LTM4603 LINER [Linear Technology], LTM4603 Datasheet - Page 14

• Manufacturer Part Number: LTM4603
• Description: 36VIN, 34VOUT High Efficiency Buck-Boost DC/DC
• Manufacturer: LINER [Linear Technology]

applications inForMation
LTM4609
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 ΔI
I
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.
14
Figure 3. Current Ripple Ratio at Different Inputs for Buck Mode
L
as follows:
R
ΔI
I
L
SENSE
L
=
(V
=
0.8
0.6
0.4
0.2
IN
0
2 • (P / V
12
V
– V
IN
V
ƒ = 400kHz
OUT
OUT
• L • ƒ • P
2 • 130mV
= 12V
18
INPUT VOLTAGE V
) • V
OUT
L
to the maximum inductor current
OUT
) − ΔI
24
2
2.5µH
L
4.7µH
3.3µH
IN
(V)
30
6µH
4609 F03
36
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:
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 V
12V, V
Set the PLLFLTR pin and R
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 ΔI
maximum inductor current I
ΔV
ΔI
I
L
L
OUT(P-P)
OUT
=
(V
= 12V and ƒ = 400kHz.
OUT
V
OUT
= ESR • ΔI
− V
• L • ƒ • P
IN
) • V
L
IN
FB
2
L
η
as in buck mode.
as follows:
IN
L
= 5V to
to the
4609fc