NSC LM2688M-ADJ

LM2688
Non-synchronous Step-down Controller for Pentium
Microprocessors
General Description
Features
The LM2688 is a low-cost non-synchronous voltage-mode
PWM controller which maintains an accurate and adjustable
output voltage for high current, low voltage DC/DC applications such as the Pentium microprocessor family. By using
different combinations of feedback resistors, the regulator
output voltage can be precisely adjusted to match different
requirements. Coming in an 8-lead surface mount package,
the LM2688 can be used to control a buck regulator with an
load current up to 10A. An on-chip precision trimmed reference allows tight voltage regulation and an internal 150 kHz
fixed-frequency oscillator eliminates extra components to set
the switching frequency. The LM2688 also provides an ON/
OFF pin which enables the CPU to shutdown the regulator
when necessary. Self protection features include a two stage
current limit for the output switch and an over temperature
shutdown for complete protection under fault conditions. The
controller also provides internal loop gain compensation,
minimizing the number of external components.
n
n
n
n
n
n
n
n
Adjustable output voltage
Available in 8-pin surface mount
Guaranteed 0.5A gate drive current
Supply voltage up to 40V
150 kHz fixed frequency internal oscillator
TTL shutdown capability
Low power standby mode, IQ typically < 85 µA
Thermal shutdown and current limit protection
Applications
n Controller for voltage regulators for Pentium, Pentium
MMX, K5, K6 MMX, Alpha and PowerPC processors
n Efficient pre-regulator for linear regulators
Typical Application
Pentium Processor Power Supply
DS100050-1
For more information about the above application, please refer to the Application Hints section.
© 1997 National Semiconductor Corporation
DS100050
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LM2688 Non-synchronous Step-down Controller for Pentium Microprocessors
July 1997
Absolute Maximum Ratings
Lead Temperature
(Note 1)
M8 Package
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Maximum Supply Volatge
45V
ON/OFF Pin Input Voltage
−0.3V ≤ V ≤ +25V
Feedback Pin Voltage
−0.3V ≤ V ≤ +25V
Vapor Phase (60 seconds)
+215˚C
Infrared (15 seconds)
+220˚C
Maximum Junction Temperature
Operating Ratings
Output Voltage to Ground
−40˚C ≤ TJ ≤ +125˚C
Temperature Range
(Steady State)
−1V
Power Dissipation
Internally Limited
Storage Temperature Range
−65˚C to +150˚C
ESD Susceptibility
Human Body Model (Note 2)
+150˚C
Supply Voltage
4.5V to 40V
2 kV
LM2688-ADJ
Electrical Characteristics
Specifications with standard typeface are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature
Range. Unless otherwise specified, VIN = 12V, IOUT = 100 mA.
Symbol
Parameter
Conditions
LM2688-ADJ
Typ
(Note 3)
Limit
(Note 4)
Units
(Limits)
SYSTEM PARAMETERS (Note 5)
VFB
Feedback Voltage
Converter Input = 5V
VCORE Programmed for 3V
η
Efficiency
Converter Input = 5V, VCORE = 2.8V, ICORE
= 10A
1.230
V
86
%
DEVICE PARAMETERS
Ib
Feedback Bias Current
VFB = 1.3V
10
fO
Oscillator Frequency
(Note 6)
150
VSAT
DC
ICL
Saturation Voltage
IOUT = 0.5A (Notes 7, 8)
50/100
127/110
kHz(min)
173/173
kHz(max)
1.1/1.2
V(max)
0.9
V
Max Duty Cycle (ON)
(Note 8)
100
%
Min Duty Cycle (OFF)
(Note 9)
0
%
Current Limit
Peak Current, (Notes 7, 8)
0.8
A
0.65/0.58
IL
IQ
ISTBY
θJA
nA
kHz
Output Leakage Current
Quiescent Current
Standby Quiescent Current
Thermal Resistance
(Notes 7, 9, 10) Output = 0V
Output = −1V
(Note 9)
1.3/1.4
A(max)
50
µA(max)
15
mA(max)
10
mA(max)
200/250
µA(max)
2
mA
5
ON/OFF Pin = 5V (OFF) (Note 10)
Junction to Ambient (Note 11)
A(min)
mA
85
µA
150
˚C/W
ON/OFF CONTROL
ON/OFF Pin Logic Input
VIH
Threshold Voltage
VIL
IH
1.3
Low (Regulator ON)
High (Regulator OFF)
ON/OFF Pin
VLOGIC = 2.5V (Regulator OFF)
Input Current
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2
V
0.6
V(min)
2.0
V(max)
15
µA(max)
5
µA
LM2688-ADJ
Electrical Characteristics
(Continued)
Specifications with standard typeface are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature
Range. Unless otherwise specified, VIN = 12V, IOUT = 100 mA.
Symbol
Parameter
Conditions
LM2688-ADJ
Typ
(Note 3)
Limit
(Note 4)
Units
(Limits)
ON/OFF CONTROL
VLOGIC = 0.5V (Regulator ON)
IL
0.02
µA
5
µA(max)
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: The human body model is a 100 pF capacitor discharged through a 1.5k resistor into each pin.
Note 3: Typical numbers are at 25˚C and represent the most likely norm.
Note 4: All limits guaranteed at room temperature (standard typeface) and at temperature extremes (bold typeface). All room temperature limits are 100% production tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).
Note 5: External components such as the catch diode, inductor, input and output capacitors, and voltage programming resistors can affect switching regulator system performance. When the LM2688 is used as shown in the typical application, system performance will be shown in system parameters section Electrical Characteristics.
Note 6: The switching frequency is reduced when the second stage current limit is activated. The amount of reduction is determined by the severity of current overload.
Note 7: No diode, inductor or capacitor connected to output pin.
Note 8: Feedback pin removed from output and connected to 0V to force the output transistor switch ON.
Note 9: Feedback pin removed from output and connected to 12V to force the output transistor switch OFF.
Note 10: VIN = 40V.
Note 11: Junction to ambient thermal resistance with approximately 1 square inch of printed circuit board copper surrounding the leads. Additional copper area will
lower thermal resistance further.
Typical Performance Characteristics
Switch Current Limit
Minimum Operating
Supply Voltage
Switching Frequency
DS100050-4
DS100050-2
DS100050-3
Quiescent Current
Standby
Quiescent Current
Feedback Pin
Bias Current
DS100050-5
DS100050-6
3
DS100050-7
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Typical Performance Characteristics
(Continued)
ON/OFF Threshold
Voltage
ON/OFF Pin Current
(Sinking)
DS100050-8
DS100050-9
Connection Diagram and Ordering Information
8-Lead Surface Mount (M)
DS100050-10
*No internal connection, but should be soldered to PC board for best heat transfer.
Top View
Order Number LM2688M-ADJ
See NS Package Number M08A
LM2688 Block Diagram
DS100050-11
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4
Output Filter
Application Hints
The output filter plays an extremely important role in meeting
the load transient requirement. From the viewpoint of load
transient response, it is desirable to have a low inductance
and a high output capacitance. However, too low an inductance causes huge ripple current through the MOSFETs
which translates into lower efficiency. Also too big a bank of
output capacitors may not be economical in terms of both
cost of the regulator and motherboard real estate. It may
also require a larger input capacitor bank to stay within input
di/dt specification. The output voltage ripple is determined by
the amount of ripple current in the inductor and the ESR of
the output capacitor bank. For the typical situation where
Pentium processor is the load, a 2.5µH inductor and a bank
of three 1500µF electrolytic capacitors are recommended.
Sanyo’s ultra low-ESR electrolytic capacitor, the
6MV1500GX, is recommended.
GENERAL
This is a cost effective non-synchronous buck solution for
powering Pentium and similar microprocessors (AMD’s K5,
Motorola’s PowerPC, etc.) that draws large current at a low
voltage. Fairly high efficiency of 86% or better at 10A load
can be achieved by properly selecting the components.
PIN FUNCTIONS
+VIN—This is the positive input supply for the controller to
work. A suitable input bypass capacitor must be present at
this pin to minimize voltage transients and to supply the current needed to drive the external MOSFET.
Ground—Circuit ground.
Output—Sources pulsed current up to 1.4A maximum to
drive external MOSFET. To minimize turn-on delay of the external MOSFET, copper trace between this pin and the gate
of the MOSFET should be kept as short as possible.
Feedback—Senses the regulator output voltage so that a
feedback control loop is achieved. A resistor divider can be
used to choose the desired output voltage.
ON/OFF—Allows the controller to be shut down by a logic
level signal. If the shutdown feature is not needed, the pin
can be either grounded or left open, both of which enable the
controller.
MOSFET Selection
It is desirable to have the on-resistance of the MOSFET as
low as possible so that its conduction loss is minimized and
high efficiency can be maintained. A good idea is to use two
low-Rdson MOSFETs in parallel so that the total conduction
loss is halved compared with using one MOSFET. In the typical application circuit, two Fairchild low Rdson (10 mΩ) MOSFETs, the NDS8410A, are used in parallel.
Diode Selection
Since current flows through the freewheeling diode when the
MOSFETs are off, efficiency is affected greatly by the forward voltage drop of the diode. Generally a Schottky diode is
used here. Motorola MBR2515L Schottky diode has a forward voltage drop of less than 0.4V at 20A which fits very
well in this application.
COMPONENTS SELECTION
Input Filter
Depending on the system requirement, an inductor may or
may not be necessary. When there is no explicit input di/dt
limitation, the filter can be composed of capacitors only. The
inductor plays an important role in limiting the input di/dt so
that the input power rail appears a quiet supply for other
loads. It also limits the inrush current during power on. The
best inductance value is too complicated to calculate but can
be easily determined by experiment. For a typical Pentium
motherboard application, 2µH is recommended. The bulk capacitors in the input filter should be of low ESR type or otherwise the input di/dt requirement may not be met. A low cost
low ESR electrolytic capacitor manufactured by United
Chemicon, LXV16VB102M, is recommended here.
Output Voltage Adjustment
Different output voltages can be obtained by using different
combinations of feedback resistors. The formula to calculate
output voltage is VCORE=(R2/R3+1)X1.235V. In the “typical
application” circuit, two jumpers are used to adjust the value
of R2 so that four output voltages can be obtained. When
necessary, C9 and C10 can be used to further compensate
the converter to result in a more stable circuit, the penalty is
a slower transient recovery speed.
Bill of Material
Label
Value
Type
Part Number
Manufacturer
C1
1000µ
Capacitor
LXV16VB102M
United Chemi-Con
C2
1000µ
Capacitor
LXV16VB102M
United Chemi-Con
C3
1000µ
Capacitor
LXV16VB102M
United Chemi-Con
C4
10µ
Capacitor, Tantalum
C5
0.1µ
Capacitor, Ceramic
C6
1500µ
Capacitor
6MV1500GX
Sanyo
C7
1500µ
Capacitor
6MV1500GX
Sanyo
C8
1500µ
Capacitor
6MV1500GX
Sanyo
C9
1n
Capacitor, Ceramic
C10
15n
Capacitor, Ceramic
L1
2.5µ
Inductor
R1
300, 0.5W
Resistor
R2
3.65k 1%
Resistor
R3
2k 1%
Resistor
5
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Bill of Material
Label
R6
(Continued)
Value
10
Type
Part Number
Manufacturer
Resistor
Q1
BJT, PNP
3906
National
U1
IC, PWM
LM2688M-ADJ
National
D1
Diode, Schottky
MBR2515L
Motorola
D2
Diode
1N4148
National
M1
MOSFET, 10 mΩ Rdson
NDS8410A
Fairchild
M2
MOSFET, 10 mΩ Rdson
NDS8410A
Fairchild
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6
7
LM2688 Non-synchronous Step-down Controller for Pentium Microprocessors
Physical Dimensions
inches (millimeters) unless otherwise noted
Order Number LM2688M-ADJ
See NS Package Number M08A
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