Datasheet

LTM4601/LTM4601-1
21
4601fd
For more information www.linear.com/LTM4601
Example for 5V Output
LTM4601 minimum on-time = 100ns
t
ON
= ((V
OUT
• 10pF)/I
fSET
), for V
OUT
> 4.8V use 4.8V.
LTM4601 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
= 170µA,
t
ON
= ((4.8 • 10pF)/I
fSET
), t
ON
= 282ns, where the internal
R
fSET
is 39.2k. Frequency = (V
OUT
/(V
IN
• t
ON
)) = (5V/(20
• 282ns)) ~ 886kHz. 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 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. An 8A ripple current is chosen, and the total
peak current is equal to 1/2 of the 8A ripple current plus
the output current. The 5V output current is limited to 8A,
so the total peak current is less than 12A. 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
20V input voltage equals 238µA. This equates to a t
ON
of
200ns. This will increase the switching frequency from
~886kHz to ~1.25MHz 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 1.25MHz 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 curve reflects
an operating range of 10V to 20V for 1.25MHz operation
with a 100k 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
LTM4601 minimum on-time = 100ns
t
ON
= ((V
OUT
• 10pF)/I
fSET
)
LTM4601 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
= 170µA,
t
ON
= ((3.3 • 10pf)/I
fSET
), t
ON
= 195ns, where the internal
R
fSET
is 39.2k. Frequency = (V
OUT
/(V
IN
• t
ON
)) = (3.3V/
(20 • 195ns)) ~ 846kHz. The minimum on-time and mini-
mum off-time are within specification at 195ns and 980ns.
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 opera-
tion. 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 20V 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