Table of Contents
Advertisement
区 JALLINCKRODT
+3.5V Supply
The +3.5V supply is used to supply power to U17 when the +5V
supply is off. It also powers the LED located on the front of the
Power Pack.
The adjustable regulator VR3 is supplied +14V unregulated DC by
rectifying (D3 and 1/4 of BR1) and filtering (C9) the main trans-
former secondary center tap voltage. VR3's output voltage is set
as follows:
Vout = 1.25V (1+ R13/R14) = 1.25V(1 + 1500/680) = 4V
This +4V is dropped to approximately 3.5V by D13 before being
connected to U17 pin 1.
+5V Supply
The +5V supply is used to power the MOTHERBOARD logic cir-
cuitry and the CPU board. The +5V regulator is a step down DC to
DC converter capable of sourcing up to 5A of output current.
At intervals of approximately 10us (100kHz) VR2 connects VR2-5
to VR2-4 impressing a voltage across inductor L2. This causes
current to build up in the inductor while also supplying current to the
load and filter capacitor C4.
VR2 determines when to disconnect VR2-5 and VR2-4 during the
10us interval to keep the output voltage at +5V. When VR2-5 and
VR2-4 are disconnected, the magnetic field in the inductor col-
lapses and the polarity of the voltage across the inductor changes
to try and maintain the current in the inductor. This current in the
inductor is now directed (due to the change in voltage polarity
across the inductor) by the diode D2, to the load. The current will
flow from the inductor until the switch turns on again or until the
inductor runs out of energy.
The divider circuit of R6 and RS is used to set the output voltage of
the supply against an internal voltage reference of 2.21VDC.
The
output voltage is calculated as follows:
Vout
= 2.21V (1 + R6/R5)
= 2.21V(1 + 2870/2210)
= 5.08V
R4 and C5 make up the frequency compensation network used to
stabilize the feedback loop.
800961-C
Nov. 2000
3-7
Advertisement
Table of Contents
