LTM4607 Datasheet, PDF (15/24 Pages) Linear Technology – 36VIN, 24VOUT High Effi ciency Buck-Boost DC/DC μModule

LTM4607
APPLICATIONS INFORMATION
0.6
1.5μH
0.4
2.5μH
3.3μH
0.2
4.7μH
0
5 6 7 8 9 10 11 12
INPUT VOLTAGE V_IN (V)
4607 F04
Figure 4. Current Ripple Ratio at Different Inputs for Boost Mode
For the input capacitor, only minimum capacitors are
needed to handle the maximum RMS current, since it
is a continuous input current at boost mode. A 100μF
capacitor is only needed if the input source impedance is
compromised by long inductive leads or traces.
Since the output capacitors at boost mode need to filter
the square wave current, more capacitors are expected
to achieve the same output ripples as the buck mode. If
assuming that the ESR dominates the output ripple, the
output ripple is as follows:
ΔV_OUT(P-P) = ESR •I_L(MAX)
If a total low ESR about 5mΩ is chosen for output capaci-
tors, the maximum output ripple of 70mV occurs at the input
voltage of 5V with the peak inductor current at 14A.
An RC snubber is recommended on SW1 to obtain low
switching noise, as shown in Figure 17.
Wide Input Mode Operation
If a wide input range is required from 5V to 36V, the module
will work in different operation modes. If input voltage
V_IN = 5V to 36V, V_OUT = 12V and f = 400kHz, the design
needs to consider the worst case in buck or boost mode
design. Therefore, the maximum output power is limited
to 60W. The sensing resistor is chosen at 7mΩ, the input
capacitor is the same as the buck mode design and the
output capacitor uses the boost mode design. Since the
maximum output ripple normally occurs at boost mode
in the wide input mode design, more inductor ripple cur-
rent, up to 150% of the inductor current, is allowed at
buck mode to meet the ripple design requirement. Thus,
a 3.3μH inductor is chosen at the wide input mode. The
maximum output ripple voltage is still 70mV if the total
ESR is about 5mΩ.
Additionally, the current limit may become very high when
the module runs at buck mode due to the low sensing
resistor used in the wide input mode operation.
Safety Considerations
The LTM4607 modules do not provide isolation from V_IN to
V_OUT. There is no internal fuse. If required, a slow blow fuse
with a rating twice the maximum input current needs to be
provided to protect each unit from catastrophic failure.
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