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LTM4606
APPLICATIONS INFORMATION
Frequency Adjustment
The LTM4606 is designed to typically operate at 800kHz
across most input conditions. The f SET pin is typically
left open. The switching frequency has been optimized
for maintaining constant output ripple noise over most
operating ranges. The 800kHz switching frequency and
the 400ns minimum off time can limit operation at higher
duty cycles like 5V to 3.3V, and produce excessive induc-
tor ripple currents for lower duty cycle applications like
28V to 5V.
Example for 5V Output
LTM4606 minimum on-time = 100ns
t ON = ((V OUT ? 10pF)/I fSET ), for V OUT > 4.8V use 4.8V
LTM4606 minimum off-time = 400ns
t OFF = t – t ON , where t = 1/Frequency
Duty Cycle = t ON /t or V OUT /V IN
Equations for setting frequency:
I fSET = (V IN /(3 ? R fSET )), where the internal R fSET is 41.2k.
For 28V input operation, I fSET = 227μA. t ON = ((4.8 ? 10pF)/
I fSET ), t ON = 211ns. Frequency = (V OUT /(V IN ? t ON )) = (5V/(28
? 211ns)) ~ 850kHz. The inductor ripple current begins to
get high at the higher input voltages due to a larger voltage
across the inductor. The current ripple is ~5A at 20% duty
cycle for the integrated 1μH inductor. The inductor ripple
current can be lowered at the higher input voltages by add-
ing an external resistor from f SET to ground to increase the
switching frequency. A 4A ripple current is chosen, and the
total peak current is equal to 1/2 of the 4A ripple current
plus the output current. For 5V output, current is limited to
5A, so the total peak current is less than 7A. This is below
the 8A peak specified value. A 150k resistor is placed from
f SET to ground, and the parallel combination of 150k and
41.2k equates to 32.3k. The I fSET calculation with 32.3k
and 28V input voltage equals 289μA. This equates to a t ON
of 166ns. This will increase the switching frequency from
850kHz to ~1MHz 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 8V for the 1MHz operation due to the
400ns minimum off time. Equation: t ON = (V OUT /V IN ) ? (1/
Frequency) equates to a 375ns on time, and a 400ns off
time. Figure 18 shows an operating range of 10V to 28V for
1MHz operation with a 150k resistor to ground, and an 8V to
16V operating range for f SET floating. These modifications
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
LTM4606 minimum on-time = 100ns
t ON = ((V OUT ? 10pF)/I fSET )
LTM4606 minimum off-time = 400ns
t OFF = t – t ON , where t = 1/Frequency
Duty Cycle (DC) = t ON /t or V OUT /V IN
Equations for setting frequency:
I fSET = (V IN /(3 ? R fSET )), for 28V input operation, I fSET =
227μA, t ON = ((3.3 ? 10pF)/I fSET ), t ON = 145ns, where the
internal R fSET is 41.2k. Frequency = (V OUT /(V IN ? t ON )) =
(3.3V/(28 ? 145ns)) ~ 810kHz. The minimum on-time and
minimum-off time are within specification at 146ns and
1089ns. But the 4.5V minimum input for converting 3.3V
output will not meet the minimum off-time specification
of 400ns. t ON = 905ns, Frequency = 810kHz, t OFF = 329ns.
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/4.5V) = ~73%. Frequency = (1 – DC)/t OFF or
(1 – 0.73)/500ns = 540kHz. The switching frequency
needs to be lowered to 540kHz at 4.5V input. t ON = DC/
frequency, or 1.35μs. The f SET pin voltage compliance
is 1/3 of V IN , and the I fSET current equates to 36μA with
the internal 41.2k. The I fSET current needs to be 24μA for
540kHz operation. A resistor can be placed from V OUT to
f SET to lower the effective I fSET current out of the f SET pin
to 24μA. The f SET pin is 4.5V/3 =1.5V and V OUT = 3.3V,
therefore a 150k resistor will source 12μA into the f SET
node and lower the I fSET current to 24μA. 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 950kHz over this
input range. This provides for an effective output current
of 5A over the input range.
4606fd
20
For more information www.linear.com/LTM4606
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