LTM4601AHV_15 Datasheet, PDF (20/30 Pages) Linear Technology – 12A, 28VIN DC/DC Module Regulator with PLL, Output Tracking and Margining

LTM4601AHV
Applications Information
Example for 5V Output
LTM4601AHV minimum on-time = 100ns
t_ON = [(V_OUT • 10pF)/I_fSET], for V_OUT > 4.8V use 4.8V
LTM4601AHV 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 operation, I_fSET = 238µA,
t_ON = [(4.8 • 10pF)/I_fSET], t_ON = 202ns, where the internal
R_fSET is 39.2k. Frequency = [V_OUT/(V_IN • t_ON)] = [5V/(28 •
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 Inductor Ripple
Current vs Duty Cycle graph (Figure 3) where I_L ≈ 10A at
20% 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 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 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
28V input voltage equals 333µA. This equates to a t_ON 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_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 curves reflect an
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_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
LTM4601AHV minimum on-time = 100ns
t_ON = [(V_OUT • 10pF)/I_fSET]
LTM4601AHV 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 operation, I_fSET = 238µA,
t_ON = [(3.3 • 10pF)/I_fSET], t_ON = 138.7ns, where the internal
R_fSET is 39.2k. Frequency = [V_OUT/(V_IN • t_ON)] = [3.3V/(28 •
138.7ns)] ~ 850kHz. The minimum on-time and minimum
off-time are within specification at 139ns and 1037ns. 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 operation.
As shown in Figure 19, 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 28V 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.
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