PRM48BH480T250A00 Datasheet, PDF (34/44 Pages) Vicor Corporation – PRM™ Regulator

PRM48BH480 X 250A00
DESIGN GUIDELINES (Remote Sense operation)
In Remote Sense operation, the PRM48BH480[x]250A00 is an
intelligent powertrain module designed to fully exploit external
output voltage feedback and current sensing sub-circuits.
These two external circuits are illustrated in Figure 36, which
shows an example of the PRM in a standalone application with
local voltage feedback and high side current sensing.
In general, these circuits include a precision voltage reference,
an operational amplifier which provides closed loop feedback
compensation, and a high side current sense circuit which
includes a shunt and current sense IC.
The following design procedures refer to the circuit shown in
Figure 36.
Setting the Output Voltage Level (Remote Sense
operation)
The output voltage setpoint is a function of the voltage
reference and the output voltage sense ratio. With reference to
Figure 36, R1 and R2 form the output voltage sensing divider
which provides the scaled output voltage to the negative input
of the error amplifier; a dedicated reference IC provides the
reference voltage to the positive input of the error amplifier.
Under normal operation, the error amplifier will keep the
voltages at the inverting and non-inverting inputs equal, and
therefore the output voltage is defined by:
V_OUT
 V_ref

R1  R2
R2
(5)
Note that the component R1 will also factor into the
compensation as described in a later section.
It is important to apply proper slew rate to the reference
voltage rise when the control loop is initially enabled. The
recommended range for reference rise time is 1 ms to 9 ms.
The lower rise time limit will ensure optimized modulator timing
performance during start up, and to allow the current limit
feature (through IFB pin) to fully protect the device during
power-up. The upper rise time limit is needed to guarantee a
sufficient factorized bus voltage is provided to any downstream
VTM input before the end of the VC pulse.
Setting the Output Current Limit and Overcurrent
Protection Level (Remote Sense operation)
In Remote Sense operation, the internal current sensing is
disabled, and an external current sense amplifier must be
implemented to provide feedback to the IFB pin.
The current limit and overcurrent protection set points are
linked, and scale together against the current sense shunt, and
the gain of the current sense amplifier. The output of the
current sense IC provides the IFB voltage which has V_{IFB_IL} and
V_{IFB_OC} thresholds for the two functions respectively. The set
points are therefore defined by:
I _IL

V_{IFB _ IL}
R_S  G_CS
(6)
and
I _OC

V_{IFB _ OC}
R_S  G_CS
(7)
where G_CS is the gain of the current sense amplifier.
- 34 -
Rev. 1.0
11/2012