lx1676 Microsemi Corporation, lx1676 Datasheet - Page 9

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

Copyright © 2000
Revision: 1.0, 4/12/2005
G
controller integrated circuit.
programmable from 200kHz to 1MHz. The device has
external compensation, for more flexibility of the loop
response. The LX1676 also makes use of a true differential
input amplifier for remote voltage sensing at the actual
processor core. This is a very important feature now that
the core voltages are in the 1 to 2 volt range. The reference
for the biphase PWM output is a 5 bit VID code DAC. The
VID code DAC can generate a reference voltage of 0.925 to
2.000 volts. The output of the DAC is a bi-directional
current source and is connected to the DACOUT pin.
Connecting a capacitor from this pin to ground will
generate a linear ramp, which will determine the rate of
change of the output voltage. The rate of change can be set
so that the current required to charge the total output
capacitance is below the maximum current limit trip point.
This will allow VID changes on the fly without tripping the
over current sensor.
P
VCC and Vin, Before both supplies reach their under-
voltage lock-out (UVLO) thresholds, a power on reset
condition will prevent soft-start from beginning, the
oscillator is disabled and all MOSFETs are kept off.
S
Enable pin is brought high, the soft-start capacitor begins to
be charged up by the reference DAC through the DACOUT
pin. The capacitor voltage at the DACOUT pin rises as a
linear ramp. The DACOUT pin is connected to the error
amplifier’s non-inverting input which controls the output
voltage. The output voltage will follow the DACOUT pin
voltage.
O
LX1676. One looks at the phase 1 lower MOSFET drain
current and the second looks at the phase 3 upper MOSFET
drain current. Both circuits have a 400 nS delay before a
current limit command is issued to the current limit latch,
once set the current limit latch will hold all three phases off
until it is reset.
VID changes.
OFT
OWER UP AND
ENERAL
VER
The LX1676 is a voltage-mode pulse-width modulation
At power up, the LX1676 monitors the supply voltage to
Once the supplies are above the UVLO threshold and the
Phase 3 (hysteretic phase) is disabled during soft-start.
There are two separate current limit circuits in the
The Over-Current Protection is disabled during positive
-S
-C
TM
TART
URRENT
D
ESCRIPTION
I
NITIALIZATION
P
ROTECTION
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
The PWM frequency is
T H E O R Y O F O P E R A T I O N
Integrated Products Division
®
Microsemi
(ENA) must be cycled low then back high or the input
power must cycle off and then back on.
O
lower MOSFET, together with a resistor (RSET) to set the
actual current limit point. The current limit comparator
senses the current 400 nS after the lower MOSFET is
switched on. A current source supplies a current (ISET),
of 50µA which flows into RSET and determines the
current limit trip point. The value of RSET is selected to
set the current limit for the application.
Phase 1 RSET is calculated by:
across RSET equals the drop across the lower MOSFET
RDS(ON)., at this time the comparator outputs a signal to
set the I limit latch and removes the enable command. The
Over-Current sensing is done on phase 1 only because
phase 2 current is always being forced to equal the phase
1 current, therefore the current trip point is set at half of the
desired current limit. For an output current limit setting of
30 amps, the current trip point for phase 1 is set at 15
amps.
phases are disabled, all MOSFETs are turned off.
current pulses exceeding the calculated values during the
delay period if a short circuit is applied during that time.
400nSec Delay
To reset the current limit latch either the enable command
Mobile AMD Athlon™ VRM Controller
50 uA
VER
The phase 1 current limit uses the RDS(ON) of the
The current limit comparator will trip when the drop
When the phase 1 over current latch is set all three
The delay before current limit is activated will result in
_
+
-C
URRENT
Current Limit
Comparator
P
Figure 3 – Phase 1 Current Limit
RODUCTION
+
R
P
SET
R
ROTECTION
SET
_
=
ILimit
R
D
DS(ON)
Q1
Q2
ATA
50
•
(P
Vin
R
µ
A
DS(ON)
S
HASE
_
+
HEET
Current
Flow
Q2
1)
Iout
Vout
LX1676
Page 9