PRM48BF480T400B00 Datasheet, PDF (15/22 Pages) Vicor Corporation – High Efficiency Remote Sense PRM Converter

PRODUCT DETAILS AND DESIGN GUIDELINES
9.1 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 0.5 mA
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 0.79 V switching
threshold, and will drive the control node through the
buffer. An internal 7.4 V 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
PRM48BF480T400B00
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 1 kΩ resistor to SG is
recommended in this case.
+
V_IN
-
+
V_PR
-
r_{EQ_IN}
C_{IN_INT}
R_PR
I_{PR_Low}
V_PR · G_PR
r_{EQ_OUT}
+
C_{OUT_INT}
V_OUT
-
Figure 25 – PRM48BF480T400B00 AC small signal model
PRM® Regulator
Rev 1.1
800 927.9474