENSE is chosen based on the required inductor current.
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:
– ?
2 ? V OUT
where Δ I L is peak-to-peak inductor ripple current.
4609ff
For more information www.linear.com/LTM4609
11
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