FEATURES
n
LT3668
40V 400mA Step-Down
Switching Regulator with Dual
Fault Protected Tracking LDOs
DESCRIPTION
The
LT
®
3668
is a monolithic triple power supply composed
of a 400mA buck switching regulator and two 200mA low
dropout linear tracking regulators (LDOs).This provides a
complete and robust power solution for applications that
require the power supply of a sensor to tightly track the
power supply of a measurement ASIC.
Each tracking LDO supplies 200mA of output current with
a typical dropout voltage of 340mV, and each LDO has an
accurate resistor programmable current limit.
Internal protection circuitry includes reverse-battery
protection, current limiting, thermal limiting and reverse
current protection.
The buck regulator includes a high efficiency switch, a
boost diode, and the necessary oscillator, control and
logic circuitry. Current mode topology is used for fast
transient response and good loop stability. Low ripple
Burst Mode operation maintains high efficiency at low
output currents while keeping output ripple below 15mV
in a typical application.
The LT3668 is available in a thermally-enhanced 16-lead
MSOP package with exposed pad for low thermal resistance.
n
n
n
n
n
n
Dual Low Dropout Linear Tracking Regulators
n
200mA Outputs with Programmable Current Limits
n
1.6V to 45V Input Range
n
Fault Protected to ±45V
Triple Output Supply from a Single Input Requires
Only One Inductor
I
Q
= 50μA at 12V
IN
to 6V and 5V with No Load
Buck Regulator:
®
n
Low Ripple (<15mV
P-P
) Burst Mode Operation
n
400mA Output with Internal Power Switch
n
4.3V to 40V Input Operation Range (60V Max)
Adjustable 250kHz to 2.2MHz Switching Frequency
Power Good Indicator
Available in a Thermally-Enhanced 16-Lead
MSOP Package
APPLICATIONS
n
n
n
Fault-Protected Sensor Supply
Automotive and Industrial Supplies
Power for Portable Instrumentation
L,
LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
TYPICAL APPLICATION
V
IN
7V TO 40V
TRANSIENT
TO 60V
No-Load Supply Current
4.7µF
IN1
BOOST
SW
DA
IN3/BD
IN2
FB1
ADJ2
ADJ3
OUT3
10µF
3668 TA01a
0.22µF 27µH
232k
100
6V
100mA
22µF
90
80
SUPPLY CURRENT (µA)
70
60
50
40
30
20
10
0
5
10
25
20
15
INPUT VOLTAGE (V)
30
35
ON OFF
EN
PG
RT
LT3668
22pF
931k
294k
f = 600kHz
5V
150mA
(FOLLOWS
OUT3)
174k
OUT2
10µF
EN2/ILIM2 GND EN3/ILIM3
5V
150mA
3668 TA01b
3668fa
For more information
www.linear.com/LT3668
1
LT3668
ABSOLUTE MAXIMUM RATINGS
(Notes 1, 2)
PIN CONFIGURATION
TOP VIEW
SW
BOOST
EN
RT
IN3/BD
OUT3
ADJ3
FB1
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
DA
IN1
PG
EN3/ILIM3
EN2/ILIM2
IN2
OUT2
ADJ2
ADJ2, ADJ3 Voltage ................................................±45V
OUT2, OUT3 Voltage ...............................................±45V
IN2 Voltage .............................................................±45V
OUT2 – IN2 Differential Voltage ..............................±45V
OUT3 – IN3/BD Differential Voltage ........................±45V
OUT2 – ADJ2 Differential Voltage ...........................±45V
OUT3 – ADJ3 Differential Voltage ...........................±45V
IN1, EN Voltage (Note 3) ...........................................60V
IN1 Reverse Voltage...............................................–0.3V
EN Pin Current .......................................................–1mA
IN3/BD Voltage .........................................................30V
BOOST Pin Voltage ...................................................50V
BOOST Pin Above SW Pin.........................................30V
RT Voltage ..................................................................2V
FB1 Voltage .................................................................6V
EN2/ILIM2, EN3/ILIM3 Voltage ...................................4V
PG Voltage ................................................................30V
Operating Junction Temperature Range (Notes 4, 5)
E-, I-Grade ......................................... −40°C to 125°C
H-Grade ............................................. −40°C to 150°C
Storage Temperature Range .................. −65°C to 150°C
Lead Temperature (Soldering, 10 sec)................... 300°C
17
GND
MSE PACKAGE
16-LEAD PLASTIC MSOP
θ
JA
= 40°C/W
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LT3668EMSE#PBF
LT3668IMSE#PBF
LT3668HMSE#PBF
TAPE AND REEL
LT3668EMSE#TRPBF
LT3668IMSE#TRPBF
LT3668HMSE#TRPBF
PART MARKING
3668
3668
3668
PACKAGE DESCRIPTION
16-Lead Plastic MSOP
16-Lead Plastic MSOP
16-Lead Plastic MSOP
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 125°C
–40°C to 150°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping
container. Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to:
http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to:
http://www.linear.com/tapeandreel/
2
3668fa
For more information
www.linear.com/LT3668
LT3668
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN1
= 12V unless otherwise noted. (Note 4)
PARAMETER
V
IN1
Undervoltage Lockout (Note 6)
V
IN1
Overvoltage Lockout
V
IN2
Undervoltage Lockout (Note 6)
Quiescent Current from IN1
Quiescent Current from IN2
Quiescent Current from IN1 + IN2
Quiescent Current from IN3/BD
EN Pin Current
EN Input Threshold
Power Good Pin PG
Leakage Current
Output Voltage Low
Threshold as % of V
FB1
PG Threshold Hysteresis
Switching Regulator
Switching Frequency
R
T
= 37.4k
R
T
= 102k
R
T
= 487k
5% Duty Cycle, V
IN
= 5V, V
FB1
= 0V
90% Duty Cycle, V
IN
= 5V, V
FB1
= 0V
I
SW
= 200mA
l
l
l
l
l
l
l
ELECTRICAL CHARACTERISTICS
CONDITIONS
MIN
l
l
TYP
4
42
4
0.01
13
0.01
38
0.01
40
0.01
25
0.6
MAX
4.3
44
4.3
1
30
1
80
1
90
1
60
2
1.1
UNITS
V
V
V
µA
µA
µA
µA
µA
µA
µA
µA
µA
V
µA
V
%
%
mV
V
IN2
= 0V, V
IN3/BD
= 0V
V
IN1
= 3.5V, V
IN3/BD
= 0V
V
EN
= 0.3V
V
EN
= 12V, V
IN2
= 0V, Not Switching
V
EN
= 0.3V
V
EN
= 12V, V
IN1
= 0V, V
IN2
= 5V
V
EN
= 0.3V, V
IN2
= 5V
V
EN
= 12V, V
IN2
= 5V, Not Switching
V
EN
= 0.3V, V
IN3/BD
= 5V
V
EN
= 12V, V
IN3/BD
= 5V
V
EN
= 12V
40
l
l
l
l
l
0.3
V
PG
= 5V
I
PG
= 40µA
Pin Voltage Falling
Pin Voltage Rising
Measured at FB1 Pin
1.8
0.8
220
600
450
420
l
0.1
0.2
88
108
90
110
30
2.0
0.94
243
120
750
550
300
500
0.05
900
0.04
l
1
0.3
92
112
2.1
1.1
300
190
950
800
650
2
4
2.5
16
1.212
1.224
20
0.005
2.2
10
6
50
15
80
MHz
MHz
kHz
ns
mA
mA
mV
mA
µA
mV
µA
V
mA
V
mV
nA
%/V
V
V
mV
mV
mV
mV
3668fa
Minimum Switch Off-Time
Switch Current Limit (Note 7)
Switch V
CESAT
DA Pin Current to Stop Switching
Switch Leakage Current
Boost Schottky Diode Forward Voltage
Boost Schottky Diode Reverse Leakage
Minimum Boost Voltage (Note 8)
BOOST Pin Current
Feedback Voltage (FB1)
I
SW
= 200mA, V
BOOST
= 15V
V
SW
= 0V
I
BOOSTDIODE
= 50mA, V
IN
= NC, V
BOOST
= 0V
V
REVERSE
= 12V, V
IN
= NC
1.7
10
1.188
1.176
1.2
1.2
0.1
0.001
l
FB1 Pin Bias Current
Reference Voltage Line Regulation
Each LDO Regulator
Minimum Input Voltage
Output Voltage Range
Tracking Error V
OUT2/3
-V
ADJ2/3
Pin Voltage = 1.2V
4.2V < V
IN1
< 40V
I
LOAD
= 200mA
1.1V ≤ V
ADJ2/3
≤ 5V, I
LOAD
= 1mA
5V < V
ADJ2/3
≤ 10V, I
LOAD
= 1mA
1.1V ≤ V
ADJ2/3
≤ 5V, I
LOAD
= 1mA
5V < V
ADJ2/3
≤ 10V, I
LOAD
= 1mA
–40°C
to 125°C
–40°C
to 125°C
–40°C
to 150°C
–40°C
to 150°C
l
l
l
l
l
l
l
1.6
1.1
–6
–20
–6
–20
For more information
www.linear.com/LT3668
3
LT3668
ELECTRICAL CHARACTERISTICS
PARAMETER
Dropout Voltage (Notes 9, 10),
V
IN
= V
OUT(NOMINAL)
CONDITIONS
I
LOAD
= 1mA
I
LOAD
= 1mA
I
LOAD
= 50mA
I
LOAD
= 50mA
I
LOAD
= 100mA
I
LOAD
= 100mA
I
LOAD
= 200mA
I
LOAD
= 200mA
GND Pin Current, V
IN
= V
OUT(NOMINAL)
+ 0.6V
(Notes 10, 11)
Quiescent Current I
IN2
with LDO2 Disabled
Quiescent Current I
IN3/BD
with LDO3 Disabled
ADJ2 Pin Bias Current (Note 10)
ADJ3 Pin Bias Current (Note 10)
Ripple Rejection
Reverse Output Current (Note 12)
Input Reverse Leakage Current LDO2
Internal Current Limit
I
LOAD
= 0mA
I
LOAD
= 50mA
I
LOAD
= 200mA
V
IN1
= 0V, V
IN2
= 12V, V
EN2/ILIM2
= 2V
V
IN1
= 16V, V
IN3/BD
= 12V, V
EN3/ILIM3
= 2V
l
V
ADJ2
≤10V, V
ADJ2
≤ V
IN2
– 0.6V, V
OUT2
≤ V
IN2
– 0.6V
V
ADJ3
≤10V, V
ADJ3
≤ V
IN3/BD
– 0.6V, V
OUT3
≤ V
IN3/BD
– 0.6V
l
l
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN1
= 12V unless otherwise noted. (Note 4)
MIN
TYP
70
230
l
MAX
165
210
300
400
400
450
650
750
90
2
10
20
2
800
800
UNITS
mV
mV
mV
mV
mV
mV
mV
mV
µA
mA
mA
µA
µA
nA
nA
dB
280
l
340
l
l
l
l
40
1
5
13
1.2
V
IN
– V
OUT
= 2V (Avg), V
RIPPLE
= 0.5V
P-P
,
f
RIPPLE
= 120Hz, I
LOAD
=200mA
V
OUT2
= 1.2V, V
IN1
= V
IN2
= V
IN3/BD
= 0V
V
OUT3
= 1.2V, V
IN1
= V
IN2
= V
IN3/BD
= 0V
V
IN2
= –45V, V
IN1
= V
IN3/BD
= V
OUT2
= 0V
V
IN2
= 2.2V, V
OUT2
= 0V, EN2/ILIM2 Pin Grounded
∆V
OUT2
= –5%
V
IN3/BD
= 2.2V, V
OUT3
= 0V, EN3/ILIM3 Pin Grounded
∆V
OUT3
= –5%
l
l
l
l
l
l
l
60
85
5
5
300
40
40
300
µA
µA
µA
mA
mA
mA
mA
220
300
220
9.5
47
48.45
176
0.3
10
51
51
197
10.5
55
53.55
230
1.2
Externally Programmed Current Limit
R
EN/ILIM
= 31.6k, V
OUT2/3
= 5V, V
IN2/3
≥ 5.6V
R
EN/ILIM
= 6.19k, V
OUT2/3
= 5V, V
IN2/3
≥ 5.6V
R
EN/ILIM
= 6.19k, V
OUT2/3
= 5V, 5.6V ≤ V
IN2/3
≤ 15V
R
EN/ILIM
= 1.54k, V
OUT2/3
= 5V, 5.6V ≤ V
IN2/3
≤ 15V
mA
mA
mA
mA
V
LDO EN/ILIM Disable Threshold
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:
Positive currents flow into pins, negative currents flow out of pins.
Minimum and maximum values refer to absolute values.
Note 3:
Absolute maximum voltage at the IN1 and EN pins is 60V for
nonrepetitive 1 second transients, and 40V for continuous operation.
Note 4:
The LT3668E is guaranteed to meet performance specifications
from 0°C to 125°C junction temperature. Specifications over the −40°C
to 125°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls. The
LT3668I is guaranteed over the full −40°C to 125°C operating junction
temperature range. The LT3668H is guaranteed over the full −40°C to
150°C operating junction temperature range.
Note 5:
This IC includes overtemperature protection that is intended to protect
the device during momentary overload conditions. Junction temperature will
exceed the maximum operating junction temperature when overtemperature
protection is active. Continuous operation above the specified maximum
operating junction temperature may impair device reliability.
Note 6:
This is the voltage necessary to keep the internal bias circuitry in
regulation.
Note 7:
Current limit guaranteed by design and/or correlation to static test.
Slope compensation reduces current limit at higher duty cycles.
Note 8:
This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the switch.
Note 9:
Dropout voltage is the minimum input-to-output voltage
differential needed for an LDO to maintain regulation at a specified output
current. When an LDO is in dropout, its output voltage will be equal to
V
IN
– V
DROP
.
Note 10:
The LT3668 is tested and specified for these conditions with
V
ADJ2/3
= 5V.
Note 11:
GND pin current is tested with V
IN
= V
OUT(NOMINAL)
+ 0.6V and a
current source load. GND pin current increases in dropout.
Note 12:
Reverse output current is tested with the IN2 (IN3/BD) pin
grounded and the OUT2 (OUT3) pin forced to the rated output voltage.
This current flows into the OUT2 (OUT3) pin and out of the GND pin.
4
3668fa
For more information
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LT3668
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency, V
OUT1
= 3.3V
90
80
70
EFFICIENCY (%)
60
50
40
30
20
10
0
0.01
0.1
FRONT PAGE APPLICATION
BUCK REGULATOR ONLY
V
OUT1
= 3.3V
L: MSS7341-223MLB
1
10
LOAD CURRENT (mA)
100
3668 G01
T
A
= 25°C, unless otherwise noted.
V
FB1
vs Temperature
1.30
Efficiency, V
OUT1
= 6V
100
90
80
EFFICIENCY (%)
70
50
40
30
20
10
0
0.01
0.1
FRONT PAGE APPLICATION
BUCK REGULATOR ONLY
V
OUT1
= 6V
L: MSS7341-273MLB
1
10
LOAD CURRENT (mA)
100
3668 G02
V
IN1
= 12V
V
IN1
= 24V
V
IN1
= 36V
V
IN1
= 24V
V
IN1
= 12V
1.25
V
FB1
(V)
V
IN1
= 36V
60
1.20
1.15
1.10
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3668 G03
No-Load Supply Current
100
90
80
SUPPLY CURRENT (µA)
SUPPLY CURRENT (A)
70
60
50
40
30
20
10
0
5
10
25
20
15
30
INPUT VOLTAGE (V)
35
40
3668 G04
No-Load Supply Current
FRONT PAGE APPLICATION
CATCH DIODE: CMMSH1-60
1m
LOAD CURRENT (mA)
INCREASED SUPPLY CURRENT
DUE TO CATCH DIODE LEAKAGE
AT HIGH TEMPERATURE
700
650
600
550
500
450
10µ
–50 –25
400
Maximum Load Current
FRONT PAGE APPLICATION
V
OUT1
= 3.3V
FRONT PAGE APPLICATION
TYPICAL
100µ
MINIMUM
0
25 50 75 100 125 150
TEMPERATURE (°C)
3668 G05
5
10
25
20
30
15
INPUT VOLTAGE (V)
35
40
3668 G06
Maximum Load Current
700
650
LOAD REGULATION (%)
LOAD CURRENT (mA)
600
550
500
450
400
MINIMUM
TYPICAL
FRONT PAGE APPLICATION
0.10
Switching Regulator
Load Regulation
FRONT PAGE APPLICATION
0.08 REFERENCED FROM V
OUT1
AT 200mA LOAD
SWITCH CURRENT LIMIT (mA)
0.06
0.04
0.02
0
–0.02
–0.04
–0.06
–0.08
800
Switch Current Limit
SWITCH PEAK
CURRENT LIMIT
700
600
CATCH DIODE VALLEY CURRENT LIMIT
500
5
10
25
20
30
15
INPUT VOLTAGE (V)
35
40
3668 G07
–0.10
0
50
100 150 200 250 300 350 400
LOAD CURRENT (mA)
400
0
20
3668 G08
60
40
DUTY CYCLE (%)
80
100
3668 G09
3668fa
For more information
www.linear.com/LT3668
5
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