ltm4601hv Linear Technology Corporation, ltm4601hv Datasheet - Page 20

• Manufacturer Part Number: ltm4601hv
• Description: 12a 28vin Dc/dc ?module With Pll, Output Tracking And Margining
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTM4601HV
Frequency Adjustment
The LTM4601HV 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 28V 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
202ns)) ~ 884kHz. 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. A 7A ripple current is chosen, and the total
peak current is equal to 1/2 of the 7A ripple current plus
the output current. The 5V output current is limited to 8A,
so the total peak current is less than 11.5A. This is below
20
fSET
ON
L
fSET
≈ 10A at 20% duty cycle. The inductor ripple current
LTM4601HV minimum on-time = 100ns;
t
LTM4601HV minimum off-time = 400ns;
t
Duty Cycle = t
ON
OFF
= ((4.8 • 10pF)/I
= (V
is 39.2k. Frequency = (V
= ((4.8 • 10pf)/I
= t – t
IN
/(3 • R
ON
, where t = 1/Frequency
SET
ON
fSET
/t or V
U
fSET
pin to allow for lower input voltage
)), for 28V operation, I
SET
fSET
), t
to ground to increase the switch-
OUT
U
)
ON
/V
= 202ns, where the internal
OUT
IN
/(V
SET
W
IN
pin is normally left
• t
ON
)) = (5V/(28 •
SET
U
= 238µA,
the 14A peak specifi ed value. A 100k resistor is placed
from f
and 39.2k equates to 28k. The I
and 28V input voltage equals 333µA. This equates to a t
of 144ns. This will increase the switching frequency from
~884kHz to ~1.24MHz 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 1.24MHz 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 28V for 1.24MHz operation with
a 100k resistor to ground as shown in Figure 18, and an
8V to 16V operation for f
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
Equations for setting frequency:
I
t
R
138.7ns)) ~ 850kHz. The minimum on-time and minimum-
off time are within specifi cation at 139ns and 1037ns. 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
fSET
ON
fSET
LTM4601HV minimum on-time = 100ns;
t
LTM4601HV minimum off-time = 400ns;
t
Duty Cycle (DC) = t
ON
OFF
= ((3.3 • 10pf)/I
= (V
is 39.2k. Frequency = (V
SET
= ((3.3 • 10pF)/I
= t – t
IN
to ground, and the parallel combination of 100k
/(3 • R
ON
, where t = 1/Frequency
IN
fSET
to V
fSET
)), for 28V operation, I
ON
), t
OUT
fSET
SET
/t or V
ON
)
Step Ratio Curve” refl ects an
fl oating. These modifi cations
= 138.7ns, where the internal
OUT
OFF
OUT
fSET
/(V
= 315ns.
/V
IN
calculation with 28k
IN
• t
ON
ON
)) = (3.3V/(28 •
fSET
= (V
= 238µA,
OUT
/V
4601hvf
ON
IN
ON
=
)