lx1676 Microsemi Corporation, lx1676 Datasheet - Page 10

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

Copyright © 2000
Revision: 1.0, 4/12/2005
O
because with it’s very fast response time with a 100 nH
inductor the upper MOSFET cannot be allowed to stay on
during an output short circuit condition. The phase 3 over-
current sensing uses the RDS(ON) of the upper MOSFET
with a resistor RSET to determine the over current limit
point. A current source draws 50uA through RSET which
determines the required drop across the MOSFET
RDS(ON) to initiate a current limit condition.
Phase 3 RSET is calculated by:
O
voltage and will latch all three phases off if an over voltage
condition (greater than 2.35 V) is detected.
MOSFETs for phase 3 will be held off and the lower
MOSFETs for phase 1 and 2 will be held on to discharge
the output capacitor till the output voltage drops below .85
volt, at .85 volts all MOSFETs will be turned off.
400nS Delay
VER
VER
The hysteretic phase has its own current limit protection
An over voltage protection circuit monitors the output
+ _
-C
V
TM
50 uA
OLTAGE
URRENT
ILIM3
Current Limit
Comparator
_
Figure 4 – Phase 3 Current Limit
R
P
P
SET
ROTECTION
ROTECTION
Rset
+
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
VS3
=
R
ILimit
DS(ON)
Q2
Q1
50
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 )
Vin
•
(P
uA
RDSon
_
HASE
+
3)
Vout
Integrated Products Division
®
Microsemi
Both
F
fault condition that will disable the output MOSFET
drivers. A fault condition will be caused by the following:
will be latched off. For an Over Voltage fault the lower
MOSFETs for phase 1 and 2 will be held on to discharge
the bulk capacitance on the output till a lower limit of .85
volts is reached then all MOSFETS will be turned off.
then back high or remove and reapply the input voltage
VIN.
output voltage has ramped up to the level commanded by
the DACOUT pin and the PWGD output in high.
PWM F
ROSC pin to ground.
The equation for ROSC is:
where ROSC is in KΩ, f is in Hz, K=105e-12
Mobile AMD Athlon™ VRM Controller
AULT
There are a number of possible states that will cause a
In all cases except Over Voltage all MOSFET drivers
To reset a fault it necessary to cycle the ENA pin low
The Under Voltage monitor is not enabled until the
An external resistor sets the PWM frequency from the
Enable (ENA) pin being pulled low
Over-current condition on either phase 1 or phase 3
Over Voltage output > 2.35V
Under Voltage output < 0.85V
L
REQUENCY
OGIC
P
RODUCTION
ROSC
=
(
K
D
•
ATA
f
)
1
+
S
100
HEET
e
−
9
LX1676
Page 10