![](http://datasheet.mmic.net.cn/300000/S39421_datasheet_16208998/S39421_22.png)
22
S39421
2024 9.0 8/8/00
The board
’
s pins should now be mated with the backplane
connector which in turn will bring the host LI/I* and
RESET* signals to the S39421. These signals should be
tied to the device
’
s HST_PWR and HST_RST inputs
respectively. Whenever HST_PWR is low the outputs
controlling the backend power on sequencing will be
inhibited; it does not impact the reset outputs or reset
timer. When low, the HST_RST input will force the reset
outputs active; once it is released the reset timer will be
started and it will keep the reset outputs active for t
PURST
.
At the same time the signal pins are making contact, the
backend voltages are applied to the card (3.3V, 5V, +12V
and -12V on short pins), but, they are blocked by FETs
under the control of the S39421 (see figure 3 ). Depending
upon the state of the VSEL pin, the S39421 will monitor
either the bussed +5V only, the bussed +3.3V only or both
the bussed +5V and +3.3V. Once the S39421 has deter-
mined these supply voltages are at or above Vtrip, (and LI/
I* has released HST_PWR) it will release the VGATE
outputs and effectively turn them on at a rate equivalent
to 250V/second. At the same time it will force DRVREN
active thus providing power to the backend circuits.
FIGURE 25: GENERAL BLOCK DIAGRAM OF S39421 HOST BUS INTERFACE AND BACKEND SIGNAL INTERFACE
VGATE3
VGATE5
DRVREN
RESET
RESET
PND1
Gnd
Gnd
Vpc
Vpc
SGNL_VLD
HST_RST
HST_PWR
CARD3V
CARD5V
LI/O*
IL/I*
RESET*
Backend
Power Circuits
See Figure
Backend Voltage
to S39421
Monitor Circuits
Reset Control
of Backend
Circuits
Ejector Switch
Circuit
S39421
System
Vcc
VCC5
2024 ILL28.0