LTM4609Y View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTM4609Y' PDF : 28 Pages View PDF
LTM4609
Applications Information
0.8
VOUT = 12V
Æ’ = 400kHz
0.6
1.5µH
0.4
2.5µH
3.3µH
4.7µH
0.2
0
5 6 7 8 9 10 11 12
INPUT VOLTAGE VIN (V)
4609 F04
Figure 4. Current Ripple Ratio at Different Inputs for Boost Mode
Figure 4 shows the current ripple ratio at different input
voltages based on the inductor values: 1.5µH, 2.5µH,
3.3µH and 4.7µH. If we need 30% ripple current ratio at
all inputs, the 3.3µH inductor can be selected.
At boost mode, sensing resistor selection is based on
the maximum input current and the allowed maximum
sensing threshold 160mV.
RSENSE
=
2
•
2 •160mV
P+
η • VIN(MIN)
∆IL
Consider the safety margin about 30%, we can choose
the sensing resistor as 8mΩ.
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:
∆VOUT(P-P) =ESR •IL(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
VIN = 5V to 36V, VOUT = 12V and Æ’ = 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 8mΩ, 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 LTM4609 modules do not provide isolation from VIN
to VOUT. 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.
For more information www.linear.com/LTM4609
4609ff
15
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