LTM4601 LINER [Linear Technology], LTM4601 Datasheet - Page 20

• Manufacturer Part Number: LTM4601
• Description: 12A DC/DC ?Modules with PLL, Output Tracking and Margining
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTM4601/LTM4601-1
Frequency Adjustment
The LTM4601 is designed to typically operate at 850kHz
across most input conditions. The f
open or decoupled with an optional 1000pF capacitor. The
switching frequency has been optimized for maintaining
constant output ripple noise over most operating ranges.
The 850kHz switching frequency and the 400ns minimum
off time can limit operation at higher duty cycles like 5V to
3.3V, and produce excessive inductor ripple currents for
lower duty cycle applications like 20V to 5V. The 5V and
3.3V drop out curves are modifi ed by adding an external
resistor on the f
operation, or higher input voltage operation.
Example for 5V Output
Equations for setting frequency:
I
t
R
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 Typical Inductor
Ripple Current verses Duty Cycle graph (Figure 3) where
I
can be lowered at the higher input voltages by adding an
external resistor from f
ing 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
20
fSET
ON
L
fSET
≈ 10A at 25% duty cycle. The inductor ripple current
LTM4601 minimum on-time = 100ns;
t
LTM4601 minimum off-time = 400ns; t
where t = 1/Frequency
Duty Cycle = t
ON
= ((4.8 • 10pF)/I
= (V
is 39.2k. Frequency = (V
= ((4.8 • 10pf)/I
IN
/(3 • R
SET
ON
fSET
/t or V
U
fSET
pin to allow for lower input voltage
)), for 20V operation, I
SET
fSET
), t
to ground to increase the switch-
OUT
U
)
ON
/V
= 282ns, where the internal
OUT
IN
/(V
SET
W
IN
pin is normally left
• t
ON
OFF
)) = (5V/(20 •
SET
= t – t
U
= 170µA,
ON
,
14A peak specifi ed value. A 100k resistor is placed from
f
39.2k equates to 28k. The I
20V input voltage equals 238µA. This equates to a t
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
• (1/Frequency) equates to a 400ns on time, and a 400ns
off time. The “V
operating range of 10V to 20V for 1.25MHz operation with
a 100k resistor to ground, and an 8V to 16V operation for
f
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
Equations for setting frequency:
I
t
R
• 195ns)) ~ 846kHz. The minimum on-time and minimum-
off time are within specifi cation at 195ns and 980ns. The
4.5V minimum input for converting 3.3V output will not
meet the minimum off-time specifi cation of 400ns. t
868ns, Frequency = 850kHz, t
SET
SET
fSET
ON
fSET
LTM4601 minimum on-time = 100ns;
t
LTM4601 minimum off-time = 400ns;
t
Duty Cycle (DC) = t
ON
OFF
= ((3.3 • 10pf)/I
to ground, and the parallel combination of 100k and
fl oating. These modifi cations are made to provide
= (V
is 39.2k. Frequency = (V
= ((3.3 • 10pF)/I
= t – t
IN
/(3 • R
ON
, where t = 1/Frequency
IN
fSET
to V
fSET
)), for 20V operation, I
ON
OUT
fSET
), t
/t or V
ON
)
Step Ratio Curve” refl ects an
fSET
= 195ns, where the internal
OUT
OFF
OUT
calculation with 28k and
/(V
= 315ns.
/V
IN
IN
• t
ON
ON
)) = (3.3V/(20
fSET
= (V
= 170µA,
OUT
ON
/V
ON
4601f
IN
of
=
)