PRD48BF480T400A00 Datasheet, PDF (15/22 Pages) Vicor Corporation – PRM™ Regulator

Not Recommended for New Designs
PRM48BF480T400A00
(Formerly VIP0001TFJ)
10.0 PRODUCT DETAILS AND DESIGN GUIDELINES
10.1 Control pins description and characteristics
Control node (PR) is the input to the control node which
determines the powertrain timing and ultimately the
module output power (Figure 21). An internal 0.5mA
current sink is always active. The bi-directional buffer
between PR and the control node has two states. In
normal operation, PR will be above the 0.79V switching
threshold, and will drive the control node through the
buffer. An internal 7.4V clamp determines the maximum
output power that can be requested of the modulator.
When PR falls below 0.79 V, the converter will stop
switching. An internal circuit clamps the modulator input
control node to 0.79 V, and a buffer will source up to 2.5
mA out of the pin at that clamp level. For this reason, the
output impedance of the amplifier driving PR must be
taken into account. A rail-to-rail operational amplifier with
low output impedance is always recommended.
The powertrain small signal (plant) response consists of a
single pole determined by the load resistance, the
powertrain equivalent output resistance, and the total
output capacitance (internal and external to the module).
Both the modulator gain and the equivalent output
resistance vary as a function of line, load and output
voltage, as shown in Figures 17, 18 and 19. As the load
increases, the powertrain pole moves to higher frequency.
As a result, the closed loop crossover frequency will be the
highest at full load and lowest at minimum load. Figure 25
shows a reference AC small-signal model.
Current feedback (IF) is the input for the module output
overcurrent protection and current limit features (see
functional block diagram in section 4.0). A voltage
proportional to the powertrain output current must be
applied to IF in order for overcurrent protection to operate
properly.
If the IF voltage exceeds the IF pin’s overcurrent
protection threshold, the powertrain will stop switching. If
the IF voltage falls below the overcurrent protection
threshold within T_BLANK time, then the powertrain will
immediately resumes switching. Otherwise a fault is
latched.
The current limit threshold for the IF pin is set lower than
the protection threshold. When the IF pin average voltage
exceeds the current limit threshold, an internal integrator
will activate a clamp amplifier which overrides the
modulator input maximum level. This causes the
powertrain to maintain a constant output current.
The bandwidth of this current limit integrator is significantly
slower than that of the PR control node input. Therefore
this current limit can not be used in lieu of properly
compensating the (external) PR control loop to avoid
exceeding maximum current or power ratings for the
device.
If the IF pin is not driven, it must be resistively terminated
to SG. A 1k resistor to SG is recommended in this case.
Figure 25 – PRM48BF480T400A00 AC small signal model
V•I CHIP CORP. (A VICOR COMPANY) 25 FRONTAGE RD. ANDOVER, MA 01810 800-735-6200
Rev. 1.2
07 / 2012
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