LT1765/LT1765-1.8/LT1765-2.5/
LT1765-3.3/LT1765-5
Monolithic 3A, 1.25MHz
Step-Down Switching Regulator
FEATURES
s
s
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s
s
s
s
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DESCRIPTIO
3A Switch in a Thermally Enhanced 16-Lead
TSSOP or 8-Lead SO Package
Constant 1.25MHz Switching Frequency
Wide Operating Voltage Range: 3V to 25V
High Efficiency 0.09Ω Switch
1.2V Feedback Reference Voltage
Uses Low Profile Surface Mount Components
Low Shutdown Current: 15µA
Synchronizable to 2MHz
Current Mode Loop Control
Constant Maximum Switch Current Rating at
All Duty Cycles*
The LT
®
1765 is a 1.25MHz monolithic buck switching
regulator. A high efficiency 3A, 0.09Ω switch is included
on the die together with all the control circuitry required
to construct a high frequency, current mode switching
regulator. Current mode control provides fast transient
response and excellent loop stability.
New design techniques achieve high efficiency at high
switching frequencies over a wide operating voltage range.
A low dropout internal regulator maintains consistent
performance over a wide range of inputs from 24V
systems to Li-Ion batteries. An operating supply current
of 1mA improves efficiency, especially at lower output
currents. Shutdown reduces quiescent current to 15µA.
Maximum switch current remains constant at all duty
cycles. Synchronization allows an external logic level
signal to increase the internal oscillator into the range of
1.6MHz to 2MHz.
Full cycle-by-cycle current control and thermal shutdown
are provided. High frequency operation allows the reduc-
tion of input and output filtering components and permits
the use of chip inductors.
APPLICATIO S
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DSL Modems
Portable Computers
Regulated Wall Adapters
Battery-Powered Systems
Distributed Power
, LTC and LT are registered trademarks of Linear Technology Corporation.
* Patent Pending
TYPICAL APPLICATIO
5V to 3.3V Step-Down Converter
CMDSH-3
0.18µF
INPUT
5V
2.2µF
CERAMIC
EFFICIENCY (%)
90
BOOST
V
IN
LT1765-3.3
OFF ON
SHDN
SYNC
FB
GND
V
C
2.2nF
V
SW
1.5µH
OUTPUT
3.3V
2.5A
UPS120
4.7µF
CERAMIC
1765 TA01
U
Efficiency vs Load Current
V
IN
= 10V
V
OUT
= 5V
85
80
75
70
0
1.0
1.5
0.5
SWITCH CURRENT (A)
2.0
1765 • TAO1a
U
U
sn1765 1765fas
1
LT1765/LT1765-1.8/LT1765-2.5/
LT1765-3.3/LT1765-5
ABSOLUTE
AXI U RATI GS
Input Voltage .......................................................... 25V
BOOST Pin Above SW ............................................ 20V
Max BOOST Pin Voltage .......................................... 35V
SHDN Pin ............................................................... 25V
FB Pin Current ....................................................... 1mA
PACKAGE/ORDER I FOR ATIO
ORDER PART
NUMBER
TOP VIEW
BOOST 1
V
IN
2
SW 3
GND 4
8 SYNC
7 V
C
6 FB
5 SHDN
LT1765ES8
S8 PACKAGE
8-LEAD PLASTIC SO
T
JMAX
= 125°C,
θ
JA
= 90°C/W,
θ
JC(PIN 4)
= 30°C/W
GROUND PIN CONNECTED TO
LARGE COPPER AREA
S8 PART
MARKING
1765
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The
q
denotes the specifications which apply over the full operating temperature range, otherwise specifications are at T
A
= 25°C.
V
IN
= 15V, V
C
= 0.8V, Boost = V
IN
+ 5V, SHDN, SYNC and switch open unless otherwise noted.
PARAMETER
Maximum Switch Current Limit
Oscillator Frequency
Switch On Voltage Drop
V
IN
Undervoltage Lockout
V
IN
Supply Current
Shutdown Supply Current
Feedback Voltage
V
SHDN
= 0V, V
IN
= 25V, V
SW
= 0V
q
CONDITIONS
3.3V < V
IN
< 25V
I = 3A
(Note 3)
q
q
q
q
3V < V
IN
< 25V, 0.4V < V
C
< 0.9V
(Note 3)
2
U
U
W
W W
U
W
(Note 1)
SYNC Pin Current .................................................. 1mA
Operating Junction Temperature Range
(Note 2) ........................................... – 40°C to 125°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
TOP VIEW
GND
BOOST
V
IN
V
IN
SW
SW
NC
GND
1
2
3
4
5
6
7
8
16 GND
15 NC
14 SYNC
13 V
C
12 FB
11 SHDN
10 NC
9
GND
ORDER PART
NUMBER
LT1765EFE
LT1765EFE-1.8
LT1765EFE-2.5
LT1765EFE-3.3
LT1765EFE-5
FE PART MARKING
1765EFE
1765EFE18
1765EFE25
1765EFE33
1765EFE-5
FE PACKAGE
16-LEAD PLASTIC TSSOP
θ
JA
= 45°C/W,
θ
JC(PAD)
= 10°C/W
EXPOSED PAD SOLDERED TO LARGE
COPPER PLANE
MIN
3
1.1
2.47
TYP
4
1.25
270
2.6
1
15
MAX
6
1.6
430
2.73
1.3
35
55
1.218
1.224
1.836
2.55
3.366
5.1
UNITS
A
MHz
mV
V
mA
µA
µA
V
V
V
V
V
V
LT1765 (Adj)
q
1.182
1.176
1.764
2.45
3.234
4.9
1.2
1.8
2.5
3.3
5
LT1765-1.8
LT1765-2.5
LT1765-3.3
LT1765-5
q
q
q
q
sn1765 1765fas
LT1765/LT1765-1.8/LT1765-2.5/
LT1765-3.3/LT1765-5
ELECTRICAL CHARACTERISTICS
The
q
denotes the specifications which apply over the full operating temperature range, otherwise specifications are at T
A
= 25°C.
V
IN
= 15V, V
C
= 0.8V, Boost = V
IN
+ 5V, SHDN, SYNC and switch open unless otherwise noted.
PARAMETER
FB Input Current
FB Input Resistance
CONDITIONS
LT1765 (Adj)
LT1765
LT1765-1.8
LT1765-3.3
LT1765-5
0.4V < V
C
< 0.9V
∆I
VC
=
±10µA
V
FB
= V
NOM
– 17%
V
FB
= V
NOM
+ 17%
Duty Cycle = 0%
V
C
= 1.2V, I
SW
= 800mA, V
IN
= 6V
q
q
q
q
q
q
q
q
q
MIN
10.5
14.7
19
29
150
500
80
70
TYP
– 0.25
15
21
27.5
42
350
850
120
110
5
0.4
0.9
MAX
– 0.5
21
30
39
60
1300
160
180
UNITS
µA
kΩ
kΩ
kΩ
kΩ
µMho
µA
µA
A/V
V
V
%
%
FB Error Amp Voltage Gain
FB Error Amp Transconductance
V
C
Pin Source Current
V
C
Pin Sink Current
V
C
Pin to Switch Current Transconductance
V
C
Pin Minimum Switching Threshold
V
C
Pin 3A I
SW
Threshold
Maximum Switch Duty Cycle
Minimum Boost Voltage Above Switch
Boost Current
SHDN Threshold Voltage
SHDN Threshold Current Hysteresis
SHDN Input Current (Shutting Down)
SYNC Threshold Voltage
SYNC Input Frequency
SYNC Pin Resistance
85
80
90
1.8
20
70
2.7
30
140
1.40
10
– 13
2.2
2
20
I
SW
= 3A
I
SW
= 1A (Note 4)
I
SW
= 3A (Note 4)
q
q
q
q
q
V
mA
mA
V
µA
µA
V
MHz
kΩ
1.27
4
–7
1.6
1.33
7
– 10
1.5
SHDN = 60mV Above Threshold
q
I
SYNC
= 1mA
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LT1765E is guaranteed to meet performance specifications
from 0°C to 125°C. Specifications over the – 40°C to 125°C operating
junction temperature range are assured by design, characterization and
correlation with statistical process controls.
Note 3:
Minimum input voltage is defined as the voltage where the internal
regulator enters lockout. Actual minimum input voltage to maintain a
regulated output will depend on output voltage and load current. See
Applications Information.
Note 4:
Current flows into the BOOST pin only during the on period of the
switch cycle.
sn1765 1765fas
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LT1765/LT1765-1.8/LT1765-2.5/
LT1765-3.3/LT1765-5
TYPICAL PERFOR A CE CHARACTERISTICS
FB vs Temperature (Adj)
1.220
1.215
350
300
T
A
= 125°C
SWITCH VOLTAGE (mV)
1.210
1.205
1.200
1.195
1.190
1.185
1.180
–50 –25
0
25
50
75
100
125
FREQUENCY (MHz)
FB VOLTAGE (V)
TEMPERATURE (°C)
1765 G01
SHDN Threshold vs Temperature
1.40
7
6
1.38
SHDN THRESHOLD (V)
1.36
V
IN
CURRENT (µA)
1.34
1.32
1
1.30
–50
0
–25
0
25
50
75
TEMPERATURE (°C)
100
125
0
5
10
15
V
IN
(V)
20
25
30
1765 G05
SHDN INPUT (µA)
4
U W
Switch On Voltage Drop
1.50
1.45
1.40
1.35
1.30
1.25
1.20
1.15
0
2
1
SWITCH CURRENT (A)
3
1765 G02
Oscillator Frequency
250
200
150
100
50
0
T
A
= 25°C
T
A
= –40°C
1.10
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
125
1765 G03
SHDN Supply Current vs V
IN
SHDN = 0V
5
4
3
2
1765 G04
SHDN I
P
Current vs Temperature
–12
–10
SHUTTING DOWN
–8
–6
–4
–2
0
–50
STARTING UP
–25
0
25
50
75
TEMPERATURE (°C)
100
125
1765 G06
sn1765 1765fas
LT1765/LT1765-1.8/LT1765-2.5/
LT1765-3.3/LT1765-5
TYPICAL PERFOR A CE CHARACTERISTICS
Minimum Input Voltage for
2.5V Out
3.5
300
V
IN
= 15V
3.3
INPUT VOLTAGE (V)
V
IN
CURRENT (µA)
250
200
150
100
50
0
1000
V
IN
CURRENT (µA)
800
600
400
200
0
0
0.2
0.4 0.6 0.8
1
1.2
SHUTDOWN VOLTAGE (V)
1.4
0
5
10
15
20
INPUT VOLTAGE (V)
25
30
1765 G09
3.1
2.9
2.7
2.5
0.001
0.1
0.01
LOAD CURRENT (A)
Current Limit Foldback
4
40
3.0
SWITCH PEAK CURRENT (A)
SWITCH CURRENT
2
20
OUTPUT CURRENT (A)
3
1
FB CURRENT
0
0
0.2
0.4
0.6
0.8
FEEDBACK VOLTAGE (V)
OUTPUT CURRENT (A)
U W
1
1765 G07
SHDN Supply Current
1200
Input Supply Current
UNDERVOLTAGE
LOCKOUT
1765 G08
Maximum Load Current,
V
OUT
= 5V
2.8
30
L = 4.7µH
2.6
FB INPUT CURRENT (µA)
2.4
L = 2.2µH
10
2.2
L = 1.5µH
1
0
1.2
1765 G10
2.0
0
5
10
15
INPUT VOLTAGE (V)
20
25
1765 G11
Maximum Load Current,
V
OUT
= 2.5V
3.0
L = 4.7µH
2.8
L = 2.2µH
2.6
L = 1.5µH
2.4
2.2
0
5
10
15
INPUT VOLTAGE (V)
20
25
1765 G12
sn1765 1765fas
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