LT3991/LT3991-3.3/LT3991-5
55V, 1.2A Step-Down
Regulator with 2.8µA
Quiescent Current
Features
Description
Ultralow Quiescent Current:
2.8µA IQ Regulating 12VIN to 3.3VOUT
n Fixed Output Voltages: 3.3V, 5V
2.1µA IQ Regulating 12VIN
n Low Ripple Burst Mode® Operation:
Output Ripple < 15mVP-P
n Wide Input Voltage Range: 4.3V to 55V
n 1.2A Maximum Output Current
n Adjustable Switching Frequency: 200kHz to 2MHz
n Synchronizable Between 250kHz to 2MHz
n Fast Transient Response
n Accurate 1V Enable Pin Threshold
n Low Shutdown Current: I = 700nA
Q
n Power Good Flag
n Soft-Start CapabilityV
n Internal Compensation
n Saturating Switch Design: 0.44Ω On-Resistance
n Output Voltage: 1.19V to 30V
n Small Thermally Enhanced 10-Pin MSOP Package
and (3mm × 3mm) DFN Packages
The LT®3991 is an adjustable frequency monolithic buck
switching regulator that accepts a wide input voltage range
up to 55V. Low quiescent current design consumes only
2.8µA of supply current while regulating with no load. 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
when VOUT reaches 91% of the programmed output voltage. The LT3991 is available in small 10-pin MSOP and
3mm × 3mm DFN packages with exposed pads for low
thermal resistance.
n
Applications
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.
Automotive Battery Regulation
Power for Portable Products
n Industrial Supplies
n
n
Typical Application
No Load Supply Current
3.0
3.3V Step Down Converter
OFF ON
EN./UVLO
VIN
BOOST
0.47µF
PG
SS
4.7µF
INPUT CURRENT (µA)
VIN
4.3V TO 55V
LT3991-3.3
12µH
SW
RT
BD
118k
f = 400kHz
SYNC
GND
VOUT
47µF
3991 TA01a
VOUT
3.3V
1.2A
OUTPUT IN REGULATION
2.5
LT3991-5
2.0
LT3991-3.3
1.5
1.0
5
10 15 20 25 30 35 40 45 50 55
INPUT VOLTAGE (V)
3991 G06
3991fa
1
LT3991/LT3991-3.3/LT3991-5
Absolute Maximum Ratings
(Note 1)
VIN, EN Voltage..........................................................55V
BOOST Pin Voltage....................................................75V
BOOST Pin Above SW Pin..........................................30V
FB, VOUT, RT, SYNC, SS Voltage..................................6V
PG, BD Voltage..........................................................30V
Boost Diode Current.....................................................1A
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)
(MSE Only)........................................................ 300°C
Pin Configuration
LT3991
LT3991
LT3991-3.3, LT3991-5
TOP VIEW
BD
1
BOOST
2
SW
3
VIN
4
EN
5
11
GND
BD
BOOST
SW
VIN
EN
9 PG
8 RT
7 SS
6 FB
DD PACKAGE
10-LEAD (3mm × 3mm) PLASTIC DFN
θJA = 45°C, θJC = 10°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
TOP VIEW
TOP VIEW
10 SYNC
1
2
3
4
5
11
GND
10
9
8
7
6
SYNC
PG
RT
SS
FB
MSE PACKAGE
10-LEAD PLASTIC MSOP
θJA = 45°C, θJC = 10°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
BD
BOOST
SW
VIN
EN
1
2
3
4
5
11
GND
10
9
8
7
6
SYNC
PG
RT
SS
VOUT
MSE PACKAGE
10-LEAD PLASTIC MSOP
θJA = 45°C, θJC = 10°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
Order Information
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3991EDD#PBF
LT3991EDD#TRPBF
LFJR
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3991IDD#PBF
LT3991IDD#TRPBF
LFJR
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3991EMSE#PBF
LT3991EMSE#TRPBF
LTFJS
10-Lead Plastic MSOP
–40°C to 125°C
LT3991IMSE#PBF
LT3991IMSE#TRPBF
LTFJS
10-Lead Plastic MSOP
–40°C to 125°C
LT3991EMSE-3.3#PBF
LT3991EMSE-3.3#TRPBF
LTFRS
10-Lead Plastic MSOP
–40°C to 125°C
LT3991IMSE-3.3#PBF
LT3991IMSE-3.3#TRPBF
LTFRS
10-Lead Plastic MSOP
–40°C to 125°C
LT3991EMSE-5#PBF
LT3991EMSE-5#TRPBF
LTFRV
10-Lead Plastic MSOP
–40°C to 125°C
LT3991IMSE-5#PBF
LT3991IMSE-5#TRPBF
LTFRV
10-Lead Plastic MSOP
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
2
3991fa
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
Minimum Input Voltage
(Note 4)
MIN
Quiescent Current from VIN
VEN Low
VEN High, VSYNC Low
VEN High, VSYNC Low
l
LT3991 FB Pin Current
VFB = 1.19V
l
l
Internal Feedback Resistor Divider
TYP
MAX
4
4.3
V
0.7
1.7
1.2
2.7
4.5
μA
μA
μA
0.1
12
10
1.175
1.165
1.19
1.19
1.205
1.215
V
V
l
3.25
3.224
3.3
3.3
3.35
3.376
V
V
l
4.93
4.89
5
5
5.07
5.11
V
V
0.0002
0.01
%/V
2
1
200
2.4
1.2
240
MHz
MHz
kHz
LT3991-3.3 Output Voltage
LT3991-5 Output Voltage
4.3V < VIN < 40V (Note 4)
Switching Frequency
RT = 11k
RT = 35.7k
RT = 255k
1.6
0.8
160
Minimum Switch On Time
110
Minimum Switch Off Time
150
200
2.3
2.9
Switch Current Limit
Switch VCESAT
nA
MΩ
l
Feedback Voltage
FB Voltage Line Regulation
UNITS
1.7
ISW = 1A
ns
440
Switch Leakage Current
0.02
ns
A
mV
1
μA
Boost Schottky Forward Voltage
ISH = 100mA
800
Boost Schottky Reverse Leakage
VREVERSE = 12V
0.02
1
Minimum Boost Voltage (Note 3)
VIN = 5V
1.4
1.8
V
BOOST Pin Current
ISW = 1A, VBOOST = 15V
25
33
mA
EN Voltage Threshold
EN Rising
1.01
1.07
l
l
0.95
mV
μA
V
EN Voltage Hysteresis
30
EN Pin Current
0.2
20
nA
100
140
mV
LT3991 PG Threshold Offset from VFB
VFB Rising
60
LT3991 PG Hysteresis
LT3991-X PG Threshold Offset from VOUT
20
VOUT Rising
5.5
LT3991X PG Hysteresis
9
mV
12.5
%
1
µA
1.3
PG Leakage
VPG = 3V
PG Sink Current
VPG = 0.4V
SYNC Threshold
0.02
l
300
570
0.6
0.8
SYNC Pin Current
SS Source Current
mV
%
μA
1.0
0.1
VSS = 1V
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The 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.
0.6
1
V
nA
1.6
μA
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 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
Typical Performance Characteristics
Efficiency, VOUT = 5V
90
VIN = 12V
90
VIN = 12V
70
EFFICIENCY (%)
VIN = 24V
70
VIN = 36V
60
VIN = 48V
40 VOUT = 5V
R1 = 1M
R2 = 309k
30
0
0.2
30
1
20
1.2
VIN = 36V
VIN = 48V
50
40
Efficiency, VOUT = 3.3V
80
0
0.2
0.4
0.6
0.8
LOAD CURRENT (A)
1
VIN = 36V
VIN = 48V
30
VIN = 36V
30
VIN = 48V
0
0.01
1.2
0.1
1
10
100
LOAD CURRENT (mA)
1000
3991 G03
No Load Supply Current
3.0
DIODES, INC.
DFLS2100
INPUT CURRENT (µA)
INPUT CURRENT (µA)
EFFICIENCY (%)
VIN = 24V
VIN = 24V
40
10
VIN = 12V
40
50
No Load Supply Current
100
50
60
3991 G02
90
60
VIN = 12V
70
20
3991 G01
70
Efficiency, VOUT = 5V
VOUT = 5V
90 R1 = 1M
R2 = 309k
80
VIN = 24V
60
50
0.4
0.6
0.8
LOAD CURRENT (A)
100
80
80
EFFICIENCY (%)
Efficiency, VOUT = 3.3V
EFFICIENCY (%)
100
TA = 25°C, unless otherwise noted.
10
20
OUTPUT IN REGULATION
2.5
LT3991-5
2.0
LT3991-3.3
1.5
10
0
0.01
0.1
1
10
100
LOAD CURRENT (mA)
1
–55
1000
–25
5
35
65
95
TEMPERATURE (°C)
125
1.0
155
3991 G05
3991 G04
5
10 15 20 25 30 35 40 45 50 55
INPUT VOLTAGE (V)
3991 G06
LT3991-3.3 Output Voltage
LT3991-5 Output Voltage
5.06
1.200
3.330
5.04
1.195
1.190
1.185
1.180
1.175
–55
OUTPUT VOLTAGE (V)
3.345
OUTPUT VOLTAGE (V)
FEEDBACK VOLTAGE (V)
LT3991 Feedback Voltage
1.205
3.315
3.300
3.285
3.270
–25
5
35
65
95
TEMPERATURE (°C)
125
155
3991 G07
3.255
–50 –25
5.00
4.98
4.96
65
35
95
5
TEMPERATURE (°C)
125
155
3991 G28
4
5.02
4.94
–50 –25
65
35
95
5
TEMPERATURE (°C)
125
155
3991 G29
3991fa
LT3991/LT3991-3.3/LT3991-5
Typical Performance Characteristics
2.5
VOUT = 3.3V
2.5
0.30
VOUT = 5V
0.20
MINIMUM
1.5
1.0
LOAD REGULATION (%)
TYPICAL
2.0
TYPICAL
1.5
MINIMUM
1.0
0.5
0.5
REFERENCED FROM VOUT AT 0.5A LOAD
0.25
2.0
LOAD CURRENT (A)
LOAD CURRENT (A)
Load Regulation
Maximum Load Current
Maximum Load Current
3.0
TA = 25°C, unless otherwise noted.
0.15
0.10
0.05
0
–0.05
–0.10
–0.15
–0.20
–0.25
0
5
0
10 15 20 25 30 35 40 45 50 55
INPUT VOLTAGE (V)
5
10 15 20 25 30 35 40 45 50 55
INPUT VOLTAGE (V)
Switching Frequency
FREQUENCY (kHz)
800
750
700
650
2.4
2.5
2.0
1.5
1.0
0.5
2.3
2.2
2.1
2.0
1.9
1.8
1.7
1.6
600
–55
–25
5
35
65
95
TEMPERATURE (°C)
125
0
155
0
20
40
60
DUTY CYCLE (%)
80
100
500
400
300
200
100
900
40
800
35
30
25
20
15
10
500
750 1000 1250
SWITCH CURRENT (mA)
1500
3991 G14
0
700
600
500
400
300
200
100
5
250
155
LT3991 Frequency Foldback
45
SWITCHING FREQUENCY (kHz)
BOOST PIN CURRENT (mA)
600
125
3991 G13
Boost Pin Current
Switch VCESAT
700
0
DUTY CYCLE = 30%
1.5
–55 –25
5
35
65
95
TEMPERATURE (°C)
3991 G12
3991 G11
VCESAT (mV)
1200
2.5
SWITCH CURRENT LIMIT (A)
SWITCH CURRENT LIMIT (A)
950
400
600
800 1000
LOAD CURRENT (mA)
Switch Current Limit
Switch Current Limit
850
200
3991 G10
3.0
1000
900
0
3991 G09
3991 G08
0
–0.30
0
250
500
750 1000 1250
SWITCH CURRENT (mA)
1500
3991 G15
0
0
0.2
0.6
0.4
0.8
FB PIN VOLTAGE (V)
1
1.2
3991 G16
3991fa
5
LT3991/LT3991-3.3/LT3991-5
Typical Performance Characteristics
Minimum Switch On-Time/
Switch Off-Time
400
800
350
700
600
500
400
300
200
Soft-Start
2.5
tOFF(MIN) 1A LOAD
300
250
SWITCH CURRENT LIMIT (A)
900
SWITCH ON/OFF TIME (ns)
SWITCHING FREQUENCY (kHz)
LT3991-X Frequency Foldback
tOFF(MIN) 0.5A LOAD
200
150
tON(MIN)
100
2.0
1.5
1.0
0.5
50
100
0
TA = 25°C, unless otherwise noted.
0
0
–55
100
80
20
60
40
VOUT (% OF REGULATION VOLTAGE)
–25
35
95
5
65
TEMPERATURE (°C)
125
0
155
0
0.25 0.5 0.75 1 1.25 1.5 1.75
SS PIN VOLTAGE (V)
3991 G17
2
3991 G18
3991 G30
Minimum Input Voltage
5.0
VOUT = 3.3V
4.8
VOUT = 5V
1.04
TO START
4.2
4.0
3.8
TO RUN
3.6
3.4
6.0
TO START
5.8
5.6
5.4
5.2
3.2
0
200
600
400
800 1000
LOAD CURRENT (mA)
5.0
1200
TO RUN
0
200
1.4
94
1.2
1.0
0.8
0.6
0.4
0.2
1200
1.00
0.99
1500
3991 G22
FALLING THRESHOLD
0.98
0.97
0.95
–55
–25
5
35
65
95
TEMPERATURE (°C)
125
155
3971 G21
Transient Load Response,
Load Current Stepped from 25mA
(Burst Mode Operation) to 525mA
93
VOUT
100mV/DIV
92
91
90
89
88
87
IL
500mA/DIV
86
250
500
750 1000 1250
BOOST DIODE CURRENT (mA)
RISING THRESHOLD
1.01
Power Good Threshold
95
THRESHOLD VOLTAGE (%)
BOOST DIODE VF (V)
Boost Diode Forward Voltage
6
1.02
3991 G20
1.6
0
1.03
0.96
400
800 1000
600
LOAD CURRENT (mA)
3991 G19
0
THRESHOLD VOLTAGE (V)
4.4
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
1.05
6.2
4.6
3.0
EN Threshold
Minimum Input Voltage
6.4
85
–55
–25
5
35
65
95
TEMPERATURE (°C)
125
155
3991 G23
10µs/DIV
VIN = 48V, VOUT = 3.3V
COUT = 47µF
3991 G24
3991fa
LT3991/LT3991-3.3/LT3991-5
Typical Performance Characteristics
Transient Load Response,
Load Current Stepped from
0.5A to 1A
TA = 25°C, unless otherwise noted.
Switching Waveforms;
Burst Mode Operation
VOUT
100mV/
DIV
Switching Waveforms; Full
Frequency Continuous Operation
VSW
5V/DIV
VSW
5V/DIV
IL
500mA/
DIV
10µs/DIV
VIN = 48V, VOUT = 3.3V
COUT = 47µF
3991 G25
IL
500mA/DIV
IL
500mA/DIV
VOUT
20mV/DIV
VOUT
20mV/DIV
5µs/DIV
VIN = 48V, VOUT = 3.3V
ILOAD = 20mA
COUT = 47µF
3971 G26
1µs/DIV
VIN = 48V, VOUT = 3.3V
ILOAD = 1A
COUT = 47µF
3971 G27
Pin Functions
BD (Pin 1): This pin connects to the anode of the boost
diode. The BD pin is normally connected to the output.
BOOST (Pin 2): This pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar
NPN power switch.
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.
VIN (Pin 4): The VIN pin supplies current to the LT3991’s
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
voltage is 1.005V going up and 0.975V going down. The EN
threshold is only accurate when VIN is above 4.3V. If VIN is
lower than 4.3V, ground EN to place the part in shutdown.
Tie to VIN if shutdown feature is not used.
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
VOUT. Typically this capacitor is 10pF.
VOUT (Pin 6, LT3991-3.3/LT3991-5 Only): The LT39913.3 and LT3991-5 regulate the VOUT pin to 3.3V and 5V
respectively. This pin connects to the internal 10MΩ
feedback divider that programs the fixed output voltage.
SS (Pin 7): A capacitor and a series resistor are tied between
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 softstart. The soft-start resistor has a typical value of 100k.
RT (Pin 8): A resistor is tied between RT and ground to
set the switching frequency.
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
valid when the LT3991 is enabled and VIN is above 4.3V.
at low output loads. Tie to a clock source for synchronization, which will include pulse-skipping at low output
loads. When in pulse-skipping mode, quiescent current
increases to 1.5mA.
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
VIN
C1
INTERNAL 1.19V REF
1V
EN
RT
–
+
VIN
+
–
Σ
SHDN
BD
SLOPE COMP
OSCILLATOR
200kHz TO 2MHz
RT
SWITCH
LATCH
BOOST
R
S
C3
Q
L1
Burst Mode
DETECT
SYNC
PG
ERROR AMP
+
–
+
–
1.09V
D1
1µA
SS
SHDN
R2
FB LT3991
ONLY
GND
R2
C2
VC CLAMP
VC
C5
VOUT
SW
C4
LT3991-3.3
LT3991-5
ONLY
R1
VOUT
R1
3991 BD
C5
8
3991fa
LT3991/LT3991-3.3/LT3991-5
Operation
The LT3991 is a constant frequency, current mode stepdown 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 the VIN and SW pins, turning the switch off when
this current reaches a level determined by the voltage at
VC (see Block Diagram). An error amplifier measures the
output voltage through an external resistor divider tied to
the FB pin and servos the VC node. If the error amplifier’s
output increases, more current is delivered to the output;
if it decreases, less current is delivered. An active clamp
on the VC node provides current limit. The VC node is
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.
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 VIN or from the
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 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
current to 1.7μA. In a typical application, 2.8μA will be consumed from the supply when regulating with no load.
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 startup and overload.
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
enabled and VIN is above 4.3V.
Applications Information
Achieving Ultralow Quiescent Current
1000
VIN = 12V
VOUT = 3.3V
Figure 1. Switching Frequency in Burst Mode Operation
As the output load decreases, the frequency of single current 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
SWITCHING FREQUENCY (kHz)
To enhance efficiency at light loads, the LT3991 operates
in low ripple Burst Mode, which keeps the output capacitor
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 capacitor 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
LOAD CURRENT (mA)
100
120
3991 F01
3991fa
9
LT3991/LT3991-3.3/LT3991-5
Applications Information
performance of the LT3991. The feedback resistors should
preferably be on the order of MΩ and the Schottky catch
diode should have less than 1µA of typical reverse leakage at room temperature. These two considerations are
reiterated in the FB Resistor Network and Catch Diode
Selection sections.
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.
VSW
5V/DIV
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the resistor
values according to:
⎛V
⎞
R1= R2 ⎜ OUT − 1⎟
⎝ 1.19V ⎠
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.
IL
500mA/DIV
VOUT
20mV/DIV
5µs/DIV
3991 F02
VIN = 48V
VOUT = 3.3V
ILOAD = 20mA
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 a 47μF output capacitor, the output ripple
is about 8mV, and with a 100μF output capacitor the output
ripple is about 4mV. The output voltage ripple can continue
to be decreased by increasing the output capacitance.
At higher output loads (above 86mA for the front page
application) the LT3991 will be running at the frequency
programmed by the RT resistor, and will be operating in
standard PWM mode. The transition between PWM and low
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 external clock, the LT3991 will pulse skip at light loads. The
10
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.
When using large FB resistors, a 10pF phase lead capacitor
should be connected from VOUT to FB.
The LT3991-3.3 and LT3991-5 control an internal 10M FB
resistor divider as well as an internal lead capacitor.
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 RT value for a desired switching
frequency is in Table 1.
Table 1. Switching Frequency vs RT Value
SWITCHING FREQUENCY (MHz)
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
RT VALUE (kΩ)
255
118
71.5
49.9
35.7
28.0
22.1
17.4
14.0
11.0
3991fa
LT3991/LT3991-3.3/LT3991-5
Applications Information
Operating Frequency Tradeoffs
Input Voltage Range
Selection of the operating frequency is a tradeoff between
efficiency, component size, minimum dropout voltage, 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, lower
maximum input voltage, and higher dropout voltage. The
highest acceptable switching frequency (fSW(MAX)) for a
given application can be calculated as follows:
The minimum input voltage is determined by either the
LT3991’s minimum operating voltage of 4.3V or by its
maximum duty cycle (see equation in Operating Frequency
Tradeoffs section). The minimum input voltage due to
duty cycle is:
fSW(MAX) =
VOUT + VD
tON(MIN)(VIN − VSW + VD )
where VIN is the typical input voltage, VOUT is the output
voltage, VD is the catch diode drop (~0.5V), and VSW is
the internal switch drop (~0.5V at max load). This equation
shows that slower switching frequency is necessary to
safely accommodate high VIN/VOUT ratio. Also, as shown
in the Input Voltage Range section, lower frequency allows
a lower dropout voltage. The input voltage range depends
on the switching frequency because the LT3991 switch has
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% for part-to-part variation. The minimum and maximum
duty cycles that can be achieved taking minimum on and
off times into account are:
DCMIN = fSW tON(MIN)
DCMAX = 1− fSW tOFF(MIN)
where fSW is the switching frequency, the tON(MIN) is the
minimum switch on-time, and the tOFF(MIN) is the minimum
switch off-time. These equations show that duty cycle
range increases when switching frequency is decreased.
See the Electrical Characteristics section for tON(MIN) and
tOFF(MIN) values.
A good choice of switching frequency should allow adequate input voltage range (see Input Voltage Range section) and keep the inductor and capacitor values small.
VIN(MIN) =
VOUT + VD
− VD + VSW
1− fSW tOFF(MIN)
where VIN(MIN) is the minimum input voltage, VOUT is
the output voltage, VD is the catch diode drop (~0.5V),
VSW is the internal switch drop (~0.5V at max load), fSW
is the switching frequency (set by RT), and tOFF(MIN) is
the minimum switch off-time. Note that higher switching frequency will increase the minimum input voltage.
If a lower dropout voltage is desired, a lower switching
frequency should be used.
The maximum input voltage for LT3991 applications
depends on switching frequency, the Absolute Maximum
Ratings of the VIN and BOOST pins, and the operating
mode. For a given application where the switching frequency and the output voltage are already selected, the
maximum input voltage (VIN(OP-MAX)) that guarantees
optimum output voltage ripple for that application can be
found by applying the following equation:
VIN(OP-MAX) =
VOUT + VD
– VD + VSW
fSW • tON(MIN)
where tON(MIN) is the minimum switch on-time. Note that
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.
The circuit will tolerate inputs above the maximum operating input voltage and up to the Absolute Maximum
Ratings of the VIN and BOOST pins, regardless of chosen
switching frequency. However, during such transients
where VIN is higher than VIN(OP-MAX), the LT3991 will enter
pulse-skipping operation where some switching pulses are
skipped to maintain output regulation. The output voltage
ripple and inductor current ripple will be higher than in
typical operation. Do not overload when VIN is greater
than VIN(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:
L=
VOUT + VD
fSW
where fSW is the switching frequency in MHz, VOUT is the
output voltage, VD is the catch diode drop (~0.5V) and L
is the inductor value in μH.
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 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.
The inductor value must be sufficient to supply the desired
maximum output current (IOUT(MAX)), which is a function
of the switch current limit (ILIM) and the ripple current.
IOUT(MAX) = ILIM –
ΔIL
2
The LT3991 limits its peak switch current in order to protect
itself and the system from overload faults. The LT3991’s
switch current limit (ILIM) is at least 2.33A at low duty
cycles and decreases linearly to 1.8A at DC = 0.8.
Table 2. Inductor Vendors
VENDOR
URL
PART SERIES
TYPE
Murata
www.murata.com
LQH55D
Open
TDK
www.componenttdk.com SLF7045
SLF10145
Shielded
Shielded
Toko
www.toko.com
D62CB
D63CB
D73C
D75F
Shielded
Shielded
Shielded
Open
Coilcraft
www.coilcraft.com
MSS7341
MSS1038
Shielded
Shielded
Sumida
www.sumida.com
CR54
CDRH74
CDRH6D38
CR75
Open
Shielded
Shielded
Open
12
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:
ΔIL =
(1−DC) • (VOUT + VD )
L • fSW
where fSW is the switching frequency of the LT3991, DC is
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
not allow sufficient maximum output current (IOUT(MAX))
given the switching frequency, and maximum input voltage
used in the desired application.
The optimum inductor for a given application may differ
from the one indicated by this simple design guide. A larger
value inductor provides a higher maximum load current and
reduces the output voltage ripple. If your load is lower than
the maximum load current, than you can relax the value of
the inductor and operate with higher ripple current. This
allows you to use a physically smaller inductor, or one with
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, low inductance may result in discontinuous mode
operation, which further reduces maximum load current.
For details of maximum output current and discontinuous
operation, see Linear Technology’s Application Note 44.
Finally, for duty cycles greater than 50% (VOUT/VIN>0.5),
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 inductors, 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
continuous. Discontinuous operation occurs when IOUT
is less than ΔIL/2.
3991fa
LT3991/LT3991-3.3/LT3991-5
Applications Information
Input Capacitor
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.
Step-down regulators draw current from the input supply 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).
A second precaution regarding the ceramic input capacitor
concerns the maximum input voltage rating of the LT3991.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank circuit. 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).
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by the
LT3991 to produce the DC output. In this role it determines
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
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
COUT =
100
VOUT fSW
where fSW is in MHz, and COUT is the recommended output
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
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
WEBSITE
AVX
www.avxcorp.com
Murata
www.murata.com
Taiyo Yuden
www.t-yuden.com
Vishay Siliconix
www.vishay.com
TDK
www.tdk.com
Catch Diode Selection
The catch diode (D1 from Block Diagram) conducts current only during switch off time. Average forward current
in normal operation can be calculated from:
ID(AVG) = IOUT
VIN – VOUT
VIN
where IOUT is the output load current. The only reason to
consider a diode with a larger current rating than necessary
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.
IAVE
(A)
MBR0520L
20
0.5
MBR0540
40
0.5
620
MBRM120E
20
1
530
MBRM140
40
1
550
B0530W
30
0.5
B0540W
40
0.5
620
1
B120
20
1
500
1.1
B130
30
1
500
1.1
B140
40
1
500
1.1
B150
50
1
700
0.4
B220
20
2
500
20
B230
30
2
500
0.6
B140HB
40
1
DFLS240L
40
2
DFLS140
40
1.1
510
1
DFLS160
60
1
500
2.5
DFLS2100
100
2
770
B240
40
2
PART NUMBER
VF at 1A
(mV)
VF at 2A
(mV)
IR at VR =
20V 25°C
(µA)
VR
(V)
On Semiconductor
30
0.4
595
0.5
20
Diodes Inc.
15
1
500
4
860
0.01
500
0.45
Central Semiconductor
CMSH1 - 40M
40
1
500
CMSH1 - 60M
60
1
700
CMSH1 - 40ML
40
1
400
CMSH2 - 40M
40
2
550
CMSH2 - 60M
60
2
700
CMSH2 - 40L
40
2
400
CMSH2 - 40
40
2
500
CMSH2 - 60M
60
2
700
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
14
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.
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3991 due to their piezoelectric nature.
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 operation, 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.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT3991. As previously mentioned, a ceramic input capacitor combined
with trace or cable inductance forms a high quality (under
damped) tank circuit. 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 rating. This situation
is easily avoided (see the Hot Plugging Safely section).
BOOST and BD Pin Considerations
Capacitor C3 and the internal boost Schottky diode (see
the 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. 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.
For outputs of 3V and above, the standard circuit (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
3991fa
LT3991/LT3991-3.3/LT3991-5
Applications Information
while using the internal boost diode. For reliable BOOST pin
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
circuit in Figure 3a is more efficient because the BOOST pin
current comes from a lower voltage source. You must also
be sure that the maximum voltage ratings of the BOOST
and BD pins are not exceeded.
The minimum operating voltage of an LT3991 application
is limited by the minimum input voltage (4.3V) and by
VIN
BD
VIN
BOOST
LT3991
4.7µF
GND
C3
SW
VOUT
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 worst-case situation where VIN is ramping very slowly.
For lower start-up voltage, the boost diode can be tied to
VIN; however, this restricts the input range to one-half of
the absolute maximum rating of the BOOST pin.
5.0
4.8
4.6
VIN
D2
BD
VIN
BOOST
LT3991
4.7µF
INPUT VOLTAGE (V)
(3a) For VOUT > 2.8V
GND
TO START
4.2
4.0
TO RUN
3.8
3.6
3.4 VOUT = 3.3V
TA = 25°C
3.2 L = 10µH
f = 400kHz
3.0
10
C3
SW
4.4
VOUT
100
LOAD CURRENT (mA)
1000
100
LOAD CURRENT (mA)
1000
6.4
(3b) For 2.5V < VOUT < 2.8V
BD
VIN
BOOST
LT3991
4.7µF
GND
C3
VOUT
SW
INPUT VOLTAGE (V)
VIN
6.2
6.0
5.8
5.6
5.4
5.2
3991 FO3
(3c) For VOUT < 2.5V; VIN(MAX) = 27V
Figure 3. Three Circuits for Generating the Boost Voltage
TO START
5.0
TO RUN
VOUT = 5V
TA = 25°C
L = 10µH
f = 400kHz
10
3991 F04
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
Enable Pin
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
can be tied to VIN if the shutdown feature is not used.
Adding a resistor divider from VIN to EN programs the
LT3991 to regulate the output only when VIN is above a
desired voltage (see Figure 5). Typically, this threshold,
VIN(EN), is used in situations 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 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 VIN(EN) threshold prevents
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:
R3
+1
R4
where output regulation should not start until VIN is above
VIN(EN). Due to the comparator’s hysteresis, regulation will
not stop until the input falls slightly below VIN(EN).
Be aware that when the input voltage is below 4.3V, the
input current may rise to several hundred μA. And the part
may be able to switch at cold or for VIN(EN) thresholds less
than 7V. Figure 6 shows the magnitude of the increased
input current in a typical application with different programmed VIN(EN).
When operating in Burst Mode for light load currents, the
current through the VIN(EN) resistor network can easily be
greater than the supply current consumed by the LT3991.
Therefore, the VIN(EN) resistors should be large to minimize
their effect on efficiency at low loads.
400
R3
EN
1V
+
–
200
0
0
1
2
3
VIN(EN) = 12V
R3 = 11M
R4 = 1M
4 5 6 7 8 9 10 11 12
INPUT VOLTAGE (V)
6V VIN(EN) Input Current
500
400
INPUT CURRENT (µA)
LT3991
300
100
VIN(EN) =
VIN
12V VIN(EN) Input Current
500
INPUT CURRENT (µA)
At light loads, the inductor current becomes discontinuous and this reduces the minimum input voltage to approximately 400mV above VOUT. At higher load currents,
the inductor current is continuous and the duty cycle is
limited by the maximum duty cycle of the LT3991, requiring
a higher input voltage to maintain regulation.
300
200
100
SHDN
0
R4
3991 F05
Figure 5. Programmed Enable Threshold
0
1
VIN(EN) = 6V
R3 = 5M
R4 = 1M
2
3
4
INPUT VOLTAGE (V)
5
6
3991 F06
Figure 6. Input Current vs Input Voltage
for a Programmed VIN(EN) of 6V and 12V
16
3991fa
LT3991/LT3991-3.3/LT3991-5
Applications Information
Soft-Start
The SS pin can be used to soft-start the LT3991 by throttling
the maximum input current during start-up. An internal 1μA
current source charges an external capacitor generating a
voltage ramp on the SS pin. The SS pin clamps the internal
VC node, which slowly ramps 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. A 100k resistor
in series with the soft-start capacitor is recommended.
Figure 7 shows start-up waveforms for a typical application
with a 10nF capacitor and a 100k resistor on SS for a 3.3Ω
load when the EN pin is pulsed high for 10ms.
The external SS capacitor is only actively discharged when
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.
VSS
0.5V/DIV
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.
The LT3991 may be synchronized over a 250kHz to 2MHz
range. The RT resistor should be chosen to set the LT3991
switching frequency 20% below the lowest synchronization
input. For example, if the synchronization signal will be
250kHz and higher, the RT should be selected for 200kHz.
To assure reliable and safe operation the LT3991 will only
synchronize when the output voltage is near regulation as
indicated by the PG flag. It is therefore necessary to choose
a large enough inductor value to supply the required output
current at the frequency set by the RT resistor (see the
Inductor Selection section). The slope compensation is set
by the RT value, while the minimum slope compensation
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 RT, than the slope compensation will be
sufficient for all synchronization frequencies.
Shorted and Reversed Input Protection
VOUT
2V/DIV
IL
0.5A/DIV
2ms/DIV
3991 F07
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
Synchronization
To select low ripple Burst Mode operation, tie the SYNC pin
below 0.6V (this can be ground or a logic low output).
Synchronizing the LT3991 oscillator to an external frequency 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).
If the inductor is chosen so that it won’t saturate excessively, 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 applications or in battery backup systems where a battery
or some other supply is diode ORed with the LT3991’s
output. If the VIN pin is allowed to float and the EN pin
is held high (either by a logic signal or because it is tied
to VIN), then the LT3991’s internal circuitry will pull its
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 VIN pin is grounded while the output
is held high, regardless of EN, parasitic diodes inside the
LT3991 can pull current from the output through the SW
pin and the VIN pin. Figure 8 shows a circuit that will run
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
VIN
VIN
BOOST
EN
SW
L1
C2
VOUT
VOUT
LT3991
GND
BD
FB
+
BACKUP
RPG
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
PCB Layout
For proper operation and minimum EMI, care must be taken
during printed circuit board layout. Figure 9 shows the
recommended component placement with trace, ground
plane and via locations. Note that large, switched currents
flow in the LT3991’s VIN and SW pins, the catch diode
(D1), and the input capacitor (C1). The loop formed by
these components 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, unbroken ground plane below these components.
The SW and BOOST nodes should be as small as possible.
Finally, keep the FB and RT nodes small so that the ground
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.
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 capacitors 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
18
C5
D1
GND
RT
C3
R1
R2
C1
GND
3991 F09
VIAS TO LOCAL GROUND PLANE
VIAS TO VOUT
VIAS TO SYNC
VIAS TO RUN/SS
VIAS TO PG
VIAS TO VIN
OUTLINE OF LOCAL
GROUND PLANE
Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation
the VIN pin of the LT3991 can ring to twice the nominal
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.
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
must be soldered to a ground plane. This ground should be
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.
Power dissipation within the LT3991 can be estimated by
calculating the total power loss from an efficiency measurement and subtracting the catch diode loss and inductor
3991fa
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.
Also keep in mind that the leakage current of the power
Schottky diode goes up exponentially with junction temperature. 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
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
2.5V Step-Down Converter
5V Step-Down Converter
VIN
4.3V TO 55V
VIN
6.6V TO 55V
VIN
VIN
EN
OFF ON
0.47µF
PG
4.7µF
SS
SW
LT3991
EN
OFF ON
BOOST
BOOST
1µF
PG
10µH
4.7µF
SS
SW
LT3991
10µH
RT
RT
BD
118k
SYNC
VOUT
5V
1.2A
1M
FB
GND
BD
10pF
47µF
309k
f = 400kHz
162k
SYNC
10pF
909k
f = 300kHz
VOUT
2.5V
1.2A
1M
FB
GND
47µF
3991 TA03
3991 TA02
3.3V Step-Down Converter
5V Step-Down Converter
VIN
4.3V TO 55V
VIN
6.6V TO 55V
EN
OFF ON
VIN
BOOST
0.47µF
PG
SS
4.7µF
OFF ON
LT3991
10µH
SW
VOUT
3.3V
1.2A
BOOST
0.47µF 15µH
SS
4.7µF
LT3991-5
SW
RT
BD
f = 400kHz
VIN
PG
RT
118k
EN./UVLO
SYNC
GND
BD
10pF
118k
1.78M
FB
47µF
1M
f = 400kHz
SYNC
GND
VOUT
5V
1.2A
VOUT
47µF
3991 TA10
3991 TA09
3991fa
19
LT3991/LT3991-3.3/LT3991-5
Typical Applications
1.8V Step-Down Converter
VIN
4.3V TO 37V
VIN
BD
EN
OFF ON
BOOST
0.47µF
PG
SS
4.7µF
SW
LT3991
6.8µH
RT
10pF
162k
SYNC
GND
1M
f = 300kHz
VOUT
1.8V
1.2A
511k
FB
100µF
3991 TA05
12V Step-Down Converter
VIN
16V TO 55V
VIN
EN
OFF ON
BOOST
0.47µF
PG
10µF
SS
SW
LT3991
10µH
RT
BD
49.9k
SYNC
GND
10pF
110k
f = 800kHz
VOUT
12V
1.2A
1M
FB
10µF
3991 TA06
3.3V Step-Down Converter with Undervoltage Lockout, Soft-Start, and Power Good
VIN
6V TO 55V
5M
VIN
BOOST
EN
0.47µF
SW
4.7µF
SS
100k
150k
LT3991
RT
PG
BD
1M
PGOOD
10pF
1nF
118k
10µH
SYNC
f = 400kHz
GND
1M
FB
562k
47µF
VOUT
3.3V
1.2A
3991 TA06
20
3991fa
LT3991/LT3991-3.3/LT3991-5
Typical Applications
4V Step-Down Converter with a High Impedance Input Source
R
+
–
10M
48V
+
VIN
EN
CBULK
100µF
432k
0.47µF
PG
SS
4.7µF
BOOST
100k
LT3991
SW
* AVERAGE OUTPUT POWER CANNOT
EXCEED THAT WHICH CAN BE PROVIDED
BY HIGH IMPEDANCE SOURCE.
NAMELY,
V2
•η
POUT(MAX) =
4R
10µH
RT
BD
2nF
10pF
118k
SYNC
f = 400kHz
GND
VOUT
4V
1.2A*
1M
FB
412k
100µF
3991 TA07a
Sourcing a Maximum Load Pulse
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.
Start-Up from High Impedance Input Source
VIN
2V/DIV
VIN
10V/DIV
VOUT
200mV/DIV
VOUT
2V/DIV
IL
1A/DIV
IL
1A/DIV
500µs/DIV
3991 TA07b
2ms/DIV
3991 TA07c
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
1.65 ±0.05
2.15 ±0.05 (2 SIDES)
1.65 ± 0.10
(2 SIDES)
3.00 ±0.10
(4 SIDES)
PIN 1 NOTCH
R = 0.20 OR
0.35 × 45°
CHAMFER
PIN 1
PACKAGE
TOP MARK
OUTLINE (SEE NOTE 6)
0.25 ± 0.05
0.50
BSC
2.38 ±0.05
(2 SIDES)
5
0.75 ±0.05
0.200 REF
0.00 – 0.05
1
(DD) DFN REV C 0310
0.25 ± 0.05
0.50 BSC
2.38 ±0.10
(2 SIDES)
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).
CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
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
5. EXPOSED PAD SHALL BE SOLDER PLATED
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 ± 0.102
(.074 ± .004)
5.23
(.206)
MIN
0.889 ± 0.127
(.035 ± .005)
1
0.05 REF
10
0.254
(.010)
3.00 ± 0.102
(.118 ± .004)
(NOTE 3)
10 9 8 7 6
0.53 ± 0.152
(.021 ± .006)
DETAIL “A”
3.00 ± 0.102
(.118 ± .004)
(NOTE 4)
SEATING
PLANE
1 2 3 4 5
0.86
(.034)
REF
1.10
(.043)
MAX
0.17 – 0.27
(.007 – .011)
TYP
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 NOT INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD
SHALL NOT EXCEED 0.254mm (.010") PER SIDE.
22
0.497 ± 0.076
(.0196 ± .003)
REF
0° – 6° TYP
GAUGE PLANE
0.18
(.007)
DETAIL “B”
CORNER TAIL IS PART OF
DETAIL “B” THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
NO MEASUREMENT PURPOSE
4.90 ± 0.152
(.193 ± .006)
DETAIL “A”
0.29
REF
1.68
(.066)
1.68 ± 0.102 3.20 – 3.45
(.066 ± .004) (.126 – .136)
0.50
0.305 ± 0.038
(.0197)
(.0120 ± .0015)
BSC
TYP
RECOMMENDED SOLDER PAD LAYOUT
1.88
(.074)
0.1016 ± 0.0508
(.004 ± .002)
MSOP (MSE) 0910 REV G
3991fa
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 representation 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
LT3970
40V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC
Converter with IQ = 2.5µA
VIN = 4.2V to 40V, VOUT(MIN) = 1.21V, IQ = 2.5µA, ISD
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