LM3488
High Efficiency Low-Side N-Channel Controller for
Switching Regulators
General Description
The LM3488 is a versatile Low-Side N-FET high perfor-
mance controller for switching regulators. It is suitable for
use in topologies requiring low side FET, such as boost,
flyback, SEPIC, etc. Moreover, the LM3488 can be operated
at extremely high switching frequency in order to reduce the
overall solution size. The switching frequency of LM3488 can
be adjusted to any value between 100kHz and 1MHz by
using a single external resistor or by synchronizing it to an
external clock. Current mode control provides superior band-
width and transient response, besides cycle-by-cycle current
limiting. Output current can be programmed with a single
external resistor.
The LM3488 has built in features such as thermal shutdown,
short-circuit protection and over voltage protection. Power
saving shutdown mode reduces the total supply current to
5A and allows power supply sequencing. Internal soft-start
limits the inrush current at start-up.
Key Specifications
n
Wide supply voltage range of 2.97V to 40V
n
100kHz to 1MHz Adjustable and Synchronizable clock
frequency
n
1.5% (over temperature) internal reference
n
5A shutdown current (over temperature)
Features
n
8-lead Mini-SO8 (MSOP-8) package
n
Internal push-pull driver with 1A peak current capability
n
Current limit and thermal shutdown
n
Frequency compensation optimized with a capacitor and
a resistor
n
Internal softstart
n
Current Mode Operation
n
Undervoltage Lockout with hysteresis
Applications
n
Distributed Power Systems
n
Notebook, PDA, Digital Camera, and other Portable
Applications
n
Offline Power Supplies
n
Set-Top Boxes
Typical Application Circuit
10138844
Typical SEPIC Converter
May 2003
LM3488
High
Efficiency
Low-Side
N-Channel
Controller
for
Switching
Regulators
2003 National Semiconductor Corporation
DS101388
www.national.com
Connection Diagram
10138802
8 Lead Mini SO8 Package (MSOP-8 Package)
Package Marking and Ordering Information
Order Number
Package Type
Package Marking
Supplied As:
LM3488MM
MSOP-8
S21B
1000 units on Tape and Reel
LM3488MMX
MSOP-8
S21B
3500 units on Tape and Reel
Pin Description
Pin Name
Pin Number
Description
I
SEN
1
Current sense input pin. Voltage generated across an external
sense resistor is fed into this pin.
COMP
2
Compensation pin. A resistor, capacitor combination connected to
this pin provides compensation for the control loop.
FB
3
Feedback pin. The output voltage should be adjusted using a
resistor divider to provide 1.26V at this pin.
AGND
4
Analog ground pin.
PGND
5
Power ground pin.
DR
6
Drive pin of the IC. The gate of the external MOSFET should be
connected to this pin.
FA/SYNC/SD
7
Frequency adjust, synchronization, and Shutdown pin. A resistor
connected to this pin sets the oscillator frequency. An external
clock signal at this pin will synchronize the controller to the
frequency of the clock. A high level on this pin for
30s will turn
the device off. The device will then draw less than 10A from the
supply.
V
IN
8
Power supply input pin.
LM3488
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2
Absolute Maximum Ratings
(Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Input Voltage
45V
FB Pin Voltage
-0.4V
<
V
FB
<
7V
FA/SYNC/SD Pin Voltage
-0.4V
<
V
FA/SYNC/SD
<
7V
Peak Driver Output Current (
<
10s)
1.0A
Power Dissipation
Internally Limited
Storage Temperature Range
-65C to +150C
Junction Temperature
+150C
ESD Susceptibilty
Human Body Model (Note 2)
2kV
Lead Temperature
MM Package
Vapor Phase (60 sec.)
Infared (15 sec.)
215C
220C
DR Pin Voltage
-0.4V
VDR 8V
I
LIM
Pin Voltage
600mV
Operating Ratings
(Note 1)
Supply Voltage
2.97V
V
IN
40V
Junction
Temperature Range
-40C
T
J
+125C
Switching Frequency
100kHz
F
SW
1MHz
Electrical Characteristics
Specifications in Standard type face are for T
J
= 25C, and in bold type face apply over the full Operating Temperature
Range. Unless otherwise specified, V
IN
= 12V, R
FA
= 40k
Symbol
Parameter
Conditions
Typical
Limit
Units
V
FB
Feedback Voltage
V
COMP
= 1.4V,
2.97
V
IN
40V
1.26
1.2507/1.24
1.2753/1.28
V
V(min)
V(max)
V
LINE
Feedback Voltage
Line Regulation
2.97
V
IN
40V
0.001
%/V
V
LOAD
Output Voltage Load
Regulation
I
EAO
Source/Sink
0.5
%/V (max)
V
UVLO
Input Undervoltage
Lock-out
2.85
2.97
V
V(max)
V
UV(HYS)
Input Undervoltage
Lock-out Hysteresis
170
130
210
mV
mV (min)
mV (max)
F
nom
Nominal Switching
Frequency
R
FA
= 40K
400
370
420
kHz
kHz(min)
kHz(max)
R
DS1 (ON)
Driver Switch On
Resistance (top)
I
DR
= 0.2A, V
IN
= 5V
16
R
DS2 (ON)
Driver Switch On
Resistance (bottom)
I
DR
= 0.2A
4.5
V
DR (max)
Maximum Drive
Voltage Swing(Note 6)
V
IN
<
7.2V
V
IN
V
V
IN
7.2V
7.2
D
max
Maximum Duty
Cycle(Note 7)
100
%
T
min
(on)
Minimum On Time
325
230
550
nsec
nsec(min)
nsec(max)
I
SUPPLY
Supply Current
(switching)
(Note 9)
2.0
2.6
mA
mA (max)
I
Q
Quiescent Current in
Shutdown Mode
V
FA/SYNC/SD
= 5V(Note
10), V
IN
= 5V
5
7
A
A (max)
V
SENSE
Current Sense
Threshold Voltage
V
IN
= 5V
165
140/ 135
195/ 200
mV
mV (min)
mV (max)
LM3488
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3
Electrical Characteristics
(Continued)
Specifications in Standard type face are for T
J
= 25C, and in bold type face apply over the full Operating Temperature
Range. Unless otherwise specified, V
IN
= 12V, R
FA
= 40k
Symbol
Parameter
Conditions
Typical
Limit
Units
V
SC
Short-Circuit Current
Limit Sense Voltage
V
IN
= 5V
325
235
395
mV
mV (min)
mV (max)
V
SL
Internal Compensation
Ramp Voltage
V
IN
= 5V
92
52
132
mV
mV(min)
mV(max)
V
OVP
Output Over-voltage
Protection (with
respect to feedback
voltage) (Note 8)
V
COMP
= 1.4V
50
32/ 25
78/ 85
mV
mV(min)
mV(max)
V
OVP(HYS)
Output Over-Voltage
Protection
Hysteresis(Note 8)
V
COMP
= 1.4V
60
20
110
mV
mV(min)
mV(max)
Gm
Error Ampifier
Transconductance
V
COMP
= 1.4V
I
EAO
= 100A
(Source/Sink)
800
600/ 365
1000/ 1265
mho
mho (min)
mho (max)
A
VOL
Error Amplifier Voltage
Gain
V
COMP
= 1.4V
I
EAO
= 100A
(Source/Sink)
38
26
44
V/V
V/V (min)
V/V (max)
I
EAO
Error Amplifier Output
Current (Source/ Sink)
Source, V
COMP
= 1.4V,
V
FB
= 0V
110
80/ 50
140/ 180
A
A (min)
A (max)
Sink, V
COMP
= 1.4V, V
FB
= 1.4V
-140
-100/ -85
-180/ -185
A
A (min)
A (max)
V
EAO
Error Amplifier Output
Voltage Swing
Upper Limit
V
FB
= 0V
COMP Pin = Floating
2.2
1.8
2.4
V
V(min)
V(max)
Lower Limit
V
FB
= 1.4V
0.56
0.2
1.0
V
V(min)
V(max)
T
SS
Internal Soft-Start
Delay
V
FB
= 1.2V, V
COMP
=
Floating
4
msec
T
r
Drive Pin Rise Time
Cgs = 3000pf, V
DR
= 0 to
3V
25
ns
T
f
Drive Pin Fall Time
Cgs = 3000pf, V
DR
= 0 to
3V
25
ns
VSD
Shutdown and
Synchronization signal
threshold (Note 5)
Output = High
1.27
1.35
V
V (max)
Output = Low
0.65
0.35
V
V (min)
I
SD
Shutdown Pin Current
V
SD
= 5V
-1
A
V
SD
= 0V
+1
TSD
Thermal Shutdown
165
C
T
sh
Thermal Shutdown
Hysteresis
10
C
JA
Thermal Resistance
MM Package
200
C/W
LM3488
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4
Electrical Characteristics
(Continued)
Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which operation of the device
is intended to be functional. 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: All limits are guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits are 100%
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 4: Typical numbers are at 25C and represent the most likely norm.
Note 5: The FA/SYNC/SD pin should be pulled to V
IN
through a resistor to turn the regulator off.
Note 6: The voltage on the drive pin, V
DR
is equal to the input voltage when input voltage is less than 7.2V. V
DR
is equal to 7.2V when the input voltage is greater
than or equal to 7.2V.
Note 7: The limits for the maximum duty cycle can not be specified since the part does not permit less than 100% maximum duty cycle operation.
Note 8: The over-voltage protection is specified with respect to the feedback voltage. This is because the over-voltage protection tracks the feedback voltage. The
over-voltage thresold can be calculated by adding the feedback voltage, V
FB
to the over-voltage protection specification.
Note 9: For this test, the FA/SYNC/SD Pin is pulled to ground using a 40K resistor .
Note 10: For this test, the FA/SYNC/SD Pin is pulled to 5V using a 40K resistor.
LM3488
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