LTM4609 LINER [Linear Technology], LTM4609 Datasheet - Page 14

• Manufacturer Part Number: LTM4609
• Description: 36VIN, 34VOUT High Effi ciency Buck-Boost DC/DC ?Module
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTM4609
at buck mode, we can get the current ripple ratio of the
current ripple ΔI
follows:
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.
Consider the safety margin about 30%, we can choose
the sensing resistor as 9mΩ.
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.
14
Figure 3. Current Ripple Ratio at Different Inputs for Buck Mode
R
ΔI
I
L
SENSE
L
=
(V
=
0.8
0.6
0.4
0.2
IN
0
2 • (P / V
12
V
– V
IN
L
OUT
• L • ƒ • P
2 • 130mV
to the maximum inductor current I
18
INPUT VOLTAGE V
) • V
OUT
OUT
) − ΔI
24
2
2.5μH
L
4.7μH
3.3μH
IN
(V)
30
6μH
4609 F03
36
L
as
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:
If a total low ESR of about 5mΩ is chosen for output ca-
pacitors, 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 V
12V, V
Set the PLLFLTR pin and R
If the maximum output power P is 50W at boost mode
and the module effi ciency η is about 90%, we can get
the current ripple ratio of the current ripple ΔI
maximum inductor current I
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.
Consider the safety margin about 30%, we can choose
the sensing resistor as 8mΩ.
R
ΔV
ΔI
I
L
SENSE
L
OUT(P-P)
OUT
=
(V
= 12V and ƒ = 400kHz.
=
OUT
V
2 •
OUT
= ESR • ΔI
− V
η • V
• L • ƒ • P
2 • 160mV
IN
IN(MIN)
P
) • V
L
IN
FB
2
L
+ ΔI
η
as in buck mode.
as follows:
L
IN
L
= 5V to
to the
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