28 Volt M-Grade Rev 1.8
Page 23 of 26 09/2018
PI31xx-00-HVMZ
Functional Description
Input Power Pins IN(+) and IN(–)
The input power pins on the PI31xx‑00‑HVMZ are connected to
the input power source which can range from 16 – 50VDC. Under
surge conditions, the PI31xx‑00‑HVMZ can withstand up to 55VDC
for 12.5ms without incurring damage. The user should take care to
avoid driving the input rails above the specified ratings. Since the
PI31xx‑00‑HVMZ is designed with high reliability in mind, the input
pins are continuously monitored. If the applied voltage exceeds
the input overvoltage trip point (typically 53.5V) the conversion
process shall be terminated immediately. The converter initiates
soft start automatically within 80ms after the input voltage is
reduced back to the appropriate value. The input pins do not have
reverse‑polarity protection. If the PI31xx‑00‑HVMZ is operated
in an environment where reverse polarity is a concern, the user
should consider using a polarity protection device such as a suitably
rated diode. To avoid the high losses of using a diode, the user
should consider the much higher efficiency family of intelligent
Cool‑ORing® solutions that can be used in reverse‑polarity
applications. Information is available at vicorpower.com.
The PI31xx‑00‑HVMZ will draw nearly zero current until the input
voltage reaches the internal start up threshold. If the ENABLE
pin is not pulled low by external circuitry, the output voltage will
begin rising to its final output value about 80ms after the input UV
lockout releases. This will occur automatically even if the ENABLE
pin is floating.
To help keep the source impedance low, the input to the
PI31xx‑00‑HVMZ should be bypassed with (2) 4.7µF 50V ceramic
capacitors of X7R dielectric in parallel with a low Q 100µF 63V
electrolytic capacitor. To reduce EMI and reflected ripple current, a
series inductor of 0.2 – 0.47µH can be added. The input traces to
the module should be low impedance configured in such a manner
as to keep stray inductance minimized.
ENABLE
The ENABLE pin serves as a multi‑function pin for the
PI31xx‑00‑HVMZ. During normal operation, it outputs the
on‑board 4.9V regulator which can be used for trimming the
module up. The ENABLE pin can also be used as a remote enable
pin either from the secondary via an optocoupler and an external
isolated bias supply or from the primary side through a small‑signal
transistor, FET, or any device that sinks 3.3mA, minimum.
If the ENABLE pin is lower than 2.35V typical, the converter will
be held off or shut down if already operating. A third feature is
offered in that during a fault condition, such as output OVP, input
UV or OV, or output current limit, the ENABLE pin is pulled low
internally. This can be used as a signal to the user that a fault has
occurred. Whenever the ENABLE pin is pulled low, the TRIM/SS pin
follows, resetting the internal and external soft‑start circuitry. All
faults will pull ENABLE low including overtemperature. If increased
turn‑on delay is desired, the ENABLE pin can be bypassed with a
small capacitor up to a maximum of 1500pF.
TRIM/SS Pin
The TRIM/SS pin serves as another multi‑purpose pin. First, it is
used as the reference for the internal error amplifier. Connecting a
resistor from TRIM/SS to SGND allows the reference to be margined
down by as much as –20%. Connecting a resistor from TRIM/
SS to ENABLE will allow the reference and output voltage to be
margined up by 10%. If the user wishes a longer start up time, a
small ceramic capacitor can be added to TRIM/SS to increase it. It
is critical to connect any device between TRIM/SS and SGND and
not –IN, otherwise high frequency noise will be introduced to the
reference and possibly cause erratic operation. Referring to the
figures below, the appropriate trim up or trim‑down resistor can be
calculated using the equivalent circuit diagram and the equations.
When trimming up, the trim‑down resistor is not populated. When
trimming down, the trim‑up resistor is not populated. The soft‑start
time is adjustable and has a default value of 500µs to reach steady
state. The internal soft‑start capacitor value is 10nF.
PI31xx
RREFI
RLOW
RHIGH
TRIM/SS
VREF
VERO
SGND
ENABLE
10kΩ
U.S. PATS. LISTED ON PACKING MATERIALS & DATASHEETS
MADE
IN
USA
DC-DC
Converter
Figure 41 — PI31xx‑00‑HVMZ shown with system fuse,
filter, decoupling and extended soft start
Figure 42 — Trim equations and equivalent circuit
RLOW = (–VDESIRED ) •
REFI
(–VOUT_NOM) + VDESIRED
RHIGH = (–RREFI ) • (–VOUT_NOM) • VERO + VDESIRED• VRE
VREF (–VOUT_NOM) + VDESIRED
CREF = TSS_DESIRED – 230 • 10-6
23000