FEATURES
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LT3489
2MHz Boost DC/DC
Converter with 2.5A Switch
and Soft-Start
DESCRIPTIO
The LT
®
3489 is a fixed frequency step-up DC/DC converter
containing an internal 2.5A, 40V switch. The LT3489 is
ideal for large TFT-LCD panel power supplies. The LT3489
switches at 2MHz, allowing the use of tiny, low profile
inductors and low value ceramic capacitors. Loop com-
pensation can be either internal or external, giving the
user flexibility in setting loop compensation and allow-
ing optimized transient response with low ESR ceramic
output capacitors. Soft-start is controlled with an external
capacitor, which determines the input current ramp rate
during start-up.
The 8-lead MS8E package and high switching frequency en-
sure a low profile overall solution less than 1.1mm high.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
2.5A, 0.12Ω, 40V Internal Switch
2MHz Switching Frequency
Integrated Soft-Start Function
V
IN
Range: 2.6V to 16V
Adjustable Output from V
IN
to 40V
Low V
CESAT
Switch: 110mV at 1A (Typical)
8V at 610mA from a 3.3V Input
Internal or External Loop Compensation
Small 8-Lead MS8E
APPLICATIO S
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TFT-LCD Bias Supplies
GPS Receivers
DSL Modems
Local Power Supply
TYPICAL APPLICATIO
Low Profile, Triple Output TFT Supply (8V, –8V, 23V)
V
ON
23V
10mA
0.1µF
0.1µF
0.1µF
90
85
80
EFFICIENCY (%)
75
70
65
60
V
IN
3.3V
V
IN
OFF ON
SHDN
SS
4.7µF
100nF
2.2µH
AV
DD
8V
610mA
SW
28.7k
+
LT3489
FB
GND
5.23k
37.4k
220pF
0.1µF
2µF
20µF
2µF
COMP
V
C
3489 TA01
V
OFF
–8V
20mA
3489f
U
U
U
Efficiency
55
V LOAD = 10mA
ON
V
OFF
LOAD = 20mA
50
100 200 300 400 500 600
0
A
VDD
LOAD CURRENT (mA)
700
3489 TA01b
1
LT3489
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
V
C
FB
SHDN
GND
1
2
3
4
8
7
6
5
SS
COMP
V
IN
SW
V
IN
Voltage ................................................................16V
SW Voltage ................................................ –0.4V to 40V
FB, V
C
, COMP, SS Voltages .........................................6V
SHDN Voltage ...........................................................16V
Current Into FB Pin ................................................±1mA
Maximum Junction Temperature .......................... 125°C
Operating Temperature Range (Note 2) ... –40°C to 85°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec) .................. 300°C
MS8E PACKAGE
8-LEAD PLASTIC MSOP
T
JMAX
= 125°C,
θ
JA
= 40°C/W,
θ
JC
= 10°C/W
EXPOSED PAD (PIN 9) IS GND (MUST BE SOLDERED TO PCB)
ORDER PART NUMBER
LT3489EMS8E
MS8E PART MARKING
LTBYF
Order Options
Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking:
http://www.linear.com/leadfree/
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
PARAMETER
Minimum Operating Voltage
Maximum Operating Voltage
Feedback Voltage
CONDITIONS
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 3V, V
SHDN
= V
IN
unless otherwise noted. (Note 2)
MIN
TYP
2.4
1.22
1.21
1.235
100
100
80
V
SHDN
= 2.5V, Not Switching
V
SHDN
= 0V, V
IN
= 3V
2.6V ≤ V
IN
≤ 16V
●
●
MAX
2.6
16
1.26
1.26
250
UNITS
V
V
V
V
nA
μmhos
V/V
●
FB Pin Bias Current
Error Amp Transconductance
Error Amp Voltage Gain
Quiescent Current
Quiescent Current in Shutdown
Reference Line Regulation
Switching Frequency
Maximum Switch Duty Cycle
Switch Current Limit
Switch V
CESAT
Switch Leakage Current
SHDN Pin Current
V
FB
= 1.25V (Note 3)
ΔI
= 10μA
●
2
0
0.01
1.8
85
2.5
2
90
3.5
0.23
0.01
100
20
2
0.3
1.5
10
4
1
0.05
2.2
5
1
(Note 4)
I
SW
= 2A
V
SW
= 5V
V
SHDN
= 5V
V
SHDN
= 1.4V
V
SHDN
= 0V
V
SS
= 0.5V
●
SHDN Pin Threshold
Soft-Start Charging Current
2
20
5
Note 1:
Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2:
The LT3489E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the 40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
Note 3:
Current flows out of the FB pin.
Note 4:
Current limit guaranteed by design and/or correlation to static test.
Current limit is independent of duty cycle and is guaranteed by design.
3489f
2
U
mA
μA
%
MHz
%
A
V
μA
μA
μA
μA
V
μA
W
U
U
W W
W
LT3489
TYPICAL PERFOR A CE CHARACTERISTICS
Feedback Pin Voltage
1.27
1.26
FEEDBACK VOLTAGE (V)
1.25
1.24
1.23
1.22
1.21
–50 –25
OSCILLATOR FREQUENCY (MHz)
2.4
CURRENT LIMIT (A)
50
25
75
0
TEMPERATURE (˚C)
Switch V
CE(SAT)
Voltage
600
500
QUIESCENT CURRENT (mA)
400
300
200
100
0
1
0.5
1.0
1.5
2.0
SWITCH CURRENT (A)
–50°C
25°C
125°C
2.5
3489 G04
V
CE(SAT)
(mV)
U W
100
3489 G01
T
A
= 25°C unless otherwise noted.
Oscillator Frequency
4.0
3.5
2.2
3.0
2.5
2.0
1.5
1.0
0.5
125
1.6
–50
–25
75
0
25
50
TEMPERATURE (°C)
100
125
Current Limit
2.0
1.8
0
–50
–25
0
50
25
TEMPERATURE (°C)
75
100
3489 03
3489 G02
Quiescent Current
4.0
3.5
3.0
2.5
2.0
1.5
1.0
–50 –25
I
L
1A/DIV
V
OUT
100mV/DIV
AC COUPLED
V
SW
10V/DIV
Switching Waveform for
the Cover Page Circuit
50
25
75
0
TEMPERATURE (˚C)
100
125
V
IN
= 3.3V
200ns/DIV
A
VDD
= 8V
I
LOAD
, A
VDD
= 400mA
V
ON
= 23V, 10mA
V
OFF
= –8V, 20mA
3489 G06
3489 G05
3489f
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LT3489
PI FU CTIO S
V
C
(Pin 1):
Error Amplifier Output Pin. Tie external compen-
sation network to this pin, or use the internal compensation
network by shorting the V
C
pin to the COMP pin.
FB (Pin 2):
Feedback Pin. Reference voltage is 1.235V.
Connect resistive divider tap here. Minimize the trace area at
FB. Set V
OUT
according to V
OUT
= 1.235 • (1 + R1/R2).
SHDN (Pin 3):
Shutdown Pin. Tie to 2V or more to enable
device. Ground to shut down. Do not float this pin.
GND (Pin 4):
Ground. Tie directly to local ground plane.
SW (Pin 5):
Switch Pin. This is the collector of the internal
NPN power switch. Minimize the metal trace area connected
to this pin to minimize EMI.
V
IN
(Pin 6):
Input Supply Pin. Must be locally bypassed.
COMP (Pin 7):
Internal Compensation Pin. Provides an
internal compensation network. Tie directly to the V
C
pin
for internal compensation. Tie to GND if not in use.
SS (Pin 8):
Soft-Start Pin. Place a soft-start capacitor here.
Upon start-up, 10μA of current charges the capacitor to
1.8V. Use a larger capacitor for slower start-up. Leave
floating if not in use.
Exposed Pad (Pin 9):
Ground. Must be soldered to
PCB.
BLOCK DIAGRA
V
IN
6
SS
8
V
OUT
R1 (EXTERNAL)
FB
R2 (EXTERNAL)
1.235V
REFERENCE
+
+
–
A1
Σ
–
RAMP
GENERATOR
4
2MHz
OSCILLATOR
SHUTDOWN
3
SHDN
2
FB
Figure 1. Block Diagram
4
+
–
W
U
U
U
V
C
1
COMP
7
100k
125pF
SW
5
COMPARATOR
DRIVER
A2
R
S
Q
Q1
+
0.01Ω
9
GND
3489 F01
GND
3489f
LT3489
OPERATIO
The LT3489 uses a constant frequency, current mode con-
trol scheme to provide excellent line and load regulation.
Please refer to Figure 1 for the following description of the
part’s operation. At the start of the oscillator cycle, the SR
latch is set, turning on the power switch, Q1. The switch
current flows through the internal current sense resistor
generating a voltage. This voltage is added to a stabilizing
ramp and the resulting sum is fed into the positive terminal
of the PWM comparator, A2. When this voltage exceeds
the level at the negative input of A2, the SR latch is reset,
turning off the power switch. The level at the negative
input of A2 (V
C
pin) is set by the error amplifier (g
m
) and
is simply an amplified version of the difference between
the feedback voltage and the reference voltage of 1.235V.
In this manner, the error amplifier sets the correct peak
current level to keep the output in regulation.
U
A soft-start function is provided to enable a clean start-up
for the LT3489. When the part is brought out of shut-
down, 10µA of current is sourced out of the SS pin. By
connecting an external capacitor to the SS pin, the rate
of voltage rise on the pin can be set. Typical values for
the soft-start capacitor range from 10nF to 200nF. The SS
pin indirectly limits the rate of rise on the V
C
pin, which
in turn limits the peak switch current. Current limit is
not shown in Figure 1. The switch current is constantly
monitored and not allowed to exceed the nominal value
of 2.5A. If the switch current reaches 2.5A, the SR latch
is reset regardless of the output of comparator A2. This
current limit helps protect the power switch as well as the
external components connected to the LT3489.
3489f
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