LT3995IMSEPBF [Linear]
60V, 3A, 2MHz Step-Down Switching Regulator with 2.7μA Quiescent Current;
| 型号： | LT3995IMSEPBF |
| 厂家： | 
Linear |
| 描述： | 60V, 3A, 2MHz Step-Down Switching Regulator with 2.7μA Quiescent Current |
| 文件： | 总24页 (文件大小：352K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT3995
60V, 3A, 2MHz Step-Down
Switching Regulator with
2.7µA Quiescent Current
FEATURES
DESCRIPTION
The LT®3995 is an adjustable frequency monolithic buck
switchingregulatorthatacceptsawideinputvoltagerange
up to 60V. Low quiescent current design consumes only
2.7µAofsupplycurrentwhileregulatingwithnoload. Low
ripple Burst Mode operation maintains high efficiency at
low output currents while keeping the output ripple below
15mV in a typical application. The LT3995 can supply up
to 3A of load current and has current limit foldback to
limit power dissipation during short circuit. A low dropout
voltage of 500mV is maintained when the input voltage
drops below the programmed output voltage, such as
during automotive cold crank.
n
Ultralow Quiescent Current:
2.7µA I at 12V to 3.3V
Q
IN
OUT
Low Ripple Burst Mode® Operation
n
Output Ripple < 15mV
P-P
n
n
n
n
n
n
n
n
n
n
n
n
n
Wide Input Range: Operation from 4.3V to 60V
3A Maximum Output Current
Excellent Start-Up and Dropout Performance
Adjustable Switching Frequency: 200kHz to 2MHz
Synchronizable Between 250kHz to 2MHz
Accurate Programmable Undervoltage Lockout
Low Shutdown Current: I = 700nA
Q
Power Good Flag
Soft-Start Capability
Thermal Shutdown Protection
An internally compensated current mode topology is used
for fast transient response and good loop stability. A high
efficiency 85mΩ switch is included on the die along with a
boostSchottkydiodeandthenecessaryoscillator,control,
andlogiccircuitry. Anaccurate1.02Vthresholdenablepin
can be driven directly from a microcontroller or used as a
programmable undervoltage lockout. A capacitor on the
SS pin provides a controlled inrush current (soft-start).
Current Limit Foldback with SS Override
Saturating Switch Design: 85mΩ On Resistance
Small, Thermally Enhanced 16-Lead MSOP Package
APPLICATIONS
n
Automotive Battery Regulation
n
Portable Products
A power good flag signals when V
reaches 91.6% of
OUT
n
Industrial Supplies
the programmed output voltage. The LT3995 is available
in a small 16-lead MSOP package with exposed pad for
low thermal resistance.
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
No-Load Supply Current
4.5
3.3V Step-Down Converter
IN REGULATION
V
4.0
3.5
3.0
2.5
2.0
1.5
1.0
IN
4.3V TO 60V
V
IN
OFF ON
EN
PG
BOOST
SW
8.2µH
0.47µF
10µF
PDS560
LT3995
SS
RT
OUT
FB
V
3.3V
3A
47µF
1210
×2
1M
OUT
10pF
SYNC
GND
10nF
0
5
10 15 20 25
60
30 35 40 45 50 55
182k
576k
INPUT VOLTAGE (V)
3995 TA01a
f = 300kHz
3995 TA01b
3995f
1
For more information www.linear.com/LT3995
LT3995
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
TOP VIEW
V , EN Voltage (Note 3) ...........................................60V
IN
1
2
3
4
5
6
7
8
SYNC
PG
FB
SS
16
15
14
13
12
11
10
9
BOOST Pin Voltage ...................................................75V
BOOST Pin Above SW Pin.........................................30V
FB, RT, SYNC, SS Voltage...........................................6V
PG Voltage................................................................30V
OUT Voltage..............................................................16V
Operating Junction Temperature Range (Note 2)
OUT
BOOST
SW
RT
17
GND
EN
V
IN
IN
IN
SW
V
V
SW
NC
NC
MSE PACKAGE
16-LEAD PLASTIC MSOP
θ
= 40°C/W
JA
LT3995E ............................................ –40°C to 125°C
LT3995I ............................................. –40°C to 125°C
LT3995H ............................................ –40°C to 150°C
Storage Temperature Range .................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec)...................300°C
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LT3995EMSE#PBF
LT3995IMSE#PBF
LT3995HMSE#PBF
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3995EMSE#TRPBF
LT3995IMSE#TRPBF
LT3995HMSE#TRPBF
3995
3995
3995
16-Lead Plastic MSOP
16-Lead Plastic MSOP
16-Lead Plastic MSOP
–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/
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Note 2)
PARAMETER
Minimum Input Voltage
CONDITIONS
(Note 3)
MIN
TYP
4
MAX
4.3
UNITS
V
l
Dropout Comparator Threshold
Dropout Comparator Threshold Hysteresis
(V – OUT) Falling
IN
430
500
25
570
mV
mV
Quiescent Current from V
V
V
V
Low
0.7
1.6
1.3
2.7
30
µA
µA
µA
IN
EN
EN
EN
High, V
High, V
Low
Low
SYNC
SYNC
l
l
FB Pin Current
V
= 1.5V
0.1
12
nA
FB
Feedback Voltage
1.183
1.173
1.197
1.197
1.212
1.222
V
V
l
FB Voltage Line Regulation
Switching Frequency
4.3V < V < 60V (Note 3)
0.0003
0.01
%/V
IN
R = 11.8k
1.8
0.8
160
2.25
1
200
2.7
1.2
240
MHz
MHz
kHz
T
R = 41.2k
T
R = 294k
T
Minimum Switch On-Time
Minimum Switch Off-Time (Note 4)
130
180
ns
ns
280
3995f
2
For more information www.linear.com/LT3995
LT3995
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Note 2)
Switch Current Limit
Foldback Switch Current Limit
V
V
= 1V
= 0V
= 1A
4.7
6.3
3.1
7.9
A
A
FB
FB
Switch V
I
100
0.02
800
0.02
1.3
22
1.02
60
mV
μA
mV
μA
V
mA
V
mV
nA
%
CESAT
SW
Switch Leakage Current
Boost Schottky Forward Voltage
Boost Schottky Reverse Leakage
Minimum Boost Voltage (Note 5)
BOOST Pin Current
EN Voltage Threshold
EN Voltage Hysteresis
EN Pin Current
PG Threshold Offset from V
1
I
= 100mA
SH
V
= 12V
2
1.8
35
REVERSE
l
l
I
= 1A, V
– V = 3V
SW
BOOST
SW
EN Falling, V ≥ 4.3V
0.92
5
1.12
IN
0.2
8.4
20
13
V
Falling
FB
FB
PG Hysteresis as % of Output Voltage
PG Leakage
PG Sink Current
SYNC Low Threshold
SYNC High Threshold
SYNC Pin Current
1.7
%
V
V
= 3V
= 0.4V
0.02
480
1.0
1.18
0.1
1
µA
μA
V
V
nA
μA
PG
PG
l
125
0.6
1.5
2.6
V
V
= 6V
SYNC
SS Source Current
= 0.5V
0.9
1.8
SS
Note 3: Minimum input voltage depends on application circuit.
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 4: The LT3995 contains circuitry that extends the maximum duty
cycle if there is sufficient voltage across the boost capacitor. See the
Application Information section for more details.
Note 2: The LT3995E 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
LT3995I is guaranteed over the full –40°C to 125°C operating junction
temperature range. The LT3995H is guaranteed over the full –40°C to
150°C operating junction temperature range. High junction temperatures
degrade operating lifetimes. Operating lifetime is derated at junction
Note 5: This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the switch.
Note 6: 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 or permanently damage the device.
temperatures greater than 125°C. The junction temperature (T , in °C) is
J
calculated from the ambient temperature (T , in °C) and power dissipation
A
(P , in Watts) according to the formula:
D
T = T + (P • θ )
JA
J
A
D
where θ (in °C/W) is the package thermal impedance.
JA
3995f
3
For more information www.linear.com/LT3995
LT3995
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
Efficiency at 5VOUT
Efficiency at 3.3VOUT
Efficiency at 5VOUT
100
95
90
85
80
75
70
65
60
55
50
90
85
80
75
70
65
60
55
50
45
40
100
90
80
70
60
50
40
30
20
10
0
L: MSS1260-682ML
f
= 500kHz
SW
12V
24V
36V
48V
12V
24V
36V
48V
12V
24V
36V
48V
L: MSS1260-682ML
L: MSS1260-822ML
f
= 500kHz
f
= 300kHz
SW
SW
0
0.5
1.5
2
2.5
3
1
0
0.5
1.5
2
2.5
3
0.01
0.1
10
100 1000 10000
1
1
LOAD CURRENT (A)
LOAD CURRENT (A)
LOAD CURRENT (mA)
3995 G01
3995 G02
3995 G03
Efficiency at 3.3VOUT
No-Load Supply Current
No-Load Supply Current
10000
1000
100
10
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
100
90
80
70
60
50
40
30
20
10
0
L: MSS1260-822ML
IN REGULATION
V = 3.3V
OUT
FRONT PAGE APPLICATION
f
= 300kHz
V
V
= 12V
SW
IN
OUT
= 3.3V
DUE TO CATCH
DIODE LEAKAGE
12V
24V
36V
48V
1
–55 –25
5
35
65
95 125 155
0.01
0.1
10
100 1000 10000
1
0
5
10 15 20 25
60
30 35 40 45 50 55
TEMPERATURE (°C)
LOAD CURRENT (mA)
INPUT VOLTAGE (V)
3975 G06
3995 G04
3995 G05
Reference Voltage
Load Regulation
Line Regulation
1.230
1.225
1.220
1.215
1.210
1.205
1.200
1.195
1.190
1.185
1.180
1.175
1.170
0.20
0.15
0.10
0.05
0
0.15
0.10
0.05
0
V
= 5V
V
V
= 12V
OUT
OUT
IN
LOAD = 1A
= 5V
–0.05
–0.10
–0.15
–0.20
–0.05
–0.10
–0.15
–55
5
35
65
95 125 155
1
1.5
2
2.5
5
10 15 20 25 30 35 40 45 50 55 60
INPUT VOLAGE (V)
–25
0
0.5
3
TEMPERATURE (°C)
LOAD CURRENT (A)
3995 G07
3995 G09
3995 G08
3995f
4
For more information www.linear.com/LT3995
LT3995
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
Thermal Derating
Thermal Derating
Switch Current Limit
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
7.0
6.5
LIMITED BY MAXIMUM JUNCTION
LIMITED BY MAXIMUM JUNCTION
TEMPERATURE θ = 40°C/W
TEMPERATURE θ = 40°C/W
JA
JA
6.0
5.5
H-GRADE
H-GRADE
12V
24V
36V
48V
60V
12V
24V
36V
48V
60V
IN
IN
IN
IN
IN
IN
IN
IN
IN
IN
5.0
4.5
4.0
V
SW
= 3.3V
V
SW
= 5V
OUT
OUT
f
= 300kHz
f
= 500kHz
2.5in × 2.5in 4-LAYER BOARD
100
TEMPERATURE (°C)
2.5in × 2.5in 4-LAYER BOARD
100
TEMPERATURE (°C)
150
150
0
0.2
0.4
0.6
0.8
1.0
0
25
50
75
125
0
25
50
75
125
DUTY CYCLE
3995 G11
3995 G10
3995 G38
Switch Current Limit
Current Limit Foldback
Soft-Start
7.0
6.5
6.0
5.5
7
6
5
4
3
2
1
0
7
6
30% DUTY CYCLE
30% DUTY CYCLE
30% DUTY CYCLE
V
= 1V
FB
V
= 3V
SS
5
4
3
2
1
0
V
= 0.2V
FB
5.0
4.5
4.0
65
TEMPERATURE (°C)
125 155
–55 –25
5
35
95
0.5
1
1.5
2.5
0
2
0.8
FB PIN VOLTAGE (V)
1.2
0
0.2
0.4
0.6
1.0
SS PIN VOLTAGE (V)
3995 G12
3995 G14
3995 G13
BOOST Pin Current
Minimum On-Time
Switch VCESAT
70
60
50
40
30
20
10
0
225
200
175
150
300
250
200
150
100
50
V
SW
= 0V
SYNC
f
= 2MHz
LOAD = 1A
LOAD = 2A
125
100
75
0
2
3
0
0.5
1
1.5
2.5
65
TEMPERATURE (°C)
125 155
–55 –25
5
35
95
0
1
1.5
2
2.5
3
0.5
SWITCH CURRENT (A)
SWITCH CURRENT (A)
3995 G16
3995 G17
3995 G15
3995f
5
For more information www.linear.com/LT3995
LT3995
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
RT Programmed Switching
Frequency
Switching Frequency
Minimum Off-Time
780
720
660
600
250
225
200
175
350
300
R
T
= 78.7k
V
SW
= 0V
SYNC
f
= 2MHz
LOAD = 2A
250
LOAD = 1A
200
150
100
50
540
480
420
150
125
100
0
65
TEMPERATURE (°C)
125 155
–55 –25
5
35
95
65
TEMPERATURE (°C)
125 155
1.0
1.2 1.4 1.6 1.8 2.0 2.2
–55 –25
5
35
95
0.2
0.8
0.4 0.6
SWITCHING FREQUENCY (MHz)
3995 G19
3995 G18
3995 G20
Internal Undervoltage Lockout
(UVLO)
Frequency Foldback
EN Threshold
700
600
500
400
300
200
100
0
6
5
4
3
1.09
R
T
= 78.7k
EN RISING
1.08
1.07
1.06
1.05
1.04
1.03
1.02
2
1
0
EN FALLING
1.01
0.8
FB PIN VOLTAGE (V)
1.2
65
TEMPERATURE (°C)
125 155
0
0.2
0.4
0.6
1
–55
35
95
–25
5
–25
5
65
95 125 155
–55
35
TEMPERATURE (°C)
3995 G21
3995 G22
3995 G23
Minimum Input Voltage,
VOUT = 3.3V
Minimum Input Voltage,
VOUT = 5V
PG Thresholds
5.0
4.5
4.0
3.5
1.12
6.5
6.0
5.5
5.0
V
= 3.3V
V
SW
= 5V
OUT
OUT
FRONT PAGE APPLICATION
f
= 500kHz
1.11
1.10
TO RUN/TO START
TO RUN/TO START
FB RISING
1.09
1.08
1.07
1.06
1.05
FB FALLING
3.0
2.5
4.5
4.0
1.04
0
1.0
1.5
2.0
2.5
3.0
–25
5
65
95 125 155
0.5
–55
35
0
1.0
1.5
2.0
2.5
3.0
0.5
LOAD CURRENT (A)
LOAD CURRENT (A)
TEMPERATURE (°C)
3995 G26
3995 G24
3995 G25
3995f
6
For more information www.linear.com/LT3995
LT3995
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
SS Pin Current
Boost Capacitor Charger
Burst Frequency
600
500
400
300
200
100
50
2.6
160
140
120
100
80
V
= 12V
V
= 0.5V
V
= V
BST IN
IN
SS
2.4
2.2
V
= 5V
OUT
f
= 500kHz
L = 10µH
SW
2.0
1.8
1.6
1.4
1.2
V
f
= 3.3V
OUT
SW
60
= 300kHz
L = 8.2µH
40
20
0
1.0
0
40
60
80
100
120
0
2
8
10
16
20
4
6
12 14
–25
5
65
95 125 155
–55
35
LOAD CURRENT (mA)
OUT PIN VOLTAGE (V)
TEMPERATURE (°C)
3995 G27
3995 G29
3995 G28
Boost Diode Forward Voltage
Dropout Comparator Thresholds
Dropout Performance
600
580
560
540
520
500
480
460
440
420
400
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
V
IN
V
IN
2V/DIV
V
OUT
V
OUT
2V/DIV
V
RISING
OUT
V
FALLING
3995 G32
OUT
1kΩ LOAD
100ms/DIV
(12mA IN REGULATION)
1
–55
5
35
65
95 125 155
0
0.5
1.5
2
–25
TEMPERATURE (°C)
BOOST DIODE CURRENT (A)
3995 G31
3995 G30
Full Frequency Switching
Waveforms
Dropout Switching
Waveforms
Burst Mode Switching Waveforms
V
V
SW
V
SW
SW
20V/DIV
20V/DIV
2V/DIV
I
L
I
L
1A/DIV
I
L
1A/DIV
1A/DIV
V
V
V
OUT
OUT
OUT
50mV/DIV
50mV/DIV
50mV/DIV
3995 G34
3995 G35
3995 G33
V
V
I
= 48V
2µs/DIV
V
V
I
= 5V
5µs/DIV
V
V
I
= 48V
5µs/DIV
IN
OUT
IN
OUT
IN
OUT
= 3.3V
= 1A
SET FOR 5V
= 0.3A
= 3.3V
= 70mA
= 47µF
LOAD
LOAD
LOAD
C
= 47µF
C
= 47µF
OUT
C
OUT
OUT
3995f
7
For more information www.linear.com/LT3995
LT3995
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
Load Transient: 0.5A to 2.5A
Load Transient: 10mA to 2A
I
I
L
1A/DIV
L
1A/DIV
V
V
OUT
200mV/DIV
OUT
200mV/DIV
3995 G36
3995 G37
V
V
C
= 48V
20µs/DIV
V
V
C
= 48V
20µs/DIV
IN
IN
= 3.3V
= 3.3V
OUT
OUT
OUT
OUT
= 47µF ×2
= 47µF ×2
PIN FUNCTIONS
FB (Pin 1): The LT3995 regulates the FB pin to 1.197V.
Connect the feedback resistor divider tap to this pin. Also,
connectaphaseleadcapacitorbetweenFBandtheoutput.
Typically, this capacitor is 10pF.
V (Pins 10, 11, 12): The V pin supplies current to the
IN IN
LT3995’sinternalcircuitryandtotheinternalpowerswitch.
These pins must be locally bypassed.
EN (Pin 13): The part is in shutdown when this pin is low
and active when this pin is high. The hysteretic threshold
voltage is 1.08V going up and 1.02V going down. The
SS (Pin 2): A capacitor is tied between SS and ground to
slowly ramp up the peak current limit of the LT3995 on
start-up. There is an internal 1.8μA pull-up on this pin.
The soft-start capacitor is actively discharged when the
EN pin goes low, during undervoltage lockout or thermal
shutdown. Float this pin to disable soft-start.
EN threshold is only accurate when V is above 4.3V. If
IN
V
is lower than 3.9V, internal UVLO will place the part
in shutdown. Tie to V if shutdown feature is not used.
IN
IN
RT (Pin 14): A resistor is tied between RT and ground to
set the switching frequency.
OUT(Pin3):Thispinisaninputtothedropoutcomparator
which maintains a minimum dropout of 500mV between
PG (Pin 15): The PG pin is the open-drain output of an
internal comparator. PGOOD remains low until the FB pin
is within 8.4% of the final regulation voltage. PGOOD is
V
and OUT. The OUT pin connects to the anode of the
IN
internal boost diode. This pin also supplies the current to
the LT3995’s internal regulator when OUT is above 3.2V.
Connect this pin to the output when the programmed
output voltage is less than 16V.
valid when V is above 2V.
IN
SYNC (Pin 16): This is the external clock synchronization
input. Ground this pin for low ripple Burst Mode operation
at low output loads. Tie to a clock source for synchroni-
zation, which will include pulse skipping at low output
loads. When in pulse-skipping mode, quiescent current
increases to 11µA in a typical application at no load. Do
not float this pin.
BOOST (Pin 4): This pin is used to provide a drive volt-
age, higher than the input voltage, to the internal bipolar
NPN power switch.
SW (Pins 5, 6, 7): The SW pin is the output of an internal
power switch. Connect these pins to the inductor, catch
diode, and boost capacitor.
GND (Exposed Pad Pin 17): Ground. The exposed pad
must be soldered to the PCB.
NC(Pins8,9):NoConnects.Thesepinsarenotconnected
to internal circuitry.
3995f
8
For more information www.linear.com/LT3995
LT3995
BLOCK DIAGRAM
OUT
V
IN
V
IN
+
0.5V
C1
–
–
+
–
INTERNAL 1.197V REF
SHDN
+
1.02V
+
–
SWITCH
LATCH
BOOST
SW
EN
RT
+
SLOPE COMP
R
C3
L1
OSCILLATOR
200kHz TO 2MHz
Q
S
R
T
V
OUT
SYNC
PG
Burst Mode
DETECT
D1 C2
ERROR AMP
V
CLAMP
C
V
+
–
+
1.097V
C
1.8µA
–
SS
C4
OPT
SHDN
GND
FB
R2
R1
3995 BD
C5
3995f
9
For more information www.linear.com/LT3995
LT3995
OPERATION
The LT3995 is a constant frequency, current mode step-
down regulator. An oscillator, with frequency set by RT,
sets an RS flip-flop, turning on the internal power switch.
An amplifier and comparator monitor the current flowing
Between bursts, all circuitry associated with controlling
the output switch is shut down reducing the input supply
current to 1.7μA. In a typical application, 2.7μA will be
consumed from the supply when regulating with no load.
between the V and SW pins, turning the switch off when
IN
The oscillator reduces the LT3995’s operating frequency
when the voltage at the FB pin is low. This frequency
foldback helps to control the output current during start-
up and overload.
this current reaches a level determined by the voltage at
V (see Block Diagram). An error amplifier measures the
C
output voltage through an external resistor divider tied
to the FB pin and servos the V node. If the error ampli-
C
The LT3995 can provide up to 3A of output current. A cur-
rent limit foldback feature throttles back the current limit
during overload conditions to limit the power dissipation.
WhenSSisbelow2V,theLT3995overridesthecurrentlimit
foldback circuit to avoid interfering with start-up. Thermal
shutdown further protects the part from excessive power
dissipation, especially in elevated ambient temperature
environments.
fier’s output increases, more current is delivered to the
output; if it decreases, less current is delivered. An active
clamp on the V pin provides current limit. The V pin is
C
C
also clamped by the voltage on the SS pin; soft-start is
implemented by generating a voltage ramp at the SS pin
using an external capacitor.
Aninternalregulatorprovidespowertothecontrolcircuitry.
The bias regulator normally draws power from the V
IN
If the input voltage decreases towards the programmed
output voltage, the LT3995 will start to skip switch-off
times and decrease the switching frequency to maintain
output regulation. As the input voltage decreases below
the programmed output voltage, the output voltage will be
regulated 500mV below the input voltage. This enforced
minimum dropout voltage limits the duty cycle and keeps
the boost capacitor charged during dropout conditions.
Since sufficient boost voltage is maintained, the internal
switchcanfullysaturateyieldinglowdropoutperformance.
pin, but if the OUT pin is connected to an external volt-
age higher than 3.2V, bias power will be drawn from the
external source (typically the regulated output voltage).
This improves efficiency.
If the EN pin is low, the LT3995 is shut down and draws
700nA from the input. When the EN pin falls below 1.02V,
the switching regulator will shut down, and when the EN
pin rises above 1.08V, the switching regulator will become
active. This accurate threshold allows programmable
undervoltage lockout.
The LT3995 contains a power good comparator which
trips when the FB pin is at 91.6% of its regulated value.
The PG output is an open-drain transistor that is off when
the output is in regulation, allowing an external resistor
The switch driver operates from either V or from the
IN
BOOST pin. An external capacitor is used to generate a
voltage at the BOOST pin that is higher than the input
supply. This allows the driver to fully saturate the internal
bipolar NPN power switch for efficient operation.
to pull the PG pin high. Power good is valid when V is
IN
above 2V. When the LT3995 is shut down the PG pin is
actively pulled low.
To further optimize efficiency, the LT3995 automatically
switches to Burst Mode operation in light load situations.
3995f
10
For more information www.linear.com/LT3995
LT3995
APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current
It is important to note that another way to decrease the
pulse frequency is to increase the magnitude of each
single current pulse. However, this increases the output
voltage ripple because each cycle delivers more power to
the output capacitor. The magnitude of the current pulses
was selected to ensure less than 30mV of output ripple
with one 47µF ceramic output capacitor in a typical ap-
plication. See Figure 2.
To enhance efficiency at light loads, the LT3995 operates
in low ripple Burst Mode operation, which keeps the out-
put capacitor charged to the desired output voltage while
minimizing the input quiescent current. In Burst Mode
operation the LT3995 delivers single pulses of current to
the output capacitor followed by sleep periods where the
output power is supplied by the output capacitor. When in
sleepmodetheLT3995consumes1.7μA,butwhenitturns
on all the circuitry to deliver a current pulse, the LT3995
consumes several mA of input current in addition to the
switch current. Therefore, the total quiescent current will
be greater than 1.7μA when regulating.
V
SW
20V/DIV
I
L
1A/DIV
V
OUT
50mV/DIV
As the output load decreases, the frequency of single cur-
rent pulses decreases (see Figure 1) and the percentage
of time the LT3995 is in sleep mode increases, resulting
in much higher light load efficiency. By maximizing the
time between pulses, the converter quiescent current
gets closer to the 1.7μA ideal. Therefore, to optimize the
quiescent current performance at light loads, the current
in the feedback resistor divider and the reverse current
in the catch diode must be minimized, as these appear
to the output as load currents. Use the largest possible
feedback resistors and a low leakage Schottky catch diode
in applications utilizing the ultralow quiescent current
performanceoftheLT3995. Thefeedbackresistorsshould
preferably be on the order of MΩ and the Schottky catch
diode should have less than a few µA of typical reverse
leakage at room temperature. These two considerations
are reiterated in the FB Resistor Network and Catch Diode
Selection sections.
3995 F02
V
V
I
= 48V
5µs/DIV
IN
OUT
= 3.3V
= 70mA
= 47µF
LOAD
C
OUT
Figure 2. Burst Mode Operation
While in Burst Mode operation, the burst frequency and
the charge delivered with each pulse will not change with
output capacitance. Therefore, the output voltage ripple
will be inversely proportional to the output capacitance.
In a typical application with two 47µF output capacitors,
the output ripple is about 15mV, and with four 47µF output
capacitors the output ripple is about 7.5mV. The output
voltage ripple can continue to be decreased by increas-
ing the output capacitance, though care must be taken
to minimize the effects of output capacitor ESR and ESL.
At higher output loads (above 90mA for the front page
application) the LT3995 will be running at the frequency
programmed by the R resistor, and will be operating in
600
T
V
= 12V
IN
standard PWM mode. The transition between PWM and
low ripple Burst Mode operation is seamless, and will not
disturb the output voltage.
500
400
300
200
100
50
V
= 5V
OUT
f
= 500kHz
L = 10µH
SW
ToensureproperBurstModeoperation,theSYNCpinmust
be grounded. When synchronized with an external clock,
theLT3995willpulseskipatlightloads. Atverylightloads,
the part will go to sleep between groups of pulses, so the
quiescent current of the part will still be low, but not as
low as in Burst Mode operation. The quiescent current in
a typical application when synchronized with an external
V
f
= 3.3V
OUT
SW
= 300kHz
L = 8.2µH
0
40
60
80
100
120
20
LOAD CURRENT (mA)
3995 F01
Figure 1. Switching Frequency in Burst Mode Operation
3995f
11
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LT3995
APPLICATIONS INFORMATION
clock is 11µA. Holding the SYNC pin DC high yields no
advantages in terms of output ripple or minimum load to
full frequency, so is not recommended.
To estimate the required R value, use the following
T
equation:
51.1
RT =
–9.27
1.09
f
FB Resistor Network
(
)
SW
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the resistor
values according to:
where f is the desired switching frequency in MHz and
SW
R is in kΩ.
T
Operating Frequency Trade-Offs
VOUT
1.197V
R1= R2
–1
Selectionoftheoperatingfrequencyisatrade-offbetween
efficiency,componentsize,minimumdropoutvoltage,and
maximum input voltage. The advantage of high frequency
operation is that smaller inductor and capacitor values
may be used. The disadvantages are lower efficiency, and
lower maximum input voltage. The highest acceptable
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
The total resistance of the FB resistor divider should be
selected to be as large as possible to enhance low current
performance. The resistor divider generates a small load
on the output, which should be minimized to optimize the
low supply current at light loads.
switching frequency (f
) for a given application
SW(MAX)
can be calculated as follows:
VOUT + VD
fSW(MAX)
=
tON(MIN) V – V + VD
(
)
OUT
IN
SW
WhenusinglargeFBresistors,a10pFphaseleadcapacitor
should be connected from V
where V is the typical input voltage, V
is the output
IN
to FB.
OUT
voltage, V is the catch diode drop (~0.5V), and V is
D
SW
the internal switch drop (~0.24V at max load). This equa-
Setting the Switching Frequency
tion shows that slower switching frequency is necessary
The LT3995 uses a constant frequency PWM architecture
that can be programmed to switch from 200kHz to 2MHz
by using a resistor tied from the RT pin to ground. A table
showing the necessary R value for a desired switching
frequency is in Table 1.
to safely accommodate high V /V
ratio. This is due
IN OUT
to the limitation on the LT3995’s minimum on-time. The
minimum on-time is a strong function of temperature.
Use the typical minimum on-time curve to design for an
application’s maximum temperature, while adding about
30%forpart-to-partvariation.Theminimumdutycyclethat
can be achieved taking minimum on time into account is:
T
Table 1. Switching Frequency vs RT Value
SWITCHING FREQUENCY (MHz)
R VALUE (kΩ)
T
0.2
0.3
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
294
182
DC
= f • t
SW ON(MIN)
MIN
where f is the switching frequency, the t
minimum switch on-time.
is the
ON(MIN)
130
SW
78.7
54.9
41.2
32.4
26.1
21.5
17.8
14.7
12.4
A good choice of switching frequency should allow ad-
equate input voltage range (see next two sections) and
keep the inductor and capacitor values small.
Maximum Input Voltage Range
The LT3995 can operate from input voltages of up to 60V.
Often the highest allowed V during normal operation
IN
(V
) is limited by the minimum duty cycle rather
IN(OP-MAX)
3995f
12
For more information www.linear.com/LT3995
LT3995
APPLICATIONS INFORMATION
than the absolute maximum ratings of the V pin. It can
whereβ isequaltothebetaoftheinternalpowerswitch.
IN
SW
be calculated using the following equation:
The beta of the power switch is typically about 50, which
leads to a DC
of about 98%. This leads to a minimum
MAX
VOUT + VD
fSW • tON(MIN)
input voltage of approximately:
V
=
– VD + VSW
IN(OP-MAX)
VOUT + VD
DCMAX
V
=
– VD + VSW
IN(MIN1)
where t
is the minimum switch on-time. A lower
ON(MIN)
switching frequency can be used to extend normal opera-
tion to higher input voltages.
where V
is the output voltage, V is the catch diode
OUT
D
drop, V is the internal switch drop and DC
is the
MAX
SW
The circuit will tolerate inputs above the maximum op-
erating input voltage and up to the absolute maximum
maximum duty cycle.
ratings of the V and BOOST pins, regardless of chosen
IN
The final factor affecting the minimum input voltage is
the minimum dropout voltage. When the OUT pin is tied
to the output, the LT3995 regulates the output such that
switching frequency. However, during such transients
where V is higher than V
, the LT3995 will enter
IN(OP-MAX)
IN
pulse-skippingoperationwheresomeswitchingpulsesare
skipped to maintain output regulation. The output voltage
ripple and inductor current ripple will be higher than in
it stays 500mV below V . This enforced minimum drop-
IN
out voltage is due to reasons that are covered in the next
section. This places a limitation on the minimum input
voltage as follows:
typical operation. Do not overload when V is greater
IN
than V
.
IN(OP-MAX)
V
= V
+ V
IN(MIN2)
OUT DROPOUT(MIN)
Minimum Input Voltage Range
where V
DROPOUT(MIN)
is the programmed output voltage and
OUT
V
is the minimum dropout voltage of 500mV.
The minimum input voltage is determined by either the
LT3995’sminimumoperatingvoltageof4.3V,itsmaximum
duty cycle, or the enforced minimum dropout voltage.
See the Typical Performance Characteristics section for
the minimum input voltage across load for outputs of
3.3V and 5V.
Combining these factors leads to the overall minimum
input voltage:
V
= Max (V
, V
, 4.3V)
IN(MIN)
IN(MIN1) IN(MIN2)
Minimum Dropout Voltage
The duty cycle is the fraction of time that the internal
switch is on during a clock cycle. Unlike many fixed fre-
quency regulators, the LT3995 can extend its duty cycle
by remaining on for multiple clock cycles. The LT3995
will not switch off at the end of each clock cycle if there
is sufficient voltage across the boost capacitor (C3 in
the Block Diagram). Eventually, the voltage on the boost
capacitor falls and requires refreshing. When this occurs,
the switch will turn off, allowing the inductor current to
recharge the boost capacitor. This places a limitation on
the maximum duty cycle as follows:
Toachievealowdropoutvoltage,theinternalpowerswitch
must always be able to fully saturate. This means that the
boost capacitor, which provides a base drive higher than
V , must always be able to charge up when the part starts
IN
up and then must also stay charged during all operating
conditions.
Duringstart-upifthereisinsufficientinductorcurrent,such
as during light load situations, the boost capacitor will be
unable to charge. When the LT3995 detects that the boost
capacitor is not charged, it activates a 100mA (typical)
pull-down on the OUT pin. If the OUT pin is connected to
theoutput, theextraloadwillincreasetheinductorcurrent
enough to sufficiently charge the boost capacitor. When
the boost capacitor is charged, the current source turns
off, and the part may re-enter Burst Mode operation.
βSW
DCMAX
=
βSW + 1
3995f
13
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LT3995
APPLICATIONS INFORMATION
To keep the boost capacitor charged regardless of load
during dropout conditions, a minimum dropout voltage
is enforced. When the OUT pin is tied to the output, the
LT3995 regulates the output such that:
Inductor Selection and Maximum Output Current
For a given input and output voltage, the inductor value
and switching frequency will determine the ripple current.
The ripple current increases with higher V or V
and
IN
OUT
V – V
> V
DROPOUT(MIN)
decreases with higher inductance and faster switching
IN
OUT
frequency. A good first choice for the inductor value is:
where V
is 500mV. The 500mV dropout volt-
DROPOUT(MIN)
VOUT + VD
L =
age limits the duty cycle and forces the switch to turn off
regularly to charge the boost capacitor. Since sufficient
voltageacrosstheboostcapacitorismaintained,theswitch
is allowed to fully saturate and the internal switch drop
stays low for good dropout performance. Figure 3 shows
1.5•fSW
where f is the switching frequency in MHz, V
is the
OUT
SW
output voltage, V is the catch diode drop (~0.5V) and L
D
the overall V to V
performances during start-up and
is the inductor value is μH.
IN
OUT
dropout conditions.
The inductor’s RMS current rating must be greater than
the maximum load current and its saturation current
should be about 30% higher. For robust operation in fault
conditions (start-up or short circuit) and high input volt-
age (>30V), the saturation current should be above 9A.
To keep the efficiency high, the series resistance (DCR)
should be less than 0.1Ω, and the core material should
be intended for high frequency applications. Table 2 lists
several inductor vendors.
V
IN
V
IN
2V/DIV
V
OUT
V
OUT
2V/DIV
3995 F03
100ms/DIV
Table 2. Inductor Vendors
Figure 3. VIN to VOUT Performance
VENDOR
Coilcraft
Sumida
URL
www.coilcraft.com
www.sumida.com
www.tokoam.com
www.we-online.com
www.cooperet.com
www.murata.com
It is important to note that the 500mV dropout voltage
specified is the minimum difference between V and
IN
Toko
V
. When measuring V to V
with a multimeter,
OUT
IN
OUT
Würth Elektronik
Coiltronics
Murata
the measured value will be higher than 500mV because
you have to add half the ripple voltage on the input and
half the ripple voltage on the output. With the normal
ceramic capacitors specified in the data sheet, this mea-
sured dropout voltage can be as high as 650mV at high
load. If some bulk electrolytic capacitance is added to the
input and output the voltage ripple, and subsequently the
measured dropout voltage, can be significantly reduced.
Additionally, when operating in dropout at high currents,
high ripple voltage on the input and output can generate
audible noise. This noise can also be significantly reduced
by adding bulk capacitance to the input and output to
reduce the voltage ripple.
Theinductorvaluemustbesufficienttosupplythedesired
maximum output current (I
), which is a function
OUT(MAX)
LIM
of the switch current limit (I ) and the ripple current.
∆IL
2
IOUT(MAX) = ILIM
–
TheLT3995limitsitspeakswitchcurrentinordertoprotect
itself and the system from overload faults. The LT3995’s
switch current limit (I ) is typically 6.3A at low duty
cycles and decreases linearly to 5.25A at DC = 0.8.
LIM
3995f
14
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LT3995
APPLICATIONS INFORMATION
When the switch is off, the potential across the inductor
is the output voltage plus the catch diode drop. This gives
the peak-to-peak ripple current in the inductor:
Current Limit Foldback and Thermal Protection
The LT3995 has a large peak current limit to ensure a 3A
max output current across duty cycle and current limit
distribution, as well as allowing a reasonable inductor
ripple current. During a short-circuit fault, having a large
current limit can lead to excessive power dissipation and
temperature rise in the LT3995, as well as the inductor and
catch diode. To limit this power dissipation, the LT3995
starts to fold back the current limit when the FB pin falls
below 0.8V. The LT3995 typically lowers the peak current
limit about 50% from 6.3A to 3.1A when FB goes to 0V.
1–DC • V + VD
(
)
(
)
OUT
∆IL =
L • fSW
where f is the switching frequency of the LT3995, DC is
SW
the duty cycle and L is the value of the inductor. Therefore,
the maximum output current that the LT3995 will deliver
depends on the switch current limit, the inductor value,
and the input and output voltages. The inductor value may
have to be increased if the inductor ripple current does
Duringstart-up,whentheoutputvoltageandFBpinarelow,
current limit foldback could hinder the LT3995’s ability to
start up into a large load. To avoid this potential problem,
the LT3995’s current limit foldback will be disabled until
the SS pin has charged above 2V. Therefore, the use of
a soft-start capacitor will keep the current limit foldback
feature out of the way while the LT3995 is starting up.
not allow sufficient maximum output current (I
)
OUT(MAX)
giventheswitchingfrequency,andmaximuminputvoltage
used in the desired application.
The optimum inductor for a given application may differ
fromtheoneindicatedbythissimpledesignguide.Alarger
valueinductorprovidesahighermaximumloadcurrentand
reducestheoutputvoltageripple.Ifyourloadislowerthan
the maximum load current, than you can relax the value of
the inductor and operate with higher ripple current. This
allowsyoutouseaphysicallysmallerinductor, oronewith
a lower DCR resulting in higher efficiency. Be aware that if
the inductance differs from the simple rule above, then the
maximum load current will depend on the input voltage. In
addition,lowinductancemayresultindiscontinuousmode
operation, which further reduces maximum load current.
For details of maximum output current and discontinuous
operation, see Linear Technology’s Application Note 44.
The LT3995 has thermal shutdown to further protect the
part during periods of high power dissipation, particularly
in high ambient temperature environments. The thermal
shutdown feature detects when the LT3995 is too hot
and shuts the part down, preventing switching. When the
thermal event passes and the LT3995 cools, the part will
restart and resume switching. A thermal shutdown event
actively discharges the soft-start capacitor.
Input Capacitor
BypasstheinputoftheLT3995circuitwithaceramiccapaci-
tor of X7R or X5R type. Y5V types have poor performance
over temperature and applied voltage, and should not be
used. A 4.7μF to 10μF ceramic capacitor is adequate to
bypass the LT3995 and will easily handle the ripple cur-
rent. Note that larger input capacitance is required when
a lower switching frequency is used (due to longer on
times). If the input power source has high impedance, or
there is significant inductance due to long wires or cables,
additional bulk capacitance may be necessary. This can
be provided with a low performance electrolytic capacitor.
Finally, for duty cycles greater than 50% (V /V > 0.5),
OUT IN
a minimum inductance is required to avoid sub-harmonic
oscillations, see Application Note 19.
One approach to choosing the inductor is to start with
the simple rule given above, look at the available induc-
tors, and choose one to meet cost or space goals. Then
use the equations above to check that the LT3995 will be
able to deliver the required output current. Note again
that these equations assume that the inductor current is
continuous. Discontinuous operation occurs when I
OUT
is less than ΔI /2.
L
3995f
15
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LT3995
APPLICATIONS INFORMATION
Step-down regulators draw current from the input sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT3995 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 4.7μF capacitor is capable of this task, but only if it is
placed close to the LT3995 (see the PCB Layout section).
Asecondprecautionregardingtheceramicinputcapacitor
concernsthemaximuminputvoltageratingoftheLT3995.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank
circuit. If the LT3995 circuit is plugged into a live supply,
the input voltage can ring to twice its nominal value, pos-
sibly exceeding the LT3995’s voltage rating. If the input
supply is poorly controlled or the user will be plugging
the LT3995 into an energized supply, the input network
should be designed to prevent this overshoot. See Linear
TechnologyApplicationNote88foracompletediscussion.
operating conditions (applied voltage and temperature).
A physically larger capacitor or one with a higher voltage
rating may be required. Table 3 lists several capacitor
vendors.
Table 3. Recommended Ceramic Capacitor Vendors
MANUFACTURER
AVX
URL
www.avxcorp.com
www.murata.com
www.t-yuden.com
www.vishay.com
www.tdk.com
Murata
Taiyo Yuden
Vishay Siliconix
TDK
Ceramic Capacitors
Whenindropout,theLT3995canexciteceramiccapacitors
at audio frequencies. At high load, this could be unaccept-
able.Simplyaddingbulkinputcapacitancetotheinputand
output will significantly reduce the voltage ripple and the
audiblenoisegeneratedatthesenodestoacceptablelevels.
Output Capacitor and Output Ripple
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT3995. As pre-
viously mentioned, a ceramic input capacitor combined
with trace or cable inductance forms a high quality (under
damped)tankcircuit. IftheLT3995circuitispluggedintoa
live supply, the input voltage can ring to twice its nominal
value, possibly exceeding the LT3995’s rating. If the input
supply is poorly controlled or the user will be plugging
the LT3995 into an energized supply, the input network
should be designed to prevent this overshoot. See Linear
TechnologyApplicationNote88foracompletediscussion.
The output capacitor has two essential functions. Along
withtheinductor,itfiltersthesquarewavegeneratedbythe
LT3995toproducetheDCoutput. Inthisroleitdetermines
the output ripple, so low impedance (at the switching
frequency) is important. The second function is to store
energy in order to satisfy transient loads and stabilize the
LT3995’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
200
VOUT •fSW
COUT
=
Catch Diode Selection
The catch diode (D1 from the Block Diagram) conducts
current only during the switch off time. Average forward
current in normal operation can be calculated from:
wheref isinMHz, andC
istherecommendedoutput
OUT
SW
capacitance in μF. Use X5R or X7R types. This choice will
provide low output ripple and good transient response.
Transientperformancecanbeimprovedwithahighervalue
capacitorifcombinedwithaphaseleadcapacitor(typically
10pF) between the output and the feedback pin. A lower
value of output capacitor can be used to save space and
cost but transient performance will suffer.
V – VOUT
IN
ID(AVG) = IOUT
V
IN
where I
is the output load current. The current rating of
OUT
the diode should be selected to be greater than or equal to
When choosing a capacitor, look carefully through the
data sheet to find out what the actual capacitance is under
the application’s output load current, so that the diode is
3995f
16
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LT3995
APPLICATIONS INFORMATION
robust for a wide input voltage range. A diode with even
higher current rating can be selected for the worst-case
scenarioofoverload,wherethemaxdiodecurrentcanthen
increase to the typical peak switch current. Short circuit is
not the worst-case condition due to current limit foldback.
Peakreversevoltageisequaltotheregulatorinputvoltage.
For inputs up to 60V, a 60V diode is adequate.
Table 4. Schottky Diodes. The Reverse Current Values Listed
Are Estimates Based Off of Typical Curves for Reverse Current
vs Reverse Voltage at 25°C
V at
I at
R
F
V at 3A 3A MAX V = 20V
F
R
TYP 25°C
25°C
(mV)
25°C
PART NUMBER V (V)
I
(A)
AVE
(mV)
570
540
600
580
(µA)
0.45
0.9
R
PDS360
PDS560
B360A
60
60
60
60
3
620
5
3
3
An additional consideration is reverse leakage current.
When the catch diode is reversed biased, any leakage
current will appear as load current. When operating under
light load conditions, the low supply current consumed
by the LT3995 will be optimized by using a catch diode
with minimum reverse leakage current. Low leakage
Schottky diodes often have larger forward voltage drops
at a given current, so a trade-off can exist between low
load and high load efficiency. Often Schottky diodes with
larger reverse bias ratings will have less leakage at a given
output voltage than a diode with a smaller reverse bias
rating. Therefore, superior leakage performance can be
achieved at the expense of diode size. Table 4 lists several
Schottky diodes and their manufacturers.
700
650
50
SBR3U60P1
1.7
For output voltages less than 2.5V, there are two options.
An external Schottky diode can charge the boost capaci-
tor from the input (Figure 4c) or from an external voltage
source (Figure 4d). Using an external voltage source is the
better option because it is more efficient than charging the
boost capacitor from the input. However, such a voltage
rail is not always available in all systems. For output volt-
ages greater than 16V, an external Schottky diode from
an external voltage source should be used to charge the
boost capacitor (Figure 4e). In applications using an ex-
ternal voltage source, the supply should be between 3.1V
and 16V. When using the input, the input voltage may not
exceed 30V. In all cases, the maximum voltage rating of
the BOOST pin must not be exceeded.
BOOST and OUT Pin Considerations
CapacitorC3andtheinternalboostSchottkydiode(seethe
Block Diagram) are used to generate a boost voltage that
is higher than the input voltage. In most cases a 0.47μF
capacitor will work well. The BOOST pin must be more
than 1.8V above the SW pin for best efficiency and more
than 2.6V above the SW pin to allow the LT3995 to skip
off times to achieve very high duty cycles. For outputs
between 3.2V and 16V, the standard circuit with the OUT
pinconnectedtotheoutput(Figure4a)isbest. Below3.2V
the internal Schottky diode may not be able to sufficiently
charge the boost capacitor. Above 16V, the OUT pin abs
max is violated. For outputs between 2.5V and 3.2V, an
external Schottky diode to the output is sufficient because
anexternalSchottkywillhavemuchlowerforwardvoltage
drop than the internal boost diode.
When the output is above 16V, the OUT pin can not be tied
to the output or the OUT pin abs max will be violated. It
should instead be tied to GND (Figure 4e). This is to pre-
vent the dropout circuitry from interfering with switching
behavior and to prevent the 100mA active pull-down from
drawingpower. Itisimportanttonotethatwhentheoutput
is above 16V and the OUT pin is grounded, the dropout
circuitry is not connected, so the minimum dropout will
be about 1.5V, rather than 500mV. If the output is less than
3.2V and an external Schottky is used to charge the boost
capacitor, the OUT pin should still be tied to the output
even though the minimum input voltage of the LT3995 will
be limited by the 4.3V minimum rather than the minimum
dropout voltage.
3995f
17
For more information www.linear.com/LT3995
LT3995
APPLICATIONS INFORMATION
BOOST
BOOST
LT3995
BOOST
LT3995
V
V
SW
V
V
SW
V
V
IN
SW
IN
IN
IN
IN
IN
LT3995
GND
OUT
OUT
OUT
V
V
V
OUT
OUT
OUT
GND
GND
(4a) For 3.2V ≤ V
≤ 16V
(4b) For 2.5V ≤ V
≤ 3.2V
(4c) For V < 2.5V, V < 30V
OUT IN
OUT
OUT
V
S
V
S
BOOST
BOOST
LT3995
V
V
SW
V
V
IN
SW
IN
IN
IN
LT3995
GND
OUT
OUT
V
V
OUT
OUT
GND
3995 F04
(4d) For V
< 2.5V, 3.1V ≤ V ≤ 16V
(4e) For V
> 16V, 3.1V ≤ V ≤ 16V
OUT S
OUT
S
Figure 4. Five Circuits for Generating the Boost Voltage
Minimum Input Voltage, VOUT = 3.3V
Minimum Input Voltage, VOUT = 5V
5.0
4.5
4.0
3.5
6.5
6.0
5.5
5.0
V
= 3.3V
V
SW
= 5V
OUT
OUT
FRONT PAGE APPLICATION
f
= 500kHz
TO RUN/TO START
TO RUN/TO START
3.0
2.5
4.5
4.0
0
1.0
1.5
2.0
2.5
3.0
0
1.0
1.5
2.0
2.5
3.0
0.5
0.5
LOAD CURRENT (A)
LOAD CURRENT (A)
3995 F05b
3995 F05a
Figure 5. The Minimum Input Voltage Depends on Output Voltage and Load Current
Enable and Undervoltage Lockout
With the OUT pin connected to the output, a 100mA ac-
tive load will charge the boost capacitor during light load
start-upandanenforced500mVminimumdropoutvoltage
will keep the boost capacitor charged across operating
conditions (see Minimum Dropout Voltage section). This
yieldsexcellentstart-upanddropoutperformance.Figure5
showstheminimuminputvoltagefor3.3Vand5Voutputs.
The LT3995 is in shutdown when the EN pin is low and
active when the pin is high. The falling threshold of the
EN comparator is 1.02V, with 60mV of hysteresis. The EN
pin can be tied to V if the shutdown feature is not used.
IN
Undervoltage lockout (UVLO) can be added to the LT3995
as shown in Figure 6. Typically, UVLO is used in situa-
3995f
18
For more information www.linear.com/LT3995
LT3995
APPLICATIONS INFORMATION
LT3995
I
V
IN
L
1A/DIV
R3
R4
1.02V
+
–
SHDN
EN
V
OUT
3.3V/DIV
V
SS
LT3995 F06
0.5V/DIV
3995 F07
1ms/DIV
Figure 6. Undervoltage Lockout
Figure 7. Soft-Start Waveforms for the Front-Page Application
with a 10nF Capaacitor on SS. EN Is Pulsed High for About
6ms with a 1.65Ω Load Resistor
tions where the input supply is current limited, or has a
relatively high source resistance. A switching regulator
draws constant power from the source, so source cur-
rent increases as source voltage drops. This looks like a
negative resistance load to the source and can cause the
sourcetocurrentlimitorlatchlowunderlowsourcevoltage
conditions. UVLO prevents the regulator from operating
at source voltages where the problems might occur. The
UVLO threshold can be adjusted by setting the values R3
and R4 such that they satisfy the following equation:
The external SS capacitor is actively discharged when the
EN pin is low, or during overvoltage lockout, or during
thermal shutdown. The active pull-down on the SS pin
has a resistance of about 150Ω.
Synchronization
To select low ripple Burst Mode operation, tie the SYNC
pin below 0.5V (this can be ground or a logic output).
R3+ R4
VUVLO = V
EN(THRESH)
R4
Synchronizing the LT3995 oscillator to an external fre-
quency can be done by connecting a square wave (with
20% to 80% duty cycle) to the SYNC pin. The square
wave amplitude should have valleys that are below 0.5V
and peaks above 1.5V (up to 6V).
where V
is the falling threshold of the EN pin,
EN(THRESH)
whichisapproximately1.02V,andwhereswitchingshould
stop when V falls below V . Note that due to the
IN
UVLO
comparator’s hysteresis, switching will not start until the
The LT3995 will pulse skip at low output loads while syn-
chronized to an external clock to maintain regulation. At
very light loads, the part will go to sleep between groups
of pulses, so the quiescent current of the part will still be
low, but not as low as in Burst Mode operation. The qui-
escent current in a typical application when synchronized
with an external clock is 11µA. Holding the SYNC pin DC
high yields no advantages in terms of output ripple or
minimum load to full frequency, so is not recommended.
Never float the SYNC pin.
input is about 6% above V
.
UVLO
When operating in Burst Mode operation for light load
currents, the current through the UVLO resistor network
can easily be greater than the supply current consumed
by the LT3995. Therefore, the UVLO resistors should be
large to minimize their effect on efficiency at low loads.
Soft-Start
The SS pin can be used to soft start the LT3995 by throt-
tlingthemaximuminputcurrentduringstart-upandreset.
An internal 1.8μA current source charges an external
capacitor generating a voltage ramp on the SS pin. The
The LT3995 may be synchronized over a 250kHz to 2MHz
range. The R resistor should be chosen to set the LT3995
T
switchingfrequency20%belowthelowestsynchronization
SS pin clamps the internal V node, which slowly ramps
C
input. For example, if the synchronization signal will be
up the current limit. Maximum current limit is reached
when the SS pin is about 1.5V or higher. By selecting a
large enough capacitor, the output can reach regulation
without overshoot. Figure 7 shows start-up waveforms
for a typical application with a 10nF capacitor on SS for
a 1.65Ω load when the EN pin is pulsed high for 6ms.
250kHz and higher, the R should be selected for 200kHz.
T
To assure reliable and safe operation the LT3995 will only
synchronize when the output voltage is near regulation
as indicated by the PG flag. It is therefore necessary to
choosealargeenoughinductorvaluetosupplytherequired
3995f
19
For more information www.linear.com/LT3995
LT3995
APPLICATIONS INFORMATION
output current at the frequency set by the R resistor (see
Shorted and Reversed Input Protection
T
InductorSelectionsection).Theslopecompensationisset
Iftheinductorischosensothatitwon’tsaturateexcessively,
a LT3995 buck regulator will tolerate a shorted output and
the power dissipation will be limited by current limit fold-
back (see Current Limit Foldback and Thermal Protection
section). There is another situation to consider in systems
where the output will be held high when the input to the
LT3995 is absent. This may occur in battery charging ap-
plications or in battery backup systems where a battery
or some other supply is diode ORed with the LT3995’s
by the R value, while the minimum slope compensation
T
required to avoid subharmonic oscillations is established
by the inductor size, input voltage and output voltage.
Since the synchronization frequency will not change the
slopes of the inductor current waveform, if the inductor
is large enough to avoid subharmonic oscillations at the
frequency set by R , than the slope compensation will be
T
sufficient for all synchronization frequencies.
output. If the V pin is allowed to float and the EN/UVLO
IN
Power Good Flag
pin is held high (either by a logic signal or because it is
ThePGpinisanopen-drainoutputwhichisusedtoindicate
to the user when the output voltage is within regulation.
When the output is lower than the regulation voltage by
more than 8.4%, as determined from the FB pin voltage,
the PG pin will pull low to indicate the power is not good.
Otherwise, the PG pin will go high impedance and can
be pulled logic high with a resistor pull-up. The PG pin is
only comparing the output voltage to an accurate refer-
tied to V ), then the LT3995’s internal circuitry will pull its
IN
quiescent current through its SW pin. This is fine if your
system can tolerate a few μA in this state. If you ground
the EN pin, the SW pin current will drop to essentially
zero. However, if the V pin is grounded while the output
IN
is held high, regardless of EN, parasitic diodes inside the
LT3995 can pull current from the output through the SW
pin and the V pin. Figure 9 shows a circuit that will run
IN
ence when the LT3995 is enabled and V is above 4.3V.
IN
only when the input voltage is present and that protects
When the part is shutdown, the PG is actively pulled low to
indicate that the LT3995 is not regulating the output. The
input voltage must be greater than 1.4V to fully turn-on
the active pull-down device. Figure 8 shows the status of
the PG pin as the input voltage is increased.
against a shorted or reversed input.
D4
PDS360
V
IN
V
BOOST
SW
IN
V
OUT
EN
4
3
2
1
0
LT3995
OUT
FB
+
GND
BACKUP
3995 F09
Figure 9. Diode D4 Prevents a Shorted Input from Discharging
a Backup Battery Tied to the Output. It Also Protects the Circuit
from a Reversed Input. The LT3995 Runs Only When the Input
Is Present
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
INPUT VOLTAGE (V)
3995 F08
Figure 8. PG Pin Voltage Versus Input Voltage when PG
Is Connected to 3V Through a 150k Resistor. The FB Pin
Voltage Is 1.15V
3995f
20
For more information www.linear.com/LT3995
LT3995
APPLICATIONS INFORMATION
PCB Layout
these layers will spread the heat dissipated by the LT3995.
Placing additional vias can reduce the thermal resistance
further. Whenoperatingathighambienttemperatures, the
maximum load current should be derated as the ambient
temperature approaches the maximum junction rating.
(SeetheThermalDeratingcurveintheTypicalPerformance
Characteristics section.)
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Figure 10 shows
a sample component placement with trace, ground plane
and via locations, which serves as a good PCB layout
example. Note that large, switched currents flow in the
LT3995’s V and SW pins, the catch diode (D1), and the
IN
input capacitor (C1). The loop formed by these compo-
nents should be as small as possible. These components,
along with the inductor and output capacitor, should be
placed on the same side of the circuit board, and their
connections should be made on that layer. Place a local,
unbrokengroundplanebelowthesecomponents. TheSW
and BOOST nodes should be as small as possible. Finally,
keep the FB and RT nodes small so that the ground traces
will shield it from the SW and BOOST nodes. The exposed
pad on the bottom of the package must be soldered to
ground so that the pad acts as a heat sink. To keep thermal
resistance low, extend the ground plane as much as pos-
sible, and add thermal vias under and near the LT3995 to
additional ground planes within the circuit board and on
the bottom side.
Power dissipation within the LT3995 can be estimated by
calculatingthetotalpowerlossfromanefficiencymeasure-
ment and subtracting the catch diode loss and inductor
loss. The die temperature is calculated by multiplying the
LT3995 power dissipation by the thermal resistance from
junction to ambient. The temperature rise of the LT3995
for a 3.3V and 5V application is measured using a thermal
camera and is shown in Figure 11.
70
V
SW
= 3.3V
OUT
f
= 300kHz
60
50
2.5in x 2.5in 4-LAYER BOARD
12V
24V
36V
48V
60V
40
30
20
10
0
SS
SYNC
1.5
2
3
1
2.5
OUTPUT CURRENT (A)
V
OUT
17
V
OUT
• • •
• • •
• • •
• • •
• • •
• • •
• • •
• • •
• • •
• • •
• • •
FB
BST
PG
3395 F11a
RT
Figure 11a. Temperature Rise of the LT3995
in the Front Page Application
OUT
EN
V
IN
90
V
f
= 5V
OUT
SW
SW
= 500kHz
2.5in x 2.5in 4-LAYER BOARD
80
70
60
50
40
30
20
10
0
12V
24V
36V
48V
60V
3995 F10
Figure 10. Layout Showing a Good PCB Design
High Temperature Considerations
For higher ambient temperatures, care should be taken in
the layout of the PCB to ensure good heat sinking of the
LT3995. The exposed pad on the bottom of the package
must be soldered to a ground plane. This ground should
be tied to large copper layers below with thermal vias;
2
1
1.5
2.5
3
OUTPUT CURRENT (A)
3995 F11b
Figure 11b. Temperature Rise of the LT3995
in a 5VOUT Application
3995f
21
For more information www.linear.com/LT3995
LT3995
APPLICATIONS INFORMATION
Also keep in mind that the leakage current of the power
Schottky diode goes up exponentially with junction tem-
perature.Whenthepowerswitchisoff,thepowerSchottky
diode is in parallel with the power converter’s output
filter stage. As a result, an increase in a diode’s leakage
current results in an effective increase in the load, and a
corresponding increase in the input quiescent current.
Therefore, the catch Schottky diode must be selected
with care to avoid excessive increase in light load supply
current at high temperatures.
Other Linear Technology Publications
Application Notes 19, 35 and 44 contain more detailed
descriptions and design information for buck regulators
and other switching regulators. The LT1376 data sheet
has a more extensive discussion of output ripple, loop
compensation and stability testing. Design Note 318
shows how to generate a bipolar output supply using a
buck regulator.
TYPICAL APPLICATIONS
5V Step-Down Converter
4V Step-Down Converter with a High Impedance Input Source
V
IN
5.7V TO 60V
+
24V
V
IN
V
V
IN
5.49M
499k
OFF ON
EN
PG
BOOST
SW
PG
EN
BOOST
SW
–
6.8µH
0.47µF
4.7µH
0.47µF
+
C
BULK
10µF
PDS360
PDS360
LT3995
LT3995
100µF
SS
RT
OUT
FB
SS
RT
OUT
FB
V
5V
3A
22µF
1210
×2
1M
V
4V
3A
47µF
1210
×2
OUT
1M
OUT
10pF
10pF
SYNC
GND
SYNC
GND
10nF
10µF
47nF
97.6k
316k
54.9k
432k
3975 TA02
3995 TA05
f = 500kHz
f = 800kHz
12V Step-Down Converter
2.5V Step-Down Converter
V
V
IN
IN
4.3V TO 60V
12.9V TO 60V
V
IN
V
BOOST
SW
IN
0.47µF
10µH
OFF ON
EN
PG
BOOST
SW
OFF ON
EN
10µH
0.47µF
PG
10µF
PDS360
10µF
PDS360
LT3995
LT3995
SS
RT
OUT
FB
SS
RT
OUT
FB
V
V
2.5V
3A
47µF
1210
×2
1M
1M
OUT
OUT
12V
10pF
10pF
2.5A (3A TRANSIENTS)
SYNC
GND
SYNC
GND
10nF
10nF
22µF
1210
×2
54.9k
110k
226k
909k
3995 TA03
3995 TA06
f = 800kHz
f = 250kHz
3995f
22
For more information www.linear.com/LT3995
LT3995
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
MSE Package
16-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1667 Rev E)
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.845 ±0.102
(.112 ±.004)
2.845 ±0.102
(.112 ±.004)
0.889 ±0.127
(.035 ±.005)
1
8
0.35
REF
5.23
(.206)
MIN
1.651 ±0.102
(.065 ±.004)
1.651 ±0.102
(.065 ±.004)
3.20 – 3.45
(.126 – .136)
0.12 REF
DETAIL “B”
CORNER TAIL IS PART OF
THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
DETAIL “B”
16
9
0.305 ±0.038
0.50
(.0197)
BSC
NO MEASUREMENT PURPOSE
4.039 ±0.102
(.159 ±.004)
(NOTE 3)
(.0120 ±.0015)
TYP
0.280 ±0.076
(.011 ±.003)
RECOMMENDED SOLDER PAD LAYOUT
16151413121110
9
REF
DETAIL “A”
0.254
(.010)
3.00 ±0.102
(.118 ±.004)
(NOTE 4)
0° – 6° TYP
4.90 ±0.152
(.193 ±.006)
GAUGE PLANE
0.53 ±0.152
(.021 ±.006)
1 2 3 4 5 6 7 8
DETAIL “A”
0.86
(.034)
REF
1.10
(.043)
MAX
0.18
(.007)
SEATING
PLANE
0.17 – 0.27
(.007 – .011)
TYP
0.1016 ±0.0508
(.004 ±.002)
MSOP (MSE16) 0911 REV E
0.50
(.0197)
BSC
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
6. EXPOSED PAD DIMENSION DOES INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD SHALL
NOT EXCEED 0.254mm (.010") PER SIDE.
3995f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
23
LT3995
TYPICAL APPLICATION
5V, 2MHz Step-Down Converter with Power Good
V
IN
5.9V TO 16V
TRANSIENT
TO 60V)
0.47µF
2.2µH
BOOST
SW
V
IN
OFF ON
EN
PDS360
150k
LT3995
4.7µF
PGOOD
PG
SS
RT
OUT
V
5V
3A
1M
OUT
FB
GND
10pF
SYNC
10nF
47µF
1210
14.7k
316k
3995 TA04
f = 2MHz
RELATED PARTS
PART NUMBER DESCRIPTION
COMMENTS
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OUT(MIN) Q SD
IN
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Q
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IN
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MSOP-16E Package
Q
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3mm × 2mm DFN-10, MSOP-10 Packages
Q
38V, 1.2A, 2.2MHz High Efficiency Micropower Step-Down
DC/DC Converter with I = 2.8µA
V : 4.3V to 38V, V = 1.2V, I = 2.8µA, I < 1µA,
IN
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3mm × 3mm DFN-10, MSOP-10E Packages
Q
55V, 1.2A, 2.2MHz High Efficiency Micropower Step-Down
DC/DC Converter with I = 2.8µA
V : 4.3V to 55V, V = 1.2V, I = 2.8µA, I < 1µA,
IN
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SD
3mm × 3mm DFN-10, MSOP-10E Packages
Q
42V, 2.5A, 96% Efficiency, 2.2MHz Synchronous Micropower
Step-Down DC/DC Converter with I = 2.5µA and Input/Output
V : 3.4V to 42V, V = 0.985V, I = 2.5µA, I < 1µA,
IN
OUT(MIN)
Q
SD
3mm × 5mm QFN-24 Package
Q
Current Limit/Monitor
LT8610
42V, 2.5A, 96% Efficiency, 2.2MHz Synchronous Micropower
V : 3.4V to 42V, V
= 0.985V, I = 2.5µA, I < 1µA,
IN
OUT(MIN)
Q
SD
Step-Down DC/DC Converter with I = 2.5µA and Input/Output
MSOP-16E Package
Q
Current Limit/Monitor
LT3480
LT3980
36V with Transient Protection to 60V, 2A (I ), 2.4MHz, High
V : 3.6V to 36V Transient to 60V, V
= 0.78V, I = 70µA,
OUT(MIN) Q
OUT
IN
Efficiency Step-Down DC/DC Converter with Burst Mode Operation
I
< 1µA, 3mm × 3mm DFN-10, MSOP-10E Packages
SD
58V with Transient Protection to 80V, 2A (I ), 2.4MHz, High
V : 3.6V to 58V Transient to 80V, V
= 0.78V, I = 85µA,
OUT
IN
OUT(MIN) Q
Efficiency Step-Down DC/DC Converter with Burst Mode Operation
I
< 1µA, 3mm × 4mm DFN-16, MSOP-16E Packages
SD
3995f
LT 0513 • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
24
●
●
(408)432-1900 FAX: (408) 434-0507 www.linear.com/LT3995
LINEAR TECHNOLOGY CORPORATION 2013
相关型号：
LT3999EDD#PBF
LT3999 - Low Noise, 1A, 1MHz Push-Pull DC/DC Driver with Duty Cycle Control; Package: DFN; Pins: 10; Temperature Range: -40°C to 85°C
Linear
LT3999EDD#TRPBF
LT3999 - Low Noise, 1A, 1MHz Push-Pull DC/DC Driver with Duty Cycle Control; Package: DFN; Pins: 10; Temperature Range: -40°C to 85°C
Linear
LT3999EMSE#PBF
LT3999 - Low Noise, 1A, 1MHz Push-Pull DC/DC Driver with Duty Cycle Control; Package: MSOP; Pins: 10; Temperature Range: -40°C to 85°C
Linear
LT3999EMSE#TRPBF
LT3999 - Low Noise, 1A, 1MHz Push-Pull DC/DC Driver with Duty Cycle Control; Package: MSOP; Pins: 10; Temperature Range: -40°C to 85°C
Linear
LT3999HMSE#PBF
LT3999 - Low Noise, 1A, 1MHz Push-Pull DC/DC Driver with Duty Cycle Control; Package: MSOP; Pins: 10; Temperature Range: -40°C to 125°C
Linear
LT3999HMSE#TRPBF
LT3999 - Low Noise, 1A, 1MHz Push-Pull DC/DC Driver with Duty Cycle Control; Package: MSOP; Pins: 10; Temperature Range: -40°C to 125°C
Linear
LT3999IDD#PBF
LT3999 - Low Noise, 1A, 1MHz Push-Pull DC/DC Driver with Duty Cycle Control; Package: DFN; Pins: 10; Temperature Range: -40°C to 85°C
Linear
LT3999IDD#TRPBF
LT3999 - Low Noise, 1A, 1MHz Push-Pull DC/DC Driver with Duty Cycle Control; Package: DFN; Pins: 10; Temperature Range: -40°C to 85°C
Linear
LT3999IMSE#PBF
LT3999 - Low Noise, 1A, 1MHz Push-Pull DC/DC Driver with Duty Cycle Control; Package: MSOP; Pins: 10; Temperature Range: -40°C to 85°C
Linear
LT3999IMSE#TRPBF
LT3999 - Low Noise, 1A, 1MHz Push-Pull DC/DC Driver with Duty Cycle Control; Package: MSOP; Pins: 10; Temperature Range: -40°C to 85°C
Linear
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