LTM4603/LTM4603-1
20
4603fb
Example for 5V Output
LTM4603 minimum on-time = 100ns
t
ON
= [(V
OUT
• 10pF)/I
fSET
], for V
OUT
> 4.8V use 4.8V
LTM4603 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 20V operation, I
fSET
= 201µA, t
ON
= [(4.8 • 10pF)/I
fSET
], t
ON
= 239ns, where the internal R
fSET
is 33.2k. Frequency = (V
OUT
/(V
IN
• t
ON
)) = (5V/(20 • 239ns))
~ 1MHz. 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 at ~5A 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 7A 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 20V input voltage equals 245µA. This equates to a t
ON
of 196ns. This will increase the switching frequency from
1MHz to ~1.28MHz for the 20V to 5V conversion. The
minimum on time is above 100ns at 20V 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 20V for 1.28MHz operation
with a 150k resistor to ground, 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
LTM4603 minimum on-time = 100ns
t
ON
= [(V
OUT
• 10pF)/I
fSET
]
LTM4603 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 20V operation, I
fSET
= 201µA,
t
ON
= [(3.3 • 10pF)/I
fSET
], t
ON
= 164ns, where the internal
R
fSET
is 33.2k. Frequency = [V
OUT
/(V
IN
• t
ON
)] = [3.3V/
(20•164ns)] ~ 1MHz. The minimum on-time and minimum
off-time are within specification at 164ns and 836ns.
However, the 4.5V input to 3.3V output circuit 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 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 82.5k 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 20V
input operation for down converting to 3.3V output. The
frequency will scale from 540kHz to 1.2MHz over this
input range. This provides for an effective output current
of 5A over the input range.
applications inForMation
