lx1676 Microsemi Corporation, lx1676 Datasheet - Page 12

• Manufacturer Part Number: lx1676
• Description: Mobile Amd Athlontm Vrm Controller
• Manufacturer: Microsemi Corporation

Copyright © 2000
Revision: 1.0, 4/12/2005
L
adjusted to keep its amplitude fixed ratio to Vin over the
range of 6 to 24 V input.
feedback networks around the error amplifiers independent
of PWM input voltage.
T
large amount of current into the output node when required
by an out of range condition. The differential feedback
summing amplifier is connected directly to the output
terminals and has sufficient bandwidth to follow any fast
changes in output voltage. The feedback error voltage is
compared to the commanded reference voltage (DACOUT)
by two high speed comparators, I-Max and I-Min. The
other inputs of these
DACOUT as shown in Fig 6. If the error in output voltage
exceeds the offset in either direction the appropriate
MOSFET will be turned on to force current into or out of
the output node to correct the voltage error. The very low
value inductor (100nH) allows large amounts of current to
be forced into or out of the output node very quickly.
forces the appropriate upper or lower MOSFETs in phases
1 and 2 to stay on (100% or 0% duty cycle) until the error
is corrected.
OOP
RANSIENT
The PWM ramp shown in Figure 5 is automatically
Phase 3 is a Transient Correction Loop that can sum a
When the Transient Correction Loop is switching it
This maintains a constant loop gain that is set by the
G
TM
AIN
C
A
ORRECTION
UTOMATIC COMPENSATION
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comparators are offset from the
L
OOP
T H E O R Y O F O P E R A T I O N ( C O N T I N U E D )
Integrated Products Division
®
Microsemi
outputs (I-Max) and (I-Min) that may be used to drive a
half bridge to correct for both low and high output voltage
conditions. This permits pulling the output
overshoot occurs due to a rapid reduction in load current.
With a conventional Buck regulator rapid changes in the
negative direction are not possible due to the low voltage
available as a forcing function.
independent. A single MOSFET and diode can be used to
correct for voltage droop only or voltage overshoot only
when driven by the appropriate output. If the I-MAX driver
is not used the VC3 and VS3 pins must be connected to +5
volts.
is only active for a very brief time during high di/dt loads
on the output.
Mobile AMD Athlon™ VRM Controller
Differential Feedback
The two drivers for the Transient Correction Loop have
The two outputs (I-MAX and I-MIN) are completely
Under normal operation the Transient Correction phase
From Output
FB -
FB +
Figure 6 – Phase 3 Transient Correction Loop
DACOUT
-
P
RODUCTION
35mV
35mV
Comparator
Comparator
-
+
+
-
D
I-Max
I-Min
ATA
VCCL
S
HEET
VC3
I-Max
PGN3
VS3
I-Min
low if an
+5
LX1676
70nH
Vout
Page 12