LX1676(2005) Просмотр технического описания (PDF) - Microsemi Corporation
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LX1676 Datasheet PDF : 16 Pages
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LX1676
TM
®
Mobile AMD Athlon™ VRM Controller
PRODUCTION DATA SHEET
THEORY OF OPERATION (CONTINUED)
LOOP GAIN AUTOMATIC COMPENSATION
The PWM ramp shown in Figure 5 is automatically
adjusted to keep its amplitude fixed ratio to Vin over the
range of 6 to 24 V input.
This maintains a constant loop gain that is set by the
feedback networks around the error amplifiers independent
of PWM input voltage.
TRANSIENT CORRECTION LOOP
Phase 3 is a Transient Correction Loop that can sum a
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 comparators are offset from the
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.
When the Transient Correction Loop is switching it
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.
The two drivers for the Transient Correction Loop have
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 low if an
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.
The two outputs (I-MAX and I-MIN) are completely
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.
Under normal operation the Transient Correction phase
is only active for a very brief time during high di/dt loads
on the output.
DACOUT
Differential Feedback
From Output
FB -
VC3
+
I-Max
35mV
I-Max
Comparator
VCCL
VS3
+5
Vout
70nH
FB +
+
I-Min
35mV
I-Min
Comparator
PGN3
Figure 6 – Phase 3 Transient Correction Loop
Copyright © 2000
Revision: 1.0, 4/12/2005
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 12
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