LTM4603HV View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTM4603HV' PDF : 24 Pages View PDF
LTM4603HV
APPLICATIO S I FOR ATIO
Example for 5V Output
LTM4603HV minimum on-time = 100ns;
tON = ((4.8 ā¢ 10pf)/IfSET)
LTM4603HV minimum off-time = 400ns;
tOFF = tā tON, where t = 1/Frequency
Duty Cycle = tON/t or VOUT/VIN
Equations for setting frequency:
IfSET = (VIN/(3 ā¢ RfSET)), for 28V input operation, IfSET =
281Ī¼A, tON = ((4.8 ā¢ 10pF)/IfSET), tON = 171ns, where the
internal RfSET is 33.2k. Frequency = (VOUT/(VIN ā¢ tON)) =
(5V/(28 ā¢ 171ns)) ~ 1MHz. The inductor ripple current
begins to get high at the higher input voltages due to a
larger voltage across the inductor. This is shown in the
āInductor Ripple Current vs Duty Cycleā graph as ~4A at
25% duty cycle. The inductor ripple current can be lowered
at the higher input voltages by adding an external resistor
from fSET to ground to increase the switching frequency.
A 3A ripple current is chosen, and the total peak current
is equal to 1/2 of the 3A ripple current plus the output
current. The 5V output current is limited to 5A, so total
peak current is less than 6.5A. This is below the 8A peak
specified value. A 150k resistor is placed from fSET to
ground, and the parallel combination of 150k and 33.2k
equates to 27.2k. The IfSET calculation with 27.2k and
28V input voltage equals 343Ī¼A. This equates to a tON of
140ns. This will increase the switching frequency from
1MHz to ~1.28MHz for the 28V to 5V conversion. The
minimum on time is above 100ns at 28V input. Since
the switching frequency is approximately constant over
input and output conditions, then the lower input voltage
range is limited to 10V for the 1.28MHz operation due to
the 400ns minimum off time. Equation: tON = (VOUT/VIN) ā¢
(1/Frequency) equates to a 382ns on time, and a 400ns off
time. The āVIN to VOUT Step-Down Ratioā curve reļ¬ects an
operating range of 10V to 28V for 1.28MHz operation with a
150k resistor to ground (shown in Figure 18), and an 8V to
16V operating range for fSET ļ¬oating. These modiļ¬cations
are made to provide wider input voltage ranges for the 5V
output designs while limiting the inductor ripple current,
and maintaining the 400ns minimum off time.
Example for 3.3V Output
LTM4603HV minimum on-time = 100ns;
tON = ((3.3 ā¢ 10pF)/IfSET)
LTM4603HV minimum off-time = 400ns;
tOFF = t ā tON, where t = 1/Frequency
Duty Cycle (DC) = tON/t or VOUT/VIN
Equations for setting frequency:
IfSET = (VIN/(3 ā¢ RfSET)), for 28V input operation, IfSET =
281Ī¼A, tON = ((3.3 ā¢ 10pf)/IfSET), tON = 117ns, where the
internal RfSET is 33.2k. Frequency = (VOUT/(VIN ā¢ tON)) =
(3.3V/(28 ā¢ 117ns)) ~ 1MHz. The minimum on-time and
minimum-off time are within speciļ¬cation at 118ns and
882ns. But the 4.5V minimum input for converting 3.3V
output will not meet the minimum off-time speciļ¬cation of
400ns. tON = 733ns, Frequency = 1MHz, tOFF = 267ns.
Solution
Lower the switching frequency at lower input voltages to
allow for higher duty cycles, and meet the 400ns mini-
mum off-time at 4.5V input voltage. The off-time should
be about 500ns with 100ns guard band. The duty cycle
for (3.3V/4.5) = ~73%. Frequency = (1 ā DC)/tOFF or
(1 ā 0.73)/500ns = 540kHz. The switching frequency needs
to be lowered to 540kHz at 4.5V input. tON = DC/frequency,
or 1.35Ī¼s. The fSET pin voltage compliance is 1/3 of VIN,
and the IfSET current equates to 45Ī¼A with the internal
33.2k. The IfSET current needs to be 24Ī¼A for 540kHz
operation. A resistor can be placed from VOUT to fSET to
lower the effective IfSET current out of the fSET pin to 24Ī¼A.
The fSET pin is 4.5V/3 =1.5V and VOUT = 3.3V, therefore
an 82.5k resistor will source 21Ī¼A into the fSET node and
lower the IfSET current to 24Ī¼A. This enables the 540kHz
operation and the 4.5V to 28V input operation for down
converting to 3.3V output as shown in Figure 19. The
frequency will scale from 540kHz to 1.27MHz over this
input range. This provides for an effective output current
of 5A over the input range.
4603hvf
19
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