PRM48BF480T500A00 Datasheet, PDF (30/45 Pages) Vicor Corporation – PRM™ Regulator

PRM48B x 480 y 500A00
Setting the Adaptive Loop Load Line (Adaptive Loop
operation)
To determine an appropriate value for the compensation slope
(R_{LL_AL}) it helps to reflect the VTM’s output resistance to the
input side of the VTM. A resistance on the output side of the
VTM is scaled by the VTMs transformer ratio (K_VTM) squared
as defined by equation (2):
_
_
__^∙
(2)
Where
R_{OUT_VTM} is the VTM output resistance at 25°C
K_VTM is the VTM transformer ratio V_IN/V_OUT
For our hypothetical VTM from above (with K_VTM = 1/4 and
R_{OUT_VTM} = 10mΩ) the output resistance reflected over to the
input would be equal to 160 mΩ. For this example, R_{LL_AL}
should be set to -160 mΩ to approximately cancel at 25°C the
inherent load line from the VTM.
R_{LL_AL} is set by the voltage difference between the AL pin and
SGND pin, V_AL, per the following formula:
R_{_}
V ∙ 0.5 Ω/V
(3)
V_AL ≤ 3.10V
Where V_AL is the voltage on the AL pin
V_AL is sampled by a 10-bit ADC, whose input is connected to
V_{CC_INT} through a 10 kΩ pull up resistor. This pull up disables
the AL engine when the AL pin is left open. V_AL can be actively
set with a DAC that is ground referenced to SGND. V_AL can be
passively set by connecting a resistor, R_AL, from AL to SGND
such that the voltage divider made with V_{CC_INT} and the 10 kΩ
pull up yields the desired V_AL. The formula for calculating this
resistor is provided in Equation (4).
∙
(4)
_
VCCINT
10K
AL
VAL
Micro
Controller
Similar to TRIM, AL is sampled during every start up to
determine if the Adaptive Loop load line is enabled or disabled.
If the AL pin is allowed to pull up to 3.20V or higher during start
up, then then the PRM will disable the Adaptive Loop load line
as long as the PRM remains operating. In this case, for all
subsequent operation the output voltage will be remain at the
set voltage, and the AL pin will be ignored.
This selection persists until the PRM is restarted with the
ENABLE pin, or due to fault auto-recovery.
When AL is enabled, the voltage at this pin is sampled at
120 µs intervals to determine the load line. The load line can
be adjusted during normal operation, however it is not
recommended to use this pin in an external analog feedback
loop.
Adaptive Loop Temperature Compensation (Adaptive
Loop operation)
By connecting the VT pin of the PRM to the VTM’s TM pin, the
PRM is able to monitor the internal temperature of the VTM.
Knowing the VTM’s internal temperature and the temperature
coefficient of the VTM’s R_OUT, which is preprogrammed into the
PRM’s microcontroller, the AL engine is able to scale the
nominal value of R_{LL_AL} (set by the AL pin) to track the VTM’s
R_OUT over temperature. In this way the output resistance of the
PRM can be tuned to cancel the output resistance of the VTM
with the addition of a single resistor across the AL pin and a
connection of the VTM’s TM pin to the PRM’s VT pin.
The VTM TM voltage is equal to the VTM internal sensed
temperature in Kelvin divided by 100. For a temperature range
of -55 °C to 125 °C the TM voltage will range from
2.18 V to 3.98 V. The Adaptive Loop temperature
compensation is pre-programed into the internal
microcontroller and is 0.3 %/°C assuming the VT pin is
connected to the TM pin of a compatible VTM
The TM pin has an internal pull down to SGND, and
temperature compensation is disabled for VT voltages less
than 1.9V. To disable temperature compensation, leave the
VT pin unconnected and open circuit. When disabled, the
temperature defaults 25 °C.
RAL
SGND
Figure 29: AL Connections
- 30 -
Rev. 1.0
11/2012