LTM4606 LINER [Linear Technology], LTM4606 Datasheet - Page 18

• Manufacturer Part Number: LTM4606
• Description: Ultralow EMI 28VIN, 6A DC/DC ?Module
• Manufacturer: LINER [Linear Technology]

LTM4606
APPLICATIONS INFORMATION
Frequency Adjustment
The LTM4606 is designed to typically operate at 800kHz
across most input conditions. The f
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 800kHz 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 28V to 5V.
Example for 5V Output
Equations for setting frequency:
I
For 28V input operation, I
I
• 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 if the integrated inductor is 1μH. The inductor ripple
current can be lowered at the higher input voltages by add-
ing an external resistor from f
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 specifi ed value. A 150k resistor is placed from
f
41.2k equates to 32.3k. The I
and 28V input voltage equals 289μA. This equates to a t
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
a 375ns on time, and a 400ns off time. Figure 18 shows an
18
fSET
fSET
SET
LTM4606 minimum on-time = 100ns;
t
LTM4606 minimum off-time = 400ns;
t
Duty Cycle = t
ON
OFF
), t
to ground, and the parallel combination of 150k and
= (V
= ((4.8 • 10pf)/I
ON
= t – t
IN
= 211ns. Frequency = (V
/(3 • R
ON
, where t = 1/Frequency
ON
ON
fSET
= (V
/t or V
)), where the internal R
fSET
OUT
fSET
OUT
/V
)
IN
SET
/V
= 227μA. t
fSET
) • (1/Frequency) equates to
IN
to ground to increase the
OUT
calculation with 32.3k
SET
/(V
pin is typically left
ON
IN
• t
= ((4.8 • 10pF)/
ON
fSET
)) = (5V/(28
is 41.2k.
ON
operating range of 10V to 28V for 1MHz operation with a
150k resistor to ground, and an 8V to 16V operating range
for f
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
Equations for setting frequency:
I
227μA, t
internal R
(3.3V/(28 • 145ns)) ~ 810kHz. The minimum on-time and
minimum-off time are within specifi cation at 146ns and
1089ns. But the 4.5V minimum input for converting 3.3V
output will not meet the minimum off-time specifi cation of
400ns. t
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
(1 – 0.73)/500ns = 540kHz. The switching frequency
needs to be lowered to 540kHz at 4.5V input. t
frequency, or 1.35μs. The f
is 1/3 of V
the internal 41.2k. The I
540kHz operation. A resistor can be placed from V
f
to 24μA. The f
therefore an 150k resistor will source 12μA into the f
node and lower the I
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.
fSET
SET
LTM4606 minimum on-time = 100ns;
t
LTM4606 minimum off-time = 400ns;
t
Duty Cycle (DC) = t
ON
OFF
SET
to lower the effective I
= (V
= ((3.3 • 10pF)/I
= t – t
fl oating. These modifi cations are made to provide
ON
ON
IN
fSET
IN
/(3 • R
= 905ns, Frequency = 810kHz, t
= ((3.3 • 10pf)/I
, and the I
ON
is 41.2k. Frequency = (V
SET
, where t = 1/Frequency
fSET
pin is 4.5V/3 =1.5V and V
fSET
)), for 28V input operation, I
ON
fSET
fSET
fSET
current to 24μA. This enables the
/t or V
)
fSET
current equates to 36μA with
fSET
current needs to be 24μA for
SET
OUT
current out of the f
), t
pin voltage compliance
ON
/V
IN
= 145ns, where the
OUT
/(V
OFF
OUT
IN
= 329ns.
ON
• t
SET
= 3.3V,
ON
OFF
OUT
fSET
= DC/
)) =
4606f
SET
pin
or
to
=