LTM4603HV
20
4603hvfa
For more information www.linear.com/LTM4603HV
applicaTions inForMaTion
Example for 5V Output
LTM4603HV minimum on-time = 100ns
t
ON
= ((V
OUT
•10pf)/I
fSET
), for V
OUT
> 4.8V use 4.8V
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
= ((4.8V•10pF)/I
fSET
), t
ON
= 171ns, where
the internal R
fSET
is 33.2k. Frequency = (V
OUT
/(V
IN
•t
ON
))
= (5V/(28V•171ns)) ~ 1MHz. The inductor ripple cur-
rent 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 over
4A at 18% 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 reflects 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
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
LTM4603HV minimum on-time = 100ns
t
ON
= ((3.3V•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
= ((3.3V•10pf)/I
fSET
), t
ON
= 117ns, where the
internal R
fSET
is 33.2k. Frequency = (V
OUT
/(V
IN
•t
ON
)) =
(3.3V/(28V•117ns)) ~ 1MHz. The minimum on-time and
minimum off-time are within specification at 117ns and
883ns. But the 4.5V minimum input for converting 3.3V
output will not meet the minimum off-time specification
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 minimum
off-time at 4.5V input voltage. The off-time should be about
500ns with 100ns guard band included. 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 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.
