LTM4607 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTM4607
Linear Technology
'LTM4607' PDF : 24 Pages View PDF
LTM4607
APPLICATIONS INFORMATION
at buck mode, we can get the current ripple ratio of the
current ripple ΔIL to the maximum inductor current IL as
follows:
ΔIL
IL
=
(VIN
– VOUT ) • VOUT
VIN • L • f • P
2
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 • 130mV
(P / VOUT ) −
ΔIL
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.
0.8
2.5μH
0.6
3.3μH
0.4
4.7μH
6μH
0.2
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:
ΔVOUT(P-P) = ESR • ΔIL
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 VIN = 5V to
12V, VOUT = 12V and f = 400kHz.
Set the PLLFLTR pin and RFB as in buck mode.
If the maximum output power P is 60W at boost mode
and the module efficiency η is about 95%, we can get
the current ripple ratio of the current ripple ΔIL to the
maximum inductor current IL as follows:
ΔIL = (VOUT − VIN ) • VIN2η
IL
VOUT • L • f • P
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) +
ΔIL
Consider the safety margin about 30%, we can choose
the sensing resistor as 7mΩ.
0
12
18
24
30
36
INPUT VOLTAGE VIN (V)
4607 F03
Figure 3. Current Ripple Ratio at Different Inputs for Buck Mode
4607f
14
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