LT3991-3.3_15 [Linear]
55V, 1.2A Step-Down Regulator with 2.8A Quiescent Current;
| 型号： | LT3991-3.3_15 |
| 厂家： | 
Linear |
| 描述： | 55V, 1.2A Step-Down Regulator with 2.8A Quiescent Current |
| 文件： | 总24页 (文件大小：394K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT3991/LT3991-3.3/LT3991-5
55V, 1.2A Step-Down
Regulator with 2.8µA
Quiescent Current
FeaTures
DescripTion
The LT®3991 is an adjustable frequency monolithic buck
switchingregulatorthatacceptsawideinputvoltagerange
up to 55V. Low quiescent current design consumes only
2.8µAofsupplycurrentwhileregulatingwithnoload. Low
ripple Burst Mode operation maintains high efficiency at
low output currents while keeping the output ripple below
15mV in a typical application. An internally compensated
current mode topology is used for fast transient response
and good loop stability. A high efficiency 0.44Ω switch
is included on the die along with a boost Schottky diode
and the necessary oscillator, control and logic circuitry.
An accurate 1V threshold enable pin can be used to shut
down the LT3991, reducing the input supply current to
700nA. A capacitor on the SS pin provides a controlled
inrush current (soft-start). A power good flag signals
n
Ultralow Quiescent Current:
2.8µA I Regulating 12V to 3.3V
Q
IN
OUT
n
n
Fixed Output Voltages: 3.3V, 5V
2.1µA I Regulating 12V
Q
IN
Low Ripple Burst Mode® Operation:
Output Ripple < 15mV
P-P
n
n
n
n
n
n
n
n
n
n
n
n
n
Wide Input Voltage Range: 4.3V to 55V
1.2A Maximum Output Current
Adjustable Switching Frequency: 200kHz to 2MHz
Synchronizable Between 250kHz to 2MHz
Fast Transient Response
Accurate 1V Enable Pin Threshold
Low Shutdown Current: I = 700nA
Q
Power Good Flag
Soft-Start CapabilityV
Internal Compensation
when V
reaches 91% of the programmed output volt-
OUT
Saturating Switch Design: 0.44Ω On-Resistance
Output Voltage: 1.19V to 30V
Small Thermally Enhanced 10-Pin MSOP Package
and (3mm × 3mm) DFN Packages
age. The LT3991 is available in small 10-pin MSOP and
3mm × 3mm DFN packages with exposed pads 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.
applicaTions
n
Automotive Battery Regulation
n
Power for Portable Products
n
Industrial Supplies
No Load Supply Current
Typical applicaTion
3.0
OUTPUT IN REGULATION
3.3V Step Down Converter
V
IN
4.3V TO 55V
2.5
V
IN
EN./UVLO
PG
BOOST
SW
OFF ON
LT3991-5
2.0
0.47µF
12µH
LT3991-3.3
SS
RT
LT3991-3.3
4.7µF
1.5
BD
V
3.3V
1.2A
OUT
118k
V
OUT
SYNC GND
47µF
1.0
5
10 15 20 25 30 35 40 45 50 55
3991 TA01a
INPUT VOLTAGE (V)
f = 400kHz
3991 G06
3991fa
1
LT3991/LT3991-3.3/LT3991-5
absoluTe MaxiMuM raTings
(Note 1)
V , EN Voltage .........................................................55V
Operating Junction Temperature Range (Note 2)
LT3991E............................................. –40°C to 125°C
LT3991I.............................................. –40°C to 125°C
Storage Temperature Range .............. –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
IN
BOOST Pin Voltage ...................................................75V
BOOST Pin Above SW Pin.........................................30V
FB, V , RT, SYNC, SS Voltage .................................6V
OUT
PG, BD Voltage .........................................................30V
Boost Diode Current....................................................1A
(MSE Only) .......................................................300°C
pin conFiguraTion
LT3991
LT3991
LT3991-3.3, LT3991-5
TOP VIEW
TOP VIEW
TOP VIEW
BD
BOOST
SW
1
2
3
4
5
10 SYNC
BD
BOOST
SW
1
2
3
4
5
10 SYNC
BD
BOOST
SW
1
2
3
4
5
10 SYNC
9
8
7
6
PG
RT
SS
FB
9
8
7
6
PG
RT
SS
FB
9
8
7
6
PG
RT
SS
11
GND
11
GND
11
GND
V
V
IN
EN
IN
EN
V
IN
V
OUT
EN
MSE PACKAGE
10-LEAD PLASTIC MSOP
MSE PACKAGE
10-LEAD PLASTIC MSOP
= 45°C, θ = 10°C/W
JA JC
DD PACKAGE
θ
JA
= 45°C, θ = 10°C/W
θ
JC
10-LEAD (3mm × 3mm) PLASTIC DFN
= 45°C, θ = 10°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
θ
JA
JC
orDer inForMaTion
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
LFJR
PACKAGE DESCRIPTION
10-Lead (3mm × 3mm) Plastic DFN
10-Lead (3mm × 3mm) Plastic DFN
10-Lead Plastic MSOP
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
LT3991EDD#PBF
LT3991EDD#TRPBF
LT3991IDD#TRPBF
LT3991EMSE#TRPBF
LT3991IMSE#TRPBF
LT3991IDD#PBF
LFJR
LT3991EMSE#PBF
LT3991IMSE#PBF
LT3991EMSE-3.3#PBF
LT3991IMSE-3.3#PBF
LT3991EMSE-5#PBF
LT3991IMSE-5#PBF
LTFJS
LTFJS
10-Lead Plastic MSOP
LT3991EMSE-3.3#TRPBF LTFRS
LT3991IMSE-3.3#TRPBF LTFRS
10-Lead Plastic MSOP
10-Lead Plastic MSOP
LT3991EMSE-5#TRPBF
LT3991IMSE-5#TRPBF
LTFRV
LTFRV
10-Lead Plastic MSOP
10-Lead Plastic MSOP
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/
3991fa
2
LT3991/LT3991-3.3/LT3991-5
elecTrical characTerisTics The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VEN = 12V, VBD = 3.3V unless otherwise noted. (Note 2)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
l
Minimum Input Voltage
Quiescent Current from V
(Note 4)
4
4.3
V
V
V
V
Low
High, V
High, V
0.7
1.7
1.2
2.7
4.5
μA
μA
μA
IN
EN
EN
EN
Low
Low
SYNC
SYNC
l
l
LT3991 FB Pin Current
V
= 1.19V
0.1
10
12
nA
FB
Internal Feedback Resistor Divider
Feedback Voltage
MΩ
1.175
1.165
1.19
1.19
1.205
1.215
V
V
l
l
l
LT3991-3.3 Output Voltage
LT3991-5 Output Voltage
3.25
3.224
3.3
3.3
3.35
3.376
V
V
4.93
4.89
5
5
5.07
5.11
V
V
FB Voltage Line Regulation
Switching Frequency
4.3V < V < 40V (Note 4)
0.0002
0.01
%/V
IN
R = 11k
1.6
0.8
160
2
1
200
2.4
1.2
240
MHz
MHz
kHz
T
R = 35.7k
T
R = 255k
T
Minimum Switch On Time
Minimum Switch Off Time
Switch Current Limit
110
150
2.3
440
0.02
800
0.02
1.4
25
ns
ns
A
200
2.9
1.7
Switch V
I
I
= 1A
mV
μA
mV
μA
V
CESAT
SW
Switch Leakage Current
Boost Schottky Forward Voltage
Boost Schottky Reverse Leakage
Minimum Boost Voltage (Note 3)
BOOST Pin Current
1
= 100mA
SH
V
V
= 12V
1
REVERSE
l
l
= 5V
1.8
33
IN
I
= 1A, V
= 15V
mA
V
SW
BOOST
EN Voltage Threshold
EN Rising
0.95
1.01
30
1.07
EN Voltage Hysteresis
mV
nA
mV
mV
%
EN Pin Current
0.2
100
20
20
LT3991 PG Threshold Offset from V
LT3991 PG Hysteresis
V
V
Rising
60
140
FB
FB
LT3991-X PG Threshold Offset from V
LT3991X PG Hysteresis
PG Leakage
Rising
5.5
9
12.5
1
OUT
OUT
1.3
0.02
570
0.8
0.1
1
%
V
V
= 3V
µA
μA
V
PG
PG
l
PG Sink Current
= 0.4V
300
0.6
SYNC Threshold
1.0
1.6
SYNC Pin Current
nA
μA
SS Source Current
V
= 1V
0.6
SS
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.
The LT3991I is guaranteed over the full –40°C to 125°C operating junction
temperature range. High junction temperatures degrade operating
lifetimes. Operating lifetime is derated at junction temperatures greater
than 125°C.
Note 2: The LT3991E 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.
Note 3: This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the switch.
Note 4: Minimum input voltage depends on application circuit.
3991fa
3
LT3991/LT3991-3.3/LT3991-5
TA = 25°C, unless otherwise noted.
Typical perForMance characTerisTics
Efficiency, VOUT = 5V
Efficiency, VOUT = 3.3V
Efficiency, VOUT = 5V
90
80
70
60
50
40
30
20
100
100
90
80
70
60
50
40
30
V
= 12V
V
= 5V
IN
OUT
V
= 12V
IN
90
80
70
60
50
40
30
20
10
0
R1 = 1M
R2 = 309k
V
= 12V
= 36V
10
IN
V
= 24V
IN
V
= 36V
V
= 24V
IN
IN
V
= 36V
V
= 48V
IN
IN
V
IN
= 24V
V
= 48V
IN
V
IN
V
= 48V
IN
V
= 5V
OUT
R1 = 1M
R2 = 309k
0
0.2
0.4
0.6
0.8
1
1.2
0.01
0.1
1
100
1000
0
0.2
0.4
0.6
0.8
1
1.2
LOAD CURRENT (A)
LOAD CURRENT (mA)
LOAD CURRENT (A)
3991 G02
3991 G03
3991 G01
Efficiency, VOUT = 3.3V
No Load Supply Current
No Load Supply Current
90
80
70
60
50
40
30
20
10
0
100
10
1
3.0
2.5
2.0
1.5
1.0
DIODES, INC.
DFLS2100
OUTPUT IN REGULATION
V
V
= 12V
= 24V
= 36V
IN
IN
IN
V
V
LT3991-5
= 48V
IN
LT3991-3.3
0.01
0.1
1
10
100
1000
–55 –25
5
35
65
95 125 155
5
10 15 20 25 30 35 40 45 50 55
LOAD CURRENT (mA)
TEMPERATURE (°C)
INPUT VOLTAGE (V)
3991 G04
3991 G05
3991 G06
LT3991 Feedback Voltage
LT3991-3.3 Output Voltage
LT3991-5 Output Voltage
1.205
1.200
1.195
1.190
1.185
1.180
1.175
3.345
3.330
3.315
3.300
5.06
5.04
5.02
5.00
3.285
3.270
3.255
4.98
4.96
4.94
–55 –25
5
35
65
95 125 155
65
TEMPERATURE (°C)
125 155
65
TEMPERATURE (°C)
125 155
–50 –25
5
35
95
–50 –25
5
35
95
TEMPERATURE (°C)
3991 G07
3991 G28
3991 G29
3991fa
4
LT3991/LT3991-3.3/LT3991-5
TA = 25°C, unless otherwise noted.
Typical perForMance characTerisTics
Load Regulation
Maximum Load Current
Maximum Load Current
2.5
2.0
1.5
1.0
0.5
0
3.0
2.5
2.0
1.5
1.0
0.5
0
0.30
0.25
0.20
0.15
0.10
0.05
0
V
OUT
= 5V
V
= 3.3V
REFERENCED FROM V
AT 0.5A LOAD
OUT
OUT
TYPICAL
TYPICAL
MINIMUM
MINIMUM
–0.05
–0.10
–0.15
–0.20
–0.25
–0.30
5
10 15 20 25 30 35 40 45 50 55
5
10 15 20 25 30 35 40 45 50 55
0
200
400
600
800 1000 1200
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
LOAD CURRENT (mA)
3991 G09
3991 G08
3991 G10
Switching Frequency
Switch Current Limit
Switch Current Limit
3.0
2.5
2.0
1.5
1.0
0.5
0
1000
950
900
850
800
750
700
650
600
2.5
2.4
2.3
2.2
2.1
2.0
1.9
1.8
1.7
1.6
1.5
DUTY CYCLE = 30%
0
20
40
60
80
100
–55 –25
5
35
65
95 125 155
–55 –25
5
35
65
95 125 155
DUTY CYCLE (%)
TEMPERATURE (°C)
TEMPERATURE (°C)
3991 G12
3991 G11
3991 G13
Boost Pin Current
Switch VCESAT
LT3991 Frequency Foldback
900
800
700
600
500
400
300
200
100
0
700
600
500
400
300
200
100
0
45
40
35
30
25
20
15
10
5
0
0
0.2
0.4
0.6
0.8
1
1.2
0
250
500
750 1000 1250 1500
0
250
500
750 1000 1250 1500
FB PIN VOLTAGE (V)
SWITCH CURRENT (mA)
SWITCH CURRENT (mA)
3991 G16
3991 G14
3991 G15
3991fa
5
LT3991/LT3991-3.3/LT3991-5
Typical perForMance characTerisTics
Minimum Switch On-Time/
TA = 25°C, unless otherwise noted.
LT3991-X Frequency Foldback
Switch Off-Time
Soft-Start
400
350
300
250
200
150
100
50
900
800
700
600
500
400
300
200
100
2.5
t
1A LOAD
2.0
1.5
1.0
0.5
0
OFF(MIN)
t
0.5A LOAD
OFF(MIN)
t
ON(MIN)
0
0
–55 –25
5
35
65
95 125 155
0
0.25 0.5 0.75
1
1.25 1.5 1.75
2
0
80
(% OF REGULATION VOLTAGE)
100
20
40
60
TEMPERATURE (°C)
SS PIN VOLTAGE (V)
V
OUT
3991 G17
3991 G18
3991 G30
Minimum Input Voltage
Minimum Input Voltage
EN Threshold
5.0
4.8
4.6
4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
6.4
6.2
6.0
5.8
5.6
5.4
5.2
5.0
1.05
1.04
1.03
1.02
1.01
1.00
0.99
0.98
0.97
0.96
0.95
V
= 3.3V
V
= 5V
OUT
OUT
TO START
RISING THRESHOLD
TO START
TO RUN
FALLING THRESHOLD
TO RUN
200
0
200
400
600
800 1000 1200
0
400
600
800 1000 1200
–55 –25
5
35
65
95 125 155
LOAD CURRENT (mA)
LOAD CURRENT (mA)
TEMPERATURE (°C)
3991 G19
3991 G20
3971 G21
Transient Load Response,
Load Current Stepped from 25mA
(Burst Mode Operation) to 525mA
Boost Diode Forward Voltage
Power Good Threshold
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
95
94
93
92
91
90
89
88
87
86
85
V
OUT
100mV/DIV
I
L
500mA/DIV
3991 G24
0
250
500
750 1000 1250 1500
–55 –25
5
35
65
95 125 155
10µs/DIV
BOOST DIODE CURRENT (mA)
TEMPERATURE (°C)
V
C
= 48V, V
OUT
= 3.3V
OUT
IN
3991 G22
3991 G23
= 47µF
3991fa
6
LT3991/LT3991-3.3/LT3991-5
TA = 25°C, unless otherwise noted.
Typical perForMance characTerisTics
Transient Load Response,
Load Current Stepped from
0.5A to 1A
Switching Waveforms;
Burst Mode Operation
Switching Waveforms; Full
Frequency Continuous Operation
V
OUT
V
SW
100mV/
DIV
V
SW
5V/DIV
5V/DIV
I
L
I
L
500mA/DIV
500mA/DIV
I
L
500mA/
DIV
V
V
OUT
OUT
20mV/DIV
20mV/DIV
3971 G26
3991 G25
3971 G27
5µs/DIV
= 3.3V
10µs/DIV
= 3.3V
1µs/DIV
= 3.3V
OUT
V
I
= 48V, V
V
C
= 48V, V
OUT
V
I
= 48V, V
= 1A
OUT
IN
OUT
IN
OUT
IN
LOAD
= 20mA
= 47µF
LOAD
C
= 47µF
C
= 47µF
OUT
pin FuncTions
BD (Pin 1): This pin connects to the anode of the boost
diode. The BD pin is normally connected to the output.
FB (Pin 6, LT3991 Only): The LT3991 regulates the FB pin
to 1.19V. Connect the feedback resistor divider tap to this
pin. Also, connect a phase lead capacitor between FB and
BOOST (Pin 2): This pin is used to provide a drive volt-
age, higher than the input voltage, to the internal bipolar
NPN power switch.
V
. Typically this capacitor is 10pF.
OUT
V
(Pin 6, LT3991-3.3/LT3991-5 Only): The LT3991-
OUT
3.3 and LT3991-5 regulate the V
pin to 3.3V and 5V
OUT
SW (Pin 3): The SW pin is the output of an internal power
switch. Connect this pin to the inductor, catch diode, and
boost capacitor.
respectively. This pin connects to the internal 10MΩ
feedback divider that programs the fixed output voltage.
SS(Pin7):Acapacitorandaseriesresistoraretiedbetween
SS and ground to slowly ramp up the peak current limit
of the LT3991 on start-up. The soft-start capacitor is only
actively discharged when EN is low. The SS pin is released
when the EN pin goes high. Float this pin to disable soft-
start. The soft-start resistor has a typical value of 100k.
V (Pin 4): The V pin supplies current to the LT3991’s
IN
IN
internal circuitry and to the internal power switch. This
pin must be locally bypassed.
EN (Pin 5): The part is in shutdown when this pin is low
and active when this pin is high. The hysteretic threshold
voltageis1.005Vgoingupand0.975Vgoingdown.TheEN
RT (Pin 8): A resistor is tied between RT and ground to
set the switching frequency.
threshold is only accurate when V is above 4.3V. If V is
IN
IN
lower than 4.3V, ground EN to place the part in shutdown.
Tie to V if shutdown feature is not used.
IN
3991fa
7
LT3991/LT3991-3.3/LT3991-5
pin FuncTions
PG (Pin 9): The PG pin is the open-drain output of an
internal comparator. PGOOD remains low until the FB pin
is within 9% of the final regulation voltage. PGOOD is
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 1.5mA.
valid when the LT3991 is enabled and V is above 4.3V.
IN
SYNC (Pin 10): This is the external clock synchronization
input. Ground this pin for low ripple Burst Mode operation
GND (Exposed Pad Pin 11): Ground. The exposed pad
must be soldered to PCB.
block DiagraM
V
IN
V
IN
–
+
C1
INTERNAL 1.19V REF
SHDN
BD
1V
+
–
SWITCH
SLOPE COMP
Σ
LATCH
BOOST
EN
RT
R
C3
OSCILLATOR
200kHz TO 2MHz
Q
S
L1
R
T
V
OUT
SW
Burst Mode
DETECT
SYNC
PG
D1
C2
ERROR AMP
V
CLAMP
C
V
+
–
+
–
1.09V
C
1µA
SS
C4
SHDN
C5
R1
LT3991-3.3
LT3991-5
ONLY
R2
LT3991
ONLY
V
OUT
GND
FB
R2
R1
3991 BD
C5
3991fa
8
LT3991/LT3991-3.3/LT3991-5
operaTion
The LT3991 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
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 further optimize efficiency, the LT3991 automatically
switches to Burst Mode operation in light load situations.
Between bursts, all circuitry associated with controlling
the output switch is shut down, reducing the input supply
currentto1.7μA.Inatypicalapplication,2.8μAwillbecon-
sumed from the supply when regulating with no load.
between the V and SW pins, turning the switch off when
IN
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 amplifier’s
C
output increases, more current is delivered to the output;
The oscillator reduces the LT3991’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.
if it decreases, less current is delivered. An active clamp
on the V node provides current limit. The V node 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 and resistor.
The LT3991 contains a power good comparator which
trips when the FB pin is at 91% 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 to pull
the PG pin high. Power good is valid when the LT3991 is
If the EN pin is low, the LT3991 is shut down and draws
700nA from the input. When the EN pin exceeds 1.01V,
the switching regulator will become active.
The switch driver operates from either V or from the
BOOST pin. An external capacitor is used to generate a
IN
enabled and V is above 4.3V.
IN
applicaTions inForMaTion
Achieving Ultralow Quiescent Current
1000
V
V
= 12V
IN
OUT
= 3.3V
To enhance efficiency at light loads, the LT3991 operates
inlowrippleBurstMode,whichkeepstheoutputcapacitor
charged to the desired output voltage while minimizing
the input quiescent current. In Burst Mode operation the
LT3991 delivers single pulses of current to the output ca-
pacitor followed by sleep periods where the output power
is supplied by the output capacitor. When in sleep mode
the LT3991 consumes 1.7μA, but when it turns on all the
circuitry to deliver a current pulse, the LT3991 consumes
1.5mA of input current in addition to the switch current.
Therefore, the total quiescent current will be greater than
1.7μA when regulating.
800
600
400
200
0
0
20
40
60
80
100
120
LOAD CURRENT (mA)
3991 F01
Figure 1. Switching Frequency in Burst Mode Operation
As the output load decreases, the frequency of single cur-
rent pulses decreases (see Figure 1) and the percentage
of time the LT3991 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
3991fa
9
LT3991/LT3991-3.3/LT3991-5
applicaTions inForMaTion
performanceoftheLT3991.Thefeedbackresistorsshould
preferably be on the order of MΩ and the Schottky catch
diode should have less than 1µA of typical reverse leak-
age at room temperature. These two considerations are
reiterated in the FB Resistor Network and Catch Diode
Selection sections.
quiescent current will significantly increase to 1.5mA in
light load situations when synchronized with an external
clock. Holding the SYNC pin high yields no advantages in
terms of output ripple or minimum load to full frequency,
so is not recommended.
FB Resistor Network
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 15mV of output ripple in
a typical application. See Figure 2.
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the resistor
values according to:
⎛
⎜
⎝
⎞
VOUT
1.19V
R1=R2
−1
⎟
⎠
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
V
SW
5V/DIV
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.
I
L
500mA/DIV
V
OUT
20mV/DIV
3991 F02
5µs/DIV
V
V
I
= 48V
WhenusinglargeFBresistors,a10pFphaseleadcapacitor
IN
OUT
= 3.3V
should be connected from V
to FB.
= 20mA
OUT
LOAD
The LT3991-3.3 and LT3991-5 control an internal 10M FB
resistor divider as well as an internal lead capacitor.
Figure 2. Burst Mode Operation
WhileinBurstModeoperation,theburstfrequencyandthe
chargedeliveredwitheachpulsewillnotchangewithoutput
capacitance. Therefore, the output voltage ripple will be
inverselyproportionaltotheoutputcapacitance.Inatypical
application with a 47μF output capacitor, the output ripple
isabout8mV,andwitha100μFoutputcapacitortheoutput
rippleisabout4mV. Theoutputvoltageripplecancontinue
to be decreased by increasing the output capacitance.
Setting the Switching Frequency
The LT3991 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
T
frequency is in Table 1.
Table 1. Switching Frequency vs RT Value
At higher output loads (above 86mA for the front page
application) the LT3991 will be running at the frequency
SWITCHING FREQUENCY (MHz)
R VALUE (kΩ)
T
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
255
118
programmed by the R resistor, and will be operating in
T
71.5
49.9
35.7
28.0
22.1
17.4
14.0
11.0
standardPWMmode.ThetransitionbetweenPWMandlow
ripple Burst Mode operation will exhibit slight frequency
jitter, but will not disturb the output voltage.
To ensure proper Burst Mode operation, the SYNC pin
must be grounded. When synchronized with an exter-
nal clock, the LT3991 will pulse skip at light loads. The
3991fa
10
LT3991/LT3991-3.3/LT3991-5
applicaTions inForMaTion
Operating Frequency Tradeoffs
Input Voltage Range
The minimum input voltage is determined by either the
LT3991’s minimum operating voltage of 4.3V or by its
maximumdutycycle(seeequationinOperatingFrequency
Tradeoffs section). The minimum input voltage due to
duty cycle is:
Selection of the operating frequency is a tradeoff between
efficiency,componentsize,minimumdropoutvoltage,and
maximum input voltage. The advantage of high frequency
operationisthatsmallerinductorandcapacitorvaluesmay
be used. The disadvantages are lower efficiency, lower
maximum input voltage, and higher dropout voltage. The
VOUT + VD
V
=
− VD + VSW
IN(MIN)
highest acceptable switching frequency (f
) for a
SW(MAX)
1− fSWtOFF(MIN)
given application can be calculated as follows:
where V
is the minimum input voltage, V
is
OUT
IN(MIN)
VOUT + VD
fSW(MAX)
=
the output voltage, V is the catch diode drop (~0.5V),
D
tON(MIN)(V − V + VD)
IN
SW
V
is the internal switch drop (~0.5V at max load), f
SW
SW
is the switching frequency (set by R ), and t
is
OFF(MIN)
T
where V is the typical input voltage, V
is the output
IN
OUT
the minimum switch off-time. Note that higher switch-
ing frequency will increase the minimum input voltage.
If a lower dropout voltage is desired, a lower switching
frequency should be used.
voltage, V is the catch diode drop (~0.5V), and V is
D
SW
theinternalswitchdrop(~0.5Vatmaxload).Thisequation
shows that slower switching frequency is necessary to
safely accommodate high V /V
ratio. Also, as shown
IN OUT
The maximum input voltage for LT3991 applications
depends on switching frequency, the Absolute Maximum
Ratings of the V and BOOST pins, and the operating
mode. For a given application where the switching fre-
quency and the output voltage are already selected, the
maximum input voltage (V
intheInputVoltageRangesection,lowerfrequencyallows
a lower dropout voltage. The input voltage range depends
ontheswitchingfrequencybecausetheLT3991switchhas
finite minimum on and off times. The minimum switch on
and off times are strong functions of temperature. Use
the typical minimum on and off curves to design for an
application’s maximum temperature, while adding about
30%forpart-to-partvariation.Theminimumandmaximum
duty cycles that can be achieved taking minimum on and
off times into account are:
IN
) that guarantees
IN(OP-MAX)
optimum output voltage ripple for that application can be
found by applying the following equation:
VOUT + VD
fSW • tON(MIN)
V
=
– VD + VSW
IN(OP-MAX)
DCMIN = fSWtON(MIN)
where t
is the minimum switch on-time. Note that
ON(MIN)
DCMAX = 1− fSWtOFF(MIN)
a higher switching frequency will decrease the maximum
operating input voltage. Conversely, a lower switching
frequency will be necessary to achieve normal operation
at higher input voltages.
where f is the switching frequency, the t
minimumswitchon-time,andthet
switch off-time. These equations show that duty cycle
range increases when switching frequency is decreased.
See the Electrical Characteristics section for t
is the
ON(MIN)
istheminimum
SW
OFF(MIN)
The circuit will tolerate inputs above the maximum op-
erating input voltage and up to the Absolute Maximum
and
ON(MIN)
Ratings of the V and BOOST pins, regardless of chosen
IN
t
values.
OFF(MIN)
switching frequency. However, during such transients
A good choice of switching frequency should allow ad-
equate input voltage range (see Input Voltage Range sec-
tion) and keep the inductor and capacitor values small.
where V is higher than V
, the LT3991 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
typical operation. Do not overload when V is greater
IN
than V
.
IN(OP-MAX)
3991fa
11
LT3991/LT3991-3.3/LT3991-5
applicaTions inForMaTion
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
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:
VOUT + VD
L =
(1−DC)•(VOUT + VD)
ΔIL =
fSW
L • fSW
where f is the switching frequency in MHz, V
is the
OUT
SW
where f is the switching frequency of the LT3991, DC is
SW
output voltage, V is the catch diode drop (~0.5V) and L
D
the duty cycle and L is the value of the inductor. Therefore,
the maximum output current that the LT3991 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
is the inductor value in μH.
Theinductor’sRMScurrentratingmustbegreaterthanthe
maximumloadcurrentanditssaturationcurrentshouldbe
about 30% higher. For robust operation in fault conditions
(start-up or short-circuit) and high input voltage (>30V),
the saturation current should be above 2.8A. 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 vendors
and suitable types.
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.
Theinductorvaluemustbesufficienttosupplythedesired
maximum output current (I
), which is a function
OUT(MAX)
of the switch current limit (I ) and the ripple current.
LIM
ΔIL
2
IOUT(MAX) =ILIM
–
TheLT3991limitsitspeakswitchcurrentinordertoprotect
itself and the system from overload faults. The LT3991’s
switch current limit (I ) is at least 2.33A at low duty
LIM
cycles and decreases linearly to 1.8A at DC = 0.8.
Table 2. Inductor Vendors
Finally, for duty cycles greater than 50% (V /V >0.5),
OUT IN
VENDOR
Murata
TDK
URL
PART SERIES
TYPE
a minimum inductance is required to avoid sub-harmonic
www.murata.com
LQH55D
Open
oscillations. See Application Note 19.
www.componenttdk.com SLF7045
SLF10145
Shielded
Shielded
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 LT3991 will be
able to deliver the required output current. Note again
that these equations assume that the inductor current is
Toko
www.toko.com
D62CB
D63CB
D73C
Shielded
Shielded
Shielded
Open
D75F
Coilcraft
Sumida
www.coilcraft.com
www.sumida.com
MSS7341
MSS1038
Shielded
Shielded
CR54
Open
continuous. Discontinuous operation occurs when I
OUT
CDRH74
CDRH6D38
CR75
Shielded
Shielded
Open
is less than ΔI /2.
L
3991fa
12
LT3991/LT3991-3.3/LT3991-5
applicaTions inForMaTion
Input Capacitor
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
Bypass the input of the LT3991 circuit with a ceramic
capacitor 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 LT3991 and will easily handle
the ripple current. 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.
100
COUT
=
VOUT SW
f
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.
Transient performance can be improved with a higher
value capacitor. Increasing the output capacitance will
also decrease the output voltage ripple. A lower value of
output capacitor can be used to save space and cost but
transient performance will suffer.
When choosing a capacitor, look carefully through the
data sheet to find out what the actual capacitance is under
operating conditions (applied voltage and temperature). A
physicallylargercapacitororonewithahighervoltagerating
may be required. Table 3 lists several capacitor vendors.
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 LT3991 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 LT3991 (see the PCB Layout section).
Asecondprecautionregardingtheceramicinputcapacitor
concernsthemaximuminputvoltageratingoftheLT3991.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank cir-
cuit. If the LT3991 circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possibly
exceeding the LT3991’s voltage rating. This situation is
easily avoided (see the Hot Plugging Safely section).
Table 3. Recommended Ceramic Capacitor Vendors
MANUFACTURER
AVX
WEBSITE
www.avxcorp.com
www.murata.com
www.t-yuden.com
www.vishay.com
www.tdk.com
Murata
Taiyo Yuden
Vishay Siliconix
TDK
Catch Diode Selection
The catch diode (D1 from Block Diagram) conducts cur-
rent only during switch off time. Average forward current
in normal operation can be calculated from:
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
withtheinductor,itfiltersthesquarewavegeneratedbythe
LT3991toproducetheDCoutput. 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
LT3991’s control loop. Ceramic capacitors have very low
V – V
IN
OUT
ID(AVG) =IOUT
V
IN
where I
is the output load current. The only reason to
OUT
consideradiodewithalargercurrentratingthannecessary
for nominal operation is for the worst-case condition of
shorted output. The diode current will then increase to the
typical peak switch current. Peak reverse voltage is equal
to the regulator input voltage. Use a diode with a reverse
voltage rating greater than the input voltage.
3991fa
13
LT3991/LT3991-3.3/LT3991-5
applicaTions inForMaTion
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.
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.
I at V =
R
R
V
I
V at 1A
V at 2A
20V 25°C
(µA)
R
AVE
F
F
PART NUMBER (V)
(A)
(mV)
(mV)
On Semiconductor
MBR0520L
MBR0540
MBRM120E
MBRM140
Diodes Inc.
B0530W
B0540W
B120
20
40
20
40
0.5
0.5
1
30
0.4
0.5
20
620
530
550
595
Ceramic Capacitors
1
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
whenusedwiththeLT3991duetotheirpiezoelectricnature.
When in Burst Mode operation, the LT3991’s switching
frequency depends on the load current, and at very light
loads the LT3991 can excite the ceramic capacitor at audio
frequencies, generating audible noise. Since the LT3991
operates at a lower current limit during Burst Mode op-
eration, the noise is typically very quiet to a casual ear. If
this is unacceptable, use a high performance tantalum or
electrolytic capacitor at the output.
30
40
20
30
40
50
20
30
40
40
40
60
100
40
0.5
0.5
1
15
1
620
500
500
500
700
1.1
1.1
1.1
0.4
20
0.6
1
B130
1
B140
1
B150
1
B220
2
500
500
B230
2
B140HB
DFLS240L
DFLS140
DFLS160
DFLS2100
B240
1
2
500
4
1.1
1
510
500
770
1
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT3991. As pre-
viously mentioned, a ceramic input capacitor combined
with trace or cable inductance forms a high quality (under
damped)tankcircuit. IftheLT3991circuitispluggedintoa
live supply, the input voltage can ring to twice its nominal
value,possiblyexceedingtheLT3991’srating.Thissituation
is easily avoided (see the Hot Plugging Safely section).
2.5
0.01
0.45
2
860
500
2
Central Semiconductor
CMSH1 - 40M
CMSH1 - 60M
40
60
1
1
1
2
2
2
2
2
500
700
400
CMSH1 - 40ML 40
CMSH2 - 40M
CMSH2 - 60M
CMSH2 - 40L
CMSH2 - 40
40
60
40
40
60
550
700
400
500
700
BOOST and BD Pin Considerations
Capacitor C3 and the internal boost Schottky diode (see
the Block Diagram) are used to generate a boost volt-
age that is higher than the input voltage. In most cases
a 0.47μF capacitor will work well. Figure 3 shows three
ways to arrange the boost circuit. The BOOST pin must
be more than 2.3V above the SW pin for best efficiency.
Foroutputsof3Vandabove,thestandardcircuit(Figure 3a)
is best. For outputs between 2.8V and 3V, use a 1μF boost
capacitor. A 2.5V output presents a special case because it
is marginally adequate to support the boosted drive stage
CMSH2 - 60M
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 LT3991 will be optimized by using a catch diode
with minimum reverse leakage current. Low leakage
3991fa
14
LT3991/LT3991-3.3/LT3991-5
applicaTions inForMaTion
whileusingtheinternalboostdiode.ForreliableBOOSTpin
operation with 2.5V outputs use a good external Schottky
diode (such as the ON Semi MBR0540), and a 1μF boost
capacitor (Figure 3b). For output voltages below 2.5V,
the boost diode can be tied to the input (Figure 3c), or to
another external supply greater than 2.8V. However, the
circuitinFigure3aismoreefficientbecausetheBOOSTpin
currentcomesfromalowervoltagesource. Youmustalso
be sure that the maximum voltage ratings of the BOOST
and BD pins are not exceeded.
the maximum duty cycle as outlined in the Input Voltage
Range section. For proper start-up, the minimum input
voltage is also limited by the boost circuit. If the input
voltage is ramped slowly, the boost capacitor may not
be fully charged. Because the boost capacitor is charged
with the energy stored in the inductor, the circuit will rely
on some minimum load current to get the boost circuit
running properly. This minimum load will depend on input
and output voltages, and on the arrangement of the boost
circuit. The minimum load generally goes to zero once the
circuit has started. Figure 4 shows a plot of minimum load
to start and to run as a function of input voltage. In many
cases the discharged output capacitor will present a load
to the switcher, which will allow it to start. The plots show
The minimum operating voltage of an LT3991 application
is limited by the minimum input voltage (4.3V) and by
theworst-casesituationwhereV isrampingveryslowly.
IN
BD
For lower start-up voltage, the boost diode can be tied to
V
IN
V
BOOST
LT3991
IN
V ; however, this restricts the input range to one-half of
IN
C3
the absolute maximum rating of the BOOST pin.
SW
V
OUT
4.7µF
GND
5.0
4.8
4.6
4.4
(3a) For V
> 2.8V
OUT
TO START
4.2
4.0
TO RUN
D2
3.8
BD
V
IN
V
BOOST
IN
3.6
V
A
= 3.3V
LT3991
OUT
3.4
3.2
3.0
C3
T
= 25°C
L = 10µH
SW
V
OUT
4.7µF
f = 400kHz
GND
10
100
1000
LOAD CURRENT (mA)
6.4
6.2
6.0
5.8
5.6
5.4
5.2
5.0
(3b) For 2.5V < V
< 2.8V
OUT
TO START
TO RUN
BD
V
V
IN
BOOST
LT3991
IN
C3
SW
V
4.7µF
OUT
GND
V
= 5V
OUT
A
T
= 25°C
L = 10µH
f = 400kHz
3991 FO3
10
100
1000
LOAD CURRENT (mA)
(3c) For V
< 2.5V; V
= 27V
IN(MAX)
OUT
3991 F04
Figure 3. Three Circuits for Generating the Boost Voltage
Figure 4. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
3991fa
15
LT3991/LT3991-3.3/LT3991-5
applicaTions inForMaTion
Be aware that when the input voltage is below 4.3V, the
input current may rise to several hundred μA. And the part
At light loads, the inductor current becomes discontinu-
ous and this reduces the minimum input voltage to ap-
may be able to switch at cold or for V
thresholds less
proximately 400mV above V . At higher load currents,
IN(EN)
OUT
than 7V. Figure 6 shows the magnitude of the increased
input current in a typical application with different pro-
the inductor current is continuous and the duty cycle is
limitedbythemaximumdutycycleoftheLT3991,requiring
a higher input voltage to maintain regulation.
grammed V
.
IN(EN)
When operating in Burst Mode for light load currents, the
current through the V resistor network can easily be
Enable Pin
IN(EN)
The LT3991 is in shutdown when the EN pin is low and
active when the pin is high. The rising threshold of the EN
comparator is 1.01V, with 30mV of hysteresis. The EN pin
greater than the supply current consumed by the LT3991.
Therefore,theV resistorsshouldbelargetominimize
IN(EN)
their effect on efficiency at low loads.
can be tied to V if the shutdown feature is not used.
IN
12V V
Input Current
IN(EN)
Adding a resistor divider from V to EN programs the
IN
500
400
300
200
100
0
LT3991 to regulate the output only when V is above a
IN
desired voltage (see Figure 5). Typically, this threshold,
V , is used in situations where the input supply is cur-
IN(EN)
rent limited, or has a relatively high source resistance. A
switchingregulatordrawsconstantpowerfromthesource,
so source current increases as source voltage drops. This
looks like a negative resistance load to the source and can
cause the source to current limit or latch low under low
source voltage conditions. The V
threshold prevents
IN(EN)
the regulator from operating at source voltages where the
problems might occur. This threshold can be adjusted by
setting the values R3 and R4 such that they satisfy the
following equation:
0
1
2
3
4
5
6
7
8
9 10 11 12
INPUT VOLTAGE (V)
V
= 12V
IN(EN)
R3 = 11M
R4 = 1M
6V V
Input Current
R3
R4
IN(EN)
V
=
+1
500
400
300
200
100
0
IN(EN)
where output regulation should not start until V is above
IN
V
. Duetothecomparator’shysteresis, regulationwill
IN(EN)
not stop until the input falls slightly below V
.
IN(EN)
LT3991
V
IN
R3
R4
1V
+
–
SHDN
EN
0
1
2
3
4
5
6
INPUT VOLTAGE (V)
3991 F05
3991 F06
V
= 6V
IN(EN)
R3 = 5M
R4 = 1M
Figure 5. Programmed Enable Threshold
Figure 6. Input Current vs Input Voltage
for a Programmed VIN(EN) of 6V and 12V
3991fa
16
LT3991/LT3991-3.3/LT3991-5
applicaTions inForMaTion
The LT3991 will not enter Burst Mode operation at low
output loads while synchronized to an external clock, but
instead will pulse skip to maintain regulation.
Soft-Start
TheSSpincanbeusedtosoft-starttheLT3991bythrottling
themaximuminputcurrentduringstart-up.Aninternal1μA
current source charges an external capacitor generating a
voltagerampontheSSpin. TheSSpinclampstheinternal
The LT3991 may be synchronized over a 250kHz to 2MHz
range. The R resistor should be chosen to set the LT3991
T
switchingfrequency20%belowthelowestsynchronization
V node,whichslowlyrampsupthecurrentlimit.Maximum
C
input. For example, if the synchronization signal will be
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. A 100k resistor
in series with the soft-start capacitor is recommended.
Figure7showsstart-upwaveformsforatypicalapplication
with a 10nF capacitor and a 100k resistor on SS for a 3.3Ω
load when the EN pin is pulsed high for 10ms.
250kHz and higher, the R should be selected for 200kHz.
T
To assure reliable and safe operation the LT3991 will only
synchronize when the output voltage is near regulation as
indicatedbythePGflag.Itisthereforenecessarytochoose
alargeenoughinductorvaluetosupplytherequiredoutput
current at the frequency set by the R resistor (see the
T
InductorSelectionsection).Theslopecompensationisset
TheexternalSScapacitorisonlyactivelydischargedwhen
EN is low. With EN low, the external SS cap is discharged
through approximately 150Ω. The EN pin needs to be low
long enough for the external cap to completely discharge
through the 150Ω pull-down and external series resistor
prior to start-up.
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.
V
SS
0.5V/DIV
Shorted and Reversed Input Protection
V
OUT
2V/DIV
If the inductor is chosen so that it won’t saturate exces-
sively, an LT3991 buck regulator will tolerate a shorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
LT3991 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 LT3991’s
I
L
0.5A/DIV
3991 F07
2ms/DIV
Figure 7. Soft-Start Waveforms for Front-Page Application
with 10nF Capacitor and 100k Series Resistor on SS.
EN is Pulsed High for About 10ms with a 3.3Ω Load Resistor
output. If the V pin is allowed to float and the EN pin
IN
is held high (either by a logic signal or because it is tied
Synchronization
to V ), then the LT3991’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
ToselectlowrippleBurstModeoperation,tietheSYNCpin
below 0.6V (this can be ground or a logic low output).
Synchronizing the LT3991 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 0.6V
and peaks above 1.0V (up to 6V).
zero. However, if the V pin is grounded while the output
IN
is held high, regardless of EN, parasitic diodes inside the
LT3991 can pull current from the output through the SW
pin and the V pin. Figure 8 shows a circuit that will run
IN
only when the input voltage is present and that protects
against a shorted or reversed input.
3991fa
17
LT3991/LT3991-3.3/LT3991-5
applicaTions inForMaTion
D4
MBRS140
V
V
BOOST
SW
L1
IN
IN
C2
V
OUT
EN
V
OUT
LT3991
BD
FB
GND
+
BACKUP
GND
R
PG
R
T
C3
C4
3991 F07
Figure 8. 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 LT3991 Runs Only When the Input is Present
R2
C5
R1
C1
D1
PCB Layout
GND
ForproperoperationandminimumEMI,caremustbetaken
during printed circuit board layout. Figure 9 shows the
recommended component placement with trace, ground
plane and via locations. Note that large, switched currents
3991 F09
VIAS TO V
VIAS TO LOCAL GROUND PLANE
VIAS TO V
VIAS TO RUN/SS
VIAS TO PG
IN
OUTLINE OF LOCAL
GROUND PLANE
VIAS TO SYNC
OUT
flow in the LT3991’s V and SW pins, the catch diode
IN
(D1), and the input capacitor (C1). The loop formed by
these components should be as small as possible. These
components,alongwiththeinductorandoutputcapacitor,
should be placed on the same side of the circuit board,
and their connections should be made on that layer. Place
a local, unbroken ground plane below these components.
The SW and BOOST nodes should be as small as possible.
Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation
the V pin of the LT3991 can ring to twice the nominal
IN
input voltage, possibly exceeding the LT3991’s rating and
damaging the part. If the input supply is poorly controlled
or the user will be plugging the LT3991 into an energized
supply, the input network should be designed to prevent
this overshoot. See Linear Technology Application Note
88 for a complete discussion.
Finally, keep the FB and R nodes small so that the ground
T
traces will shield them 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 possible, and add thermal vias under and near
the LT3991 to additional ground planes within the circuit
board and on the bottom side.
High Temperature Considerations
For higher ambient temperatures, care should be taken in
the layout of the PCB to ensure good heat sinking of the
LT3991. The Exposed Pad on the bottom of the package
mustbesolderedtoagroundplane.Thisgroundshouldbe
tied to large copper layers below with thermal vias; these
layers will spread heat dissipated by the LT3991. Placing
additional vias can reduce thermal resistance further. The
maximum load current should be derated as the ambient
temperature approaches the maximum junction rating.
Hot Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3991 circuits. However, these ca-
pacitors can cause problems if the LT3991 is plugged into
a live supply. The low loss ceramic capacitor, combined
with stray inductance in series with the power source,
forms an under damped tank circuit, and the voltage at
Power dissipation within the LT3991 can be estimated by
calculatingthetotalpowerlossfromanefficiencymeasure-
ment and subtracting the catch diode loss and inductor
3991fa
18
LT3991/LT3991-3.3/LT3991-5
applicaTions inForMaTion
loss. The die temperature is calculated by multiplying the
LT3991 power dissipation by the thermal resistance from
junction to ambient.
avoid excessive increase in light load supply current at
high temperatures.
Other Linear Technology Publications
Also keep in mind that the leakage current of the power
Schottky diode goes up exponentially with junction tem-
perature. When the power switch is closed, the power
Schottky 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 input power. Therefore,
the catch Schottky diode must be selected with care to
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
2.5V Step-Down Converter
5V Step-Down Converter
V
IN
4.3V TO 55V
V
IN
6.6V TO 55V
V
IN
V
IN
EN
BOOST
SW
EN
BOOST
SW
OFF ON
4.7µF
OFF ON
4.7µF
1µF
0.47µF
10pF
PG
SS
PG
SS
10µH
10µH
LT3991
LT3991
RT
RT
BD
FB
BD
FB
10pF
V
2.5V
1.2A
1M
47µF
V
1M
47µF
162k
OUT
118k
OUT
5V
SYNC GND
SYNC GND
1.2A
909k
309k
f = 300kHz
f = 400kHz
3991 TA03
3991 TA02
3.3V Step-Down Converter
5V Step-Down Converter
V
V
IN
4.3V TO 55V
IN
6.6V TO 55V
V
V
IN
IN
EN
BOOST
SW
EN./UVLO
PG
BOOST
SW
OFF ON
OFF ON
4.7µF
0.47µF
0.47µF
15µH
10µH
10pF
V
3.3V
1.2A
PG
SS
OUT
SS
RT
LT3991
LT3991-5
4.7µF
RT
BD
FB
BD
V
OUT
118k
5V
V
OUT
SYNC GND
1.2A
47µF
1.78M
118k
SYNC GND
3991 TA10
f = 400kHz
1M
47µF
3991 TA09
f = 400kHz
3991fa
19
LT3991/LT3991-3.3/LT3991-5
Typical applicaTions
1.8V Step-Down Converter
V
IN
4.3V TO 37V
V
IN
BD
BOOST
EN
OFF ON
4.7µF
0.47µF
10pF
PG
SS
6.8µH
SW
LT3991
RT
V
1.8V
1.2A
162k
511k
OUT
FB
SYNC GND
1M
100µF
f = 300kHz
3991 TA05
12V Step-Down Converter
V
IN
16V TO 55V
V
IN
EN
BOOST
SW
OFF ON
10µF
0.47µF
10pF
PG
SS
10µH
1M
LT3991
RT
BD
FB
V
OUT
49.9k
f = 800kHz
12V
SYNC GND
1.2A
110k
10µF
3991 TA06
3.3V Step-Down Converter with Undervoltage Lockout, Soft-Start, and Power Good
V
IN
6V TO 55V
5M
V
IN
BOOST
SW
EN
0.47µF
10µH
4.7µF
SS
RT
150k
LT3991
100k
1nF
PG
BD
PGOOD
1M
10pF
1M
V
3.3V
1.2A
OUT
118k
FB
SYNC GND
562k
47µF
f = 400kHz
3991 TA06
3991fa
20
LT3991/LT3991-3.3/LT3991-5
Typical applicaTions
4V Step-Down Converter with a High Impedance Input Source
R
+
–
10M
432k
V
IN
48V
EN
BOOST
SW
+
C
BULK
0.47µF
* AVERAGE OUTPUT POWER CANNOT
10µH
PG
SS
100µF
EXCEED THAT WHICH CAN BE PROVIDED
BY HIGH IMPEDANCE SOURCE.
NAMELY,
LT3991
4.7µF
100k
2nF
2
RT
V
P
=
• η
OUT(MAX)
4R
BD
FB
10pF
WHERE V IS VOLTAGE OF SOURCE, R IS
INTERNAL SOURCE IMPEDANCE, AND η IS
LT3971 EFFICIENCY. MAXIMUM OUTPUT
CURRENT OF 1.2A CAN BE SUPPLIED FOR A
SHORT TIME BASED ON THE ENERGY
WHICH CAN BE SOURCED BY THE BULK
INPUT CAPACITANCE.
V
OUT
118k
1M
4V
1.2A*
SYNC GND
412k
100µF
f = 400kHz
3991 TA07a
Sourcing a Maximum Load Pulse
Start-Up from High Impedance Input Source
V
V
IN
IN
2V/DIV
10V/DIV
V
OUT
V
OUT
200mV/DIV
2V/DIV
I
I
L
L
1A/DIV
1A/DIV
3991 TA07c
3991 TA07b
2ms/DIV
500µs/DIV
3991fa
21
LT3991/LT3991-3.3/LT3991-5
package DescripTion
DD Package
10-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1699 Rev C)
R = 0.125
TYP
6
0.40 ± 0.10
10
0.70 ±0.05
3.55 ±0.05
2.15 ±0.05 (2 SIDES)
1.65 ±0.05
3.00 ±0.10
(4 SIDES)
1.65 ± 0.10
(2 SIDES)
PIN 1 NOTCH
R = 0.20 OR
PIN 1
TOP MARK
(SEE NOTE 6)
0.35 × 45°
PACKAGE
OUTLINE
CHAMFER
(DD) DFN REV C 0310
5
1
0.25 ± 0.05
0.50 BSC
0.75 ±0.05
0.200 REF
0.25 ± 0.05
0.50
BSC
2.38 ±0.10
(2 SIDES)
2.38 ±0.05
(2 SIDES)
0.00 – 0.05
BOTTOM VIEW—EXPOSED PAD
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
NOTE:
1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2).
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT 5. EXPOSED PAD SHALL BE SOLDER PLATED
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE
MSE Package
10-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1664 Rev G)
BOTTOM VIEW OF
EXPOSED PAD OPTION
1.88
(.074)
1.88 ± 0.102
(.074 ± .004)
0.889 ± 0.127
(.035 ± .005)
1
0.29
REF
1.68
(.066)
0.05 REF
5.23
(.206)
MIN
1.68 ± 0.102 3.20 – 3.45
(.066 ± .004) (.126 – .136)
DETAIL “B”
CORNER TAIL IS PART OF
THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
DETAIL “B”
10
NO MEASUREMENT PURPOSE
0.50
(.0197)
BSC
3.00 ± 0.102
(.118 ± .004)
(NOTE 3)
0.305 ± 0.038
(.0120 ± .0015)
TYP
0.497 ± 0.076
(.0196 ± .003)
10 9
8
7 6
REF
RECOMMENDED SOLDER PAD LAYOUT
3.00 ± 0.102
(.118 ± .004)
(NOTE 4)
4.90 ± 0.152
(.193 ± .006)
DETAIL “A”
0.254
(.010)
0° – 6° TYP
GAUGE PLANE
1
2
3
4 5
0.53 ± 0.152
(.021 ± .006)
0.86
(.034)
REF
1.10
(.043)
MAX
DETAIL “A”
0.18
(.007)
SEATING
PLANE
0.17 – 0.27
(.007 – .011)
TYP
0.1016 ± 0.0508
(.004 ± .002)
0.50
(.0197)
BSC
MSOP (MSE) 0910 REV G
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 NOT INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD
SHALL NOT EXCEED 0.254mm (.010") PER SIDE.
3991fa
22
LT3991/LT3991-3.3/LT3991-5
revision hisTory
REV
DATE
DESCRIPTION
PAGE NUMBER
A
01/11 Added 3.3V and 5V fixed voltage options reflected throughout the data sheet.
1-24
3991fa
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
LT3991/LT3991-3.3/LT3991-5
relaTeD parTs
PART
DESCRIPTION
COMMENTS
= 4.2V to 40V, V
LT3970
40V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC
Converter with I = 2.5µA
V
= 1.21V, I = 2.5µA, I <1 µA,
OUT(MIN) Q SD
IN
3mm × 2mm DFN-10, MSOP-10 Packages
Q
LT3990
LT3971
LT3682
LT3689
62V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC
V
IN
= 4.2V to 62V, V = 1.21V, I = 2.5µA, I <1 µA,
OUT(MIN)
Q
SD
Converter with I = 2.5µA
3mm × 2mm DFN-10, MSOP-10 Packages
Q
38V, 1.2A, 2.2MHz High Efficiency Micropower Step-Down DC/DC
V
IN
= 4.3V to 38V, V = 1.21V, I = 2.8µA, I <1 µA,
OUT(MIN)
Q
SD
Converter with I = 2.8µA
3mm × 3mm DFN-10, MSOP-10E Packages
V = 3.6V to 36V, V = 0.8V, I = 75µA, I <1 µA,
IN
Q
36V, 60V Max, 1A, 2.2MHz High Efficiency Micropower Step-Down
DC/DC Converter
OUT(MIN)
Q
SD
3mm × 3mm DFN-12 Package
36V, 60V Transient Protection, 800mA, 2.2MHz High Efficiency
Micropower Step-Down DC/DC Converter with POR Reset and
Watchdog Timer
V
SD
= 3.6V to 36V (Transient to 60V), V
= 0.8V, I = 75µA,
OUT(MIN) Q
IN
I
<1 µA, 3mm × 3mm QFN-16 Package
LT3480
LT3980
36V with Transient Protection to 60V, 2A (I ), 2.4MHz, High
V
SD
= 3.6V to 36V (Transient to 60V), V
= 0.78V, I = 70µA,
OUT
IN
OUT(MIN) Q
Efficiency Step-Down DC/DC Converter with Burst Mode Operation
I
<1 µA, 3mm × 3mm DFN-10, MSOP-10E Packages
58V with Transient Protection to 80V, 2A (I ), 2.4MHz, High
V
SD
= 3.6V to 58V (Transient to 60V), V
= 0.78V, I = 85µA,
OUT
IN
OUT(MIN) Q
Efficiency Step-Down DC/DC Converter with Burst Mode Operation
I
<1 µA, 3mm × 4mm DFN-16, MSOP-16E Packages
3991fa
LT 0211 REV A • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
24
●
●
LINEAR TECHNOLOGY CORPORATION 2009
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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