LTM4605_15 Datasheet, PDF (11/26 Pages) Linear Technology – High Efficiency Buck-Boost DC/DC Module Regulator

LTM4605
Applications Information
For a buck converter, the switching duty-cycle can be
estimated as:
D = ^VOUT
V_IN
Without considering the inductor current ripple, the RMS
current of the input capacitor can be estimated as:
I_CIN(RMS)
=
I_{OUT(MAX )}
η
•
D • (1−D)
In the above equation, η is the estimated efficiency 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
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:
( ) _{VRIPPLE,BOOST}
=
I_OUT(MAX) •
C_OUT
V_OUT − V_IN(MIN)
• V_OUT • f
( ) _VRIPPLE,BUCK
=
V_OUT
8 •L •
• V_IN(MAX) − V_OUT
C_OUT • V_IN(MAX) • f²
The steady ripple due to the voltage drop across the ESR
(effective series resistance) is given by:
V_ESR,BUCK = ΔI_L(MAX) • ESR
V_ESR,BOOST = I_L(MAX) • ESR
The LTM4605 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 a low ESR tanta-
lum capacitor, a low ESR polymer capacitor or a 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 reduc-
tion 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
ripple ΔI_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:
( ) _LBOOST
≥
V_IN • V_OUT(MAX) − V_IN
V_OUT(MAX) • f • I_OUT(MAX) • Ripple%
( ) _LBUCK
≥
V_OUT •
V_IN(MAX) • f
V_IN(MAX) − V_OUT
• I_OUT(MAX) • Ripple%
where:
f is operating frequency, Hz
Ripple% is allowable inductor current ripple, %
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) is maximum output load current, A
The inductor should have low DC resistance to reduce the
I²R 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.
For more information www.linear.com/LTM4605
4605fd
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