HV100/HV101
Functional Block Diagram
VPP
Regulator
UVLO
Reference
Generator
UV
POR
Timer
Logic
Restart
Timer
GATE
VNN
Functional Description
Insertion into Hot Backplanes
Telecom, data network and some computer applications require
the ability to insert and remove circuit cards from systems
without powering down the entire system. Since all circuit cards
have some filter capacitance on the power rails, which is espe-
cially true in circuit cards or network terminal equipment utilizing
distributed power systems, the insertion can result in high inrush
currents that can cause damage to connector and circuit cards
and may result in unacceptable disturbances on the system
backplane power rails.
The HV100/HV101 are designed to facilitate the insertion and
removal of these circuit cards or connection of terminal equip-
ment by eliminating these inrush currents and powering up these
circuits in a controlled manner after full connector insertion has
been achieved. The HV100/HV101 are intended to provide this
control function on the negative supply rail.
After completion of a full POR period, the MOSFET gate Auto-
Adapt operation begins. A reference current source is turned on
which begins to charge an internal capacitor generating a ramp
voltage which rises at a slew rate of 2.5 V/ms. This reference
slew rate is used by a closed loop system to generate a GATE
output current to drive the gate of the external N-channel
MOSFET with a slew rate that matches the reference slew rate.
Before the gate crosses a reference voltage, which is well below
the V
TH
of industry standard MOSFETs, the pull-up current value
is stored and the Auto-Adapt loop is opened. This stored pull-up
current value is used to drive the gate during the remainder of the
hot swap period. The result is a normalization with C
ISS
, which
for most MOSFETs scales with C
RSS
.
The MOSFET gate is charged with a current source until it
reaches its turn on threshold and starts to charge the load
capacitor. At this point the onset of the Miller Effect causes the
effective capacitance looking into the gate to rise, and the current
source charging the gate will have little effect on the gate voltage.
The gate voltage remains essentially constant until the output
capacitor is fully charged. At this point the voltage on the gate of
the MOSFET continues to rise to a voltage level that guarantees
full turn on of the MOSFET. It will remain in the full on state until
an input under voltage condition is detected.
If the circuit attempts turn on into a shorted load, then the Miller
Effect will not occur. The gate voltage will continue to rise
essentially at the same rate as the reference ramp indicating that
a short circuit exists. This is detected by the control circuit and
results in turning off the MOSFET initiating a 2.5 second delay,
after which a normal restart is attempted.
If at any time during the start up cycle or thereafter, the input
voltage falls below the UV threshold the GATE output will be
pulled down to V
NN
, turning off the N-channel MOSFET and all
internal circuitry is reset. A normal restart sequence will be
initiated once the input voltage rises above the UVLO threshold
plus hysteresis.
Description of Operation
On initial power application the high input voltage internal regu-
lator seeks to provide a regulated supply for the internal circuitry.
Until the proper internal voltage is achieved all circuits are held
reset by the internal UVLO and the gate to source voltage of the
external N-channel MOSFET is held off. Once the internal
regulator voltage exceeds the UVLO threshold, the input
undervoltage detection circuit (UV) senses the input voltage to
confirm that it is above the internally programmed threshold. If
at any time the input voltage falls below the UV threshold, all
internal circuitry is reset and the GATE output is pulled down to
V
NN
. UVLO detection works in conjunction with a power on reset
(POR) timer of approximately 3.5ms to overcome contact bounce.
Once the UVLO is satisfied the gate is held to V
NN
until a POR
timer expires. Should the UV monitor toggle before the POR
timer expires, the POR timer will be reset. This process will be
repeated each time UVLO is satisfied until a full POR period has
been achieved.
3
SUPERTEX [ Supertex, Inc ]
