Datasheet

LTM4603HV
19
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APPLICATIO S I FOR ATIO
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Example for 5V Output
LTM4603HV minimum on-time = 100ns;
t
ON
= ((4.8 • 10pf)/I
fSET
)
LTM4603HV 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 input operation, I
fSET
=
281μA, t
ON
= ((4.8 • 10pF)/I
fSET
), t
ON
= 171ns, where the
internal R
fSET
is 33.2k. Frequency = (V
OUT
/(V
IN
• t
ON
)) =
(5V/(28 • 171ns)) ~ 1MHz. The inductor ripple current
begins to get high at the higher input voltages due to a
larger voltage across the inductor. This is shown in the
“Inductor Ripple Current vs Duty Cycle” graph as ~4A at
25% 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 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 8A peak
specified value. A 150k resistor is placed from f
SET
to
ground, and the parallel combination of 150k and 33.2k
equates to 27.2k. The I
fSET
calculation with 27.2k and
28V input voltage equals 343μA. This equates to a t
ON
of
140ns. This will increase the switching frequency from
1MHz to ~1.28MHz 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 1.28MHz operation due to
the 400ns minimum off time. Equation: t
ON
= (V
OUT
/V
IN
) •
(1/Frequency) equates to a 382ns on time, and a 400ns off
time. The “V
IN
to V
OUT
Step-Down Ratio” curve refl ects an
operating range of 10V to 28V for 1.28MHz operation with a
150k resistor to ground (shown in Figure 18), and an 8V to
16V operating range for f
SET
fl oating. These modifi 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
LTM4603HV minimum on-time = 100ns;
t
ON
= ((3.3 • 10pF)/I
fSET
)
LTM4603HV 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
=
281μA, t
ON
= ((3.3 • 10pf)/I
fSET
), t
ON
= 117ns, where the
internal R
fSET
is 33.2k. Frequency = (V
OUT
/(V
IN
• t
ON
)) =
(3.3V/(28 • 117ns)) ~ 1MHz. The minimum on-time and
minimum-off time are within specifi cation at 118ns and
882ns. But the 4.5V minimum input for converting 3.3V
output will not meet the minimum off-time specifi cation of
400ns. t
ON
= 733ns, Frequency = 1MHz, t
OFF
= 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 – 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 45μA with the internal
33.2k. 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
an 82.5k resistor will source 21μA 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 1.27MHz over this
input range. This provides for an effective output current
of 5A over the input range.