LTM4601A View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTM4601A' PDF : 28 Pages View PDF
LTM4601A/LTM4601A-1
APPLICATIONS INFORMATION
Frequency Adjustment
The LTM4601A is designed to typically operate at 850kHz
across most input conditions. The fSET pin is normally left
open or decoupled with an optional 1000pF capacitor. The
switching frequency has been optimized for maintaining
constant output ripple noise over most operating ranges.
The 850kHz switching frequency and the 400ns minimum
off time can limit operation at higher duty cycles like 5V to
3.3V, and produce excessive inductor ripple currents for
lower duty cycle applications like 20V to 5V. The 5V and
3.3V drop out curves are modiļ¬ed by adding an external
resistor on the fSET pin to allow for lower input voltage
operation, or higher input voltage operation.
Example for 5V Output
LTM4601A minimum on time = 100ns;
tON = ((4.8 ā¢ 10pf)/IfSET)
LTM4601A 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, IfSET = 170Ī¼A,
tON = ((4.8 ā¢ 10pF)/IfSET), tON = 282ns, where the internal
RfSET is 39.2k. Frequency = (VOUT/(VIN ā¢ tON)) = (5V/(20
ā¢ 282ns)) ā 886kHz. The inductor ripple current begins
to get high at the higher input voltages due to a larger
voltage across the inductor. This is noted in the āInduc-
tor Ripple Current vs Duty Cycleā graph (Figure 3) where
IL ā 10A 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 switch-
ing frequency. An 8A ripple current is chosen, and the total
peak current is equal to 1/2 of the 8A ripple current plus
the output current. The 5V output current is limited to 8A,
so the total peak current is less than 12A. This is below the
14A peak speciļ¬ed value. A 100k resistor is placed from
fSET to ground, and the parallel combination of 100k and
39.2k equates to 28k. The IfSET calculation with 28k and
20V input voltage equals 238Ī¼A. This equates to a tON of
200ns. This will increase the switching frequency from
~886kHz to ~1.25MHz 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 1.25MHz operation due to
the 400ns minimum off time. Equation: tON = (VOUT/VIN)
ā¢ (1/Frequency) equates to a 400ns on time, and a 400ns
off time. The āVIN to VOUT Step-Down Ratioā curve reļ¬ects
an operating range of 10V to 20V for 1.25MHz operation
with a 100k 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
LTM4601A minimum on time = 100ns;
tON = ((3.3 ā¢ 10pF)/IfSET)
LTM4601A 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 = 170Ī¼A,
tON = ((3.3 ā¢ 10pf)/IfSET), tON = 195ns, where the internal
RfSET is 39.2k. Frequency = (VOUT/(VIN ā¢ tON)) = (3.3V/(20
ā¢ 195ns)) ā 846kHz. The minimum on time and minimum
off time are within speciļ¬cation at 195ns and 980ns. The
4.5V minimum input for converting 3.3V output will not
meet the minimum off-time speciļ¬cation of 400ns. tON =
868ns, Frequency = 850kHz, tOFF = 315ns.
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