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LTM4605
APPLICATIONS INFORMATION
For a buck converter, the switching duty-cycle can be
estimated as:
The LTM4605 is designed for low output voltage ripple.
The bulk output capacitors de?ned as C OUT are chosen
D =
V OUT
V IN
with low enough ESR to meet the output voltage ripple
and transient requirements. C OUT can be a low ESR tanta-
lum capacitor, a low ESR polymer capacitor or a ceramic
Without considering the inductor current ripple, the RMS
current of the input capacitor can be estimated as:
capacitor. Multiple capacitors can be placed in parallel to
meet the ESR and RMS current handling requirements.
The typical capacitance is 300μF. Additional output ?ltering
I CIN(RMS) =
I OUT(MAX)
η
? D ? (1 ? D)
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
In the above equation, η is the estimated ef?ciency of the
power module. C IN can be a switcher-rated electrolytic
aluminum capacitor, OS-CON capacitor or high volume
ceramic capacitors. Note the capacitor ripple current rat-
ings are often based on temperature and hours of life. This
makes it advisable to properly derate the input capacitor,
or choose a capacitor rated at a higher temperature than
required. Always contact the capacitor manufacturer for
derating requirements.
output capacitors to minimize the voltage droop and
overshoot at a current transient.
Inductor Selection
The inductor is chie?y decided by the required ripple cur-
rent and the operating frequency. The inductor current
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:
Output Capacitors
In boost mode, the discontinuous current shifts from the
L BOOST ≥
V IN ? ( V OUT(MAX) ? V IN )
V OUT(MAX) ? f ? I OUT(MAX) ? Ripple%
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-
L BUCK ≥
V OUT ? ( V IN(MAX) ? V OUT )
V IN(MAX) ? f ? I OUT(MAX) ? Ripple%
I OUT(MAX) ? ( V OUT ? V IN(MIN) )
ing and discharging the bulk capacitance is given by:
V RIPPLE,BOOST =
C OUT ? V OUT ? f
where:
f is operating frequency, Hz
Ripple% is allowable inductor current ripple, %
8 ? L ? C OUT ? V IN(MAX) ? f
V RIPPLE,BUCK =
V OUT ? ( V IN(MAX) ? V OUT )
2
V OUT(MAX) is maximum output voltage, V
V IN(MAX) is maximum input voltage, V
The steady ripple due to the voltage drop across the ESR
(effective series resistance) is given by:
V ESR,BUCK = Δ I L(MAX) ? ESR
V ESR,BOOST = I L(MAX) ? ESR
V OUT is output voltage, V
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.
4605fc
11
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