Datasheet

LTM4601AHV
20
4601ahvfc
For more information www.linear.com/LTM4601AHV
Example for 5V Output
LTM4601AHV minimum on-time = 100ns
t
ON
= [(V
OUT
• 10pF)/I
fSET
], for V
OUT
> 4.8V use 4.8V
LTM4601AHV 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
)], for 28V operation, I
fSET
= 238µA,
t
ON
= [(4.8 • 10pF)/I
fSET
], t
ON
= 202ns, where the internal
R
fSET
is 39.2k. Frequency = [V
OUT
/(V
IN
• t
ON
)] = [5V/(28 •
202ns)] ~ 884kHz. 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 Inductor Ripple
Current vs Duty Cycle graph (Figure 3) where I
L
≈ 10A at
20% duty cycle. The inductor ripple current can be lowered
at the higher input voltages by adding an external resistor
from f
SET
to ground to increase the switching frequency.
A 7A ripple current is chosen, and the total peak current
is equal to 1/2 of the 7A ripple current plus the output
current. The 5V output current is limited to 8A, so the
total peak current is less than 11.5A. This is below the
14A peak specified value. A 100k resistor is placed from
f
SET
to ground, and the parallel combination of 100k and
39.2k equates to 28k. The I
fSET
calculation with 28k and
28V input voltage equals 333µA. This equates to a t
ON
of
144ns. This will increase the switching frequency from
~884kHz to ~1.24MHz 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 1.24MHz operation due to the
400ns
minimum
off time. Equation: t
ON
= (V
OUT
/V
IN
) • (1/
Frequency) equates to a 400ns on time, and a 400ns off
time. The V
IN
to V
OUT
Step-Down Ratio curves reflect an
operating range of 10V to 28V for 1.24MHz operation with
a 100k resistor to ground as shown in Figure 18, and an
8V to 16V operation 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
LTM4601AHV minimum on-time = 100ns
t
ON
= [(V
OUT
• 10pF)/I
fSET
]
LTM4601AHV 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 operation, I
fSET
= 238µA,
t
ON
= [(3.3 • 10pF)/I
fSET
], t
ON
= 138.7ns, where the internal
R
fSET
is 39.2k. Frequency = [V
OUT
/(V
IN
• t
ON
)] = [3.3V/(28 •
138.7ns)] ~ 850kHz. The minimum on-time and minimum
off-time are within specification at 139ns and 1037ns. The
4.5V minimum input for converting 3.3V output will not
meet the minimum off-time specification of 400ns. t
ON
=
868ns, Frequency = 850kHz, t
OFF
= 315ns.
Solution
Lower the switching frequency at lower input voltages to
allow for higher duty cycles, and meet the 400ns minimum
off-time at 4.5V input voltage. The off-time should be about
500ns, which includes a 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 is 1/3 of V
IN
, and
the I
fSET
current equates to 38µA with the internal 39.2k.
The I
fSET
current needs to be 24µA for 540kHz operation.
As shown in Figure 19, 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 130k will source 14µA 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. The frequency will scale from
540kHz to 1.1 MHz over this input range. This provides
for an effective output current of 8A over the input range.
applicaTions inForMaTion