LTM4609Y View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTM4609Y' PDF : 28 Pages View PDF
LTM4609
Applications Information
To choose a proper inductor, we need to know the current
ripple at different input voltages. The inductor should
be chosen by considering the worst case in the practi-
cal operating region. If the maximum output power P is
120W at buck mode, we can get the current ripple ratio
of the current ripple ΔIL to the maximum inductor current
IL as follows:
∆IL = (VIN – VOUT )• VOUT2
IL
VIN •L • ƒ •P
Figure 3 shows the current ripple ratio at different input
voltages based on the inductor values: 2.5µH, 3.3µH, 4.7µH
and 6µH. If we need about 40% ripple current ratio at all
inputs, the 4.7µH inductor can be selected.
At buck mode, sensing resistor selection is based on
the maximum output current and the allowed maximum
sensing threshold 130mV.
RSENSE
=
2
•
2
(P
•
/
130mV
VOUT )–
∆IL
Consider the safety margin about 30%, we can choose
the sensing resistor as 9mΩ.
0.8
VOUT = 12V
Æ’ = 400kHz
0.6
0.4
0.2
2.5µH
3.3µH
4.7µH
6µH
For the input capacitor, use a low ESR sized capacitor to
handle the maximum RMS current. Input capacitors are
required to be placed adjacent to the module. In Figure 16,
the 10µF ceramic input capacitors are selected for their
ability to handle the large RMS current into the converter.
The 100µF bulk capacitor is only needed if the input source
impedance is compromised by long inductive leads or
traces.
For the output capacitor, the output voltage ripple and
transient requirements require low ESR capacitors. If
assuming that the ESR dominates the output ripple, the
output ripple is as follows:
∆VOUT(P-P) =ESR • ∆IL
If a total low ESR of about 5mΩ is chosen for output
capacitors, the maximum output ripple of 21.5mV occurs
at the input voltage of 36V with the current ripple at 4.3A.
Boost Mode Operation
For boost mode operation, use input voltage VIN = 5V to
12V, VOUT = 12V and Æ’ = 400kHz.
Set the PLLFLTR pin and RFB as in buck mode.
If the maximum output power P is 50W at boost mode
and the module efficiency η is about 90%, we can get
the current ripple ratio of the current ripple ΔIL to the
maximum inductor current IL as follows:
∆IL = (VOUT – VIN )• VIN2η
IL
VOUT •L • ƒ •P
0
12
18
24
30
36
INPUT VOLTAGE VIN (V)
4609 F03
Figure 3. Current Ripple Ratio at Different Inputs for Buck Mode
4609ff
14
For more information www.linear.com/LTM4609
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