LTM4603V-1 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTM4603V-1' PDF : 24 Pages View PDF
LTM4603/LTM4603-1
APPLICATIO S I FOR ATIO
Example for 5V Output
LTM4603 minimum on-time = 100ns;
tON = ((4.8 ā¢ 10pf)/IfSET)
LTM4603 minimum off-time = 400ns; tOFF = t ā tON,
where t = 1/Frequency
Duty Cycle = tON/t or VOUT/VIN
Equations for setting frequency:
IfSET = (VIN/(3 ā¢ RfSET)), for 20V operation, ISET = 201ĀµA, tON
= ((4.8 ā¢ 10pF)/IfSET), tON = 239ns, where the internal RfSET
is 33.2k. Frequency = (VOUT/(VIN ā¢ tON)) = (5V/(20 ā¢ 239ns))
~ 1MHz. The inductor ripple current begins to get high at the
higher input voltages due to a larger voltage across the in-
ductor. This is noted in the āTypical Inductor Ripple Current
verses Duty Cycle graphā at ~4.5A 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 7A peak speciļ¬ed 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 20V input voltage equals 245ĀµA. This equates to a tON
of 196ns. This will increase the switching frequency from
1MHz to ~1.28MHz for the 20V to 5V conversion. The
minimum on time is above 100ns at 20V 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 Ratio Curveā reļ¬ects an
operating range of 10V to 20V for 1.28MHz operation with
a 150k resistor to ground, and an 8V to 16V operation 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
LTM4603 minimum on-time = 100ns;
tON = ((3.3 ā¢ 10pF)/IfSET)
LTM4603 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 20V operation, IfSET = 201ĀµA,
tON = ((3.3 ā¢ 10pf)/IfSET), tON = 164ns, where the internal
RfSET is 33.2k. Frequency = (VOUT/(VIN ā¢ tON)) = (3.3V/(20
ā¢ 164ns)) ~ 1MHz. The minimum on-time and minimum-
off time are within speciļ¬cation at 164ns and 836ns. 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 1.35Āµs. The fSET pin voltage compliance is 1/3 of VIN,
and the IfSET current equates to 45ĀµA with the internal
33.2k. The IfSET current needs to be 24ĀµA for 540kHz
operation. A resistor can be placed from VOUT to fSET to
lower the effective IfSET current out of the fSET pin to 24ĀµA.
The fSET pin is 4.5V/3 =1.5V and VOUT = 3.3V, therefore
82.5k will source 21ĀµA into the fSET node and lower the
IfSET current to 24ĀµA. This enables the 540kHz operation
and the 4.5V to 20V input operation for down converting
to 3.3V output. The frequency will scale from 540kHz to
1.2MHz over this input range. This provides for an effective
output current of 5A over the input range.
4603f
19
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