LINER LT3500 36v, 4a, 1.5mhz synchronous step-down switching regulator Datasheet

LT3690
36V, 4A, 1.5MHz Synchronous
Step-Down Switching Regulator
with 70µA Quiescent Current
DESCRIPTION
FEATURES
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Wide Input Range:
– Operation from 3.9V to 36V
– Overvoltage Lockout Protects Circuits
Through 60V Transients
4A Maximum Output Current
Integrated 30mΩ N-Channel Synchronous Switch
Low Ripple (<15mVP-P) Burst Mode® Operation:
IQ = 70µA at 12VIN to 3.3VOUT
Programmable Input Undervoltage Lockout
0.8V Feedback Reference Voltage
Output Voltage: 0.8V to 20V
Programmable and Synchronizable Oscillator
(170kHz to 1.5MHz)
Soft-Startup and Output Voltage Tracking
Short-Circuit Robust
Power Good Flag
Small Thermally Enhanced 4mm × 6mm QFN Package
APPLICATIONS
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Automotive Systems
Industrial Supplies
Distributed Supply Regulation
The LT®3690 is an adjustable frequency monolithic buck
switching regulator that accepts input voltages up to 36V.
A high efficiency 90mΩ switch is included on the device
along with the boost diode and the necessary oscillator,
control, and logic circuitry. The internal synchronous power
switch of 30mΩ increases efficiency and eliminates the
need for an external Schottky catch diode. Current mode
topology is used for fast transient response and good
loop stability. Shutdown reduces input supply current to
less than 1µA. The low ripple Burst Mode maintains high
efficiency at low output currents while keeping output
ripple below 15mV in typical applications.
The LT3690 features robust operation and is easily configurable. Using a resistor divider on the UVLO pin provides a
programmable undervoltage lockout. A power good flag
signals when VOUT reaches 90% of the programmed output voltage. Protection circuitry senses the current in the
power switches to protect the LT3690 against short-circuit
conditions. Frequency foldback and thermal shutdown
provide additional protection. The LT3690 is available in
a 4mm × 6mm QFN package with exposed pads for low
thermal resistance.
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks of
Linear Technology Corporation. All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
3.3V Step-Down Converter
Efficiency and Power Loss
VIN
4.5V TO 36V
100
VOUT = 5V
0.68µF
VIN
EN
90
BST
UVLO
SS
22k
680pF
0.47µF
SW
LT3690
3.3V
4A
BIAS
VC
PG
VCCINT
FB
SYNC
RT
GND
ƒ = 600kHz
80
1.5
VOUT = 5V
70
60
100µF
102k
50
1.0
VOUT = 3.3V
316k
32.4k
2.0
VOUT = 3.3V
0.5
VIN = 12V
L = 4.7µH
ƒ = 600kHz
0
0.5
1
1.5 2 2.5 3
LOAD CURRENT (A)
3.5
POWER LOSS (W)
3.3µH
EFFICIENCY (%)
10µF
2.5
4
0
3690 TA01b
3690 TA01a
3690f
1
LT3690
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
EN, UVLO, VIN Voltage (Note 2).................................60V
BST Voltage ..............................................................55V
BST Voltage Above SW Voltage ................................30V
BIAS, PG Voltage ......................................................30V
FB, RT, SS, SYNC, VC, VCCINT Voltage .........................6V
SW
1
SW
2
SW
3
26 SW
25 SW
27
SW
24 SW
SW
4
23 SW
SYNC
5
22 BST
GND
6
21 GND
RT
7
VC
8
FB
9
18 PG
GND 10
17 EN
20 VCCINT
GND 16
19 BIAS
VIN 15
VIN 14
VIN 13
SS 11
28
GND
UVLO 12
Operating Junction Temperature Range (Notes 3 and 4)
LT3690E ............................................ –40°C to 125°C
LT3690I ............................................. –40°C to 125°C
Storage Temperature Range .................. –65°C to 150°C
TOP VIEW
UFE PACKAGE
26-LEAD (4mm × 6mm) PLASTIC QFN
θJA = 40°C/W, θJC = 2.7°C/W
EXPOSED PAD (PIN 27) IS SW, MUST BE SOLDERED TO PCB
EXPOSED PAD (PIN 28) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3690EUFE#PBF
LT3690EUFE#TRPBF
3690
26-Lead (4mm × 6mm) Plastic QFN
–40°C to 125°C
LT3690IUFE#PBF
LT3690IUFE#TRPBF
3690
26-Lead (4mm × 6mm) Plastic QFN
–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/
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C, VIN = 12V unless otherwise noted (Notes 3, 7).
PARAMETER
CONDITIONS
TYP
MAX
3.0
3.9
38.2
40
V
0.1
1
µA
35
60
µA
VBIAS = 0V, VFB = 0.85V Not Switching
110
150
µA
VEN = 0.2V
0.1
1
µA
70
110
µA
–3
–10
µA
VIN Fixed Undervoltage Lockout
VIN Overvoltage Lockout OVLO
VIN Rising
Quiescent Current from VIN
VEN = 0.2V
VBIAS = 3V, VFB = 0.85V Not Switching
Quiescent Current from BIAS Pin
MIN
l
VBIAS = 3V, VFB = 0.85V Not Switching
VBIAS = 0V, VFB = 0.85V Not Switching
l
l
l
36
UNITS
V
3690f
2
LT3690
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C, VIN = 12V unless otherwise noted (Notes 3, 7).
PARAMETER
CONDITIONS
Boost Schottky Diode Drop (VBIAS – VBST)
IBIAS = 200mA
BST Voltage (Note 5) (VBST – VSW)
Minimum BOOST Voltage Above SW, ISW = 4A
MIN
l
TYP
MAX
UNITS
820
950
mV
1.6
2.3
V
BST Pin Current
ISW = 4A
70
140
mA
BST Pin Leakage
VSW = 12V, VBIAS = 0V
0.1
6
µA
HS Switch Drop (VIN – VSW)
ISW = 4A
370
600
mV
HS Switch Current Limit (Note 6)
HS Switch Leakage Current
5.5
VSW = 0V
HS Minimum Switch Off-Time
6.6
8
A
0.1
2
µA
210
ns
LS Switch Off Voltage Drop
ISW = 4A
700
850
mV
LS Switch On-Resistance
ISW = 4A, VCCINT = 5V
30
60
mΩ
LS Switch On-Resistance
ISW = 4A, VCCINT = 4V
30
90
mΩ
5
6.5
A
0.1
10
µA
l
LS Switch Current Threshold
LS Switch Leakage Current
4
VEN = 0V, VSW = 12V, VBST =12V
95
µA
VCCINT Pin Output Voltage
VEN = 0V, VSW = 12V, VBST = 12V, TJ = 125°C
IVCCINT = 0µA
4.3
4.9
5.3
V
VCCINT Pin Output Voltage
IVCCINT = –10mA
4.2
4.8
5.3
V
EN Input Current
VEN = 12V
8
15
µA
VEN = 2.5V
2.5
6
µA
EN Input Voltage, Enable
1.5
V
EN Input Voltage, Disable
0.4
UVLO Threshold Voltage
1.1
V
1.33
V
UVLO Pin Current
VUVLO = 1.33V
–2.0
–3.8
µA
UVLO Pin Current
VUVLO = 1.1V
0.1
1
µA
UVLO Pin Current Hysteresis
IUVH – IUVL
1.2
2
2.8
µA
Pull-Up Current at SS Pin
VSS = 0.8V
–1.2
–2
–2.8
µA
Tracking Offset (VSS – VFB)
VSS = 0.4V
–4
7
15
mV
0.4
V
SYNC Input Voltage High
0.8
V
SYNC Input Voltage Low
SYNC Input Resistance to GND
150
SYNC Input Frequency
0.17
Feedback Reference Voltage
l
FB Pin Bias Current Flows Out of Pin
VFB = 800mV
FB Voltage Line Regulation
3.6V < VIN < 36V
PG Threshold as Percentage of VFB
VFB Rising
786
l
88
PG Hysteresis
PG Sink Current
VPG = 0.3V
PG Leakage
VPG = 5V
l
100
300
800
600
kΩ
1.5
MHz
814
mV
–8
–40
nA
0.001
0.01
%/V
90
92
%
12
mV
500
µA
0.1
1
µA
Error Amplifier Transconductance
400
µA/V
Error Amp Voltage Gain
60
dB
3690f
3
LT3690
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C, VIN = 12V unless otherwise noted (Notes 3, 7).
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
VC Source Current
–50
µA
VC Sink Current
50
µA
VC Pin to Switch Current Gain
Transconductance
4.6
A/V
VC Switching Threshold
0.7
V
VC Clamp Voltage
2.0
V
Programmable Switching Frequency
RT = 10kΩ
1.32
1.5
1.68
MHz
RT = 24.9kΩ
660
750
840
kHz
RT = 180kΩ
122
138
154
kHz
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: Absolute Maximum Voltage at the EN, UVLO and VIN pins is 60V
for non-repetitive 1 second transients, and 40V for continuous operation.
Note 3: The LT3690E is guaranteed to meet performance specifications
from 0°C to 125°C junction temperature. Specifications over the –40°C
to 125°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls. The
LT3690I is guaranteed over the full –40°C to 125°C operating junction
temperature range.
Note 4: This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed the maximum operating junction temperature
when overtemperature protection is active. Continuous operation above
the specified maximum operating junction temperature may impair device
reliability.
Note 5: This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the switch.
Note 6: Current limit guaranteed by design and/or correlation to static test.
Slope compensation reduces current limit at higher duty cycles. Current
limit reduced when feedback voltage is below the reference voltage.
Note 7: The voltages are referred to GND and currents are assumed
positive, when the current flows into the pin. Negative magnitudes are
shown as maximum.
3690f
4
LT3690
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency and Power Loss
100
3.0
90
2.5
90
2.5
80
2.0
80
2.0
70
1.5
70
1.5
60
VOUT = 3.3V 1.0
L = 3.3µH
ƒ = 500kHz
VIN = 12V 0.5
VIN = 24V
VIN = 34V
0
3 3.5 4
60
50
40
0
0.5
1
1.5 2 2.5
LOAD CURRENT (A)
VOUT = 5V
L = 4.7µH 1.0
ƒ = 600kHz
VIN = 12V 0.5
VIN = 24V
VIN = 34V
0
3 3.5
4
EFFICIENCY (%)
3.0
50
40
0
0.5
1
1.5 2 2.5
LOAD CURRENT (A)
3690 G01
3690 G02
Efficiency and Power Loss
3.0
2.5
80
2.0
70
1.5
60
VOUT = 3.3V 1.0
L = 3.3µH
ƒ = 600kHz
VIN = 12V 0.5
VIN = 24V
VIN = 34V
0
3 3.5
50
40
0
0.5
1
1.5 2 2.5
LOAD CURRENT (A)
No Load Supply Current vs VIN
VOUT = 3.3V
140
INPUT CURRENT (µA)
90
160
POWER LOSS (W)
EFFICIENCY (%)
100
120
100
80
60
40
20
0
0
5
10
15
20
25
INPUT VOLTAGE (V)
7.0
Maximum Load Current vs VIN
100
50
7.0
VOUT = 5V
L = 4.7µH
ƒ = 600kHz
6.5
150
3690 G04
6.0
TYPICAL
5.5
5.0
MINIMUM
4.5
0
25 50 75 100 125 150
TEMPERATURE (°C)
3690 G05
3.5
VOUT = 3.3V
L = 4.7µH
ƒ = 600kHz
6.0
TYPICAL
5.5
5.0
MINIMUM
4.5
4.0
4.0
0
–50 –25
Maximum Load Current vs VIN
6.5
LOAD CURRENT (A)
VIN = 12V
VOUT = 3.3V
LOAD CURRENT (A)
SUPPLY CURRENT (µA)
200
35
30
3690 G03
No Load Supply Current
vs Temperature
POWER LOSS (W)
100
POWER LOSS (W)
EFFICIENCY (%)
Efficiency and Power Loss
TA = 25°C, unless otherwise noted.
5
10
15
20
25
INPUT VOLTAGE (V)
30
35
3690 G06
3.5
5
10
15
20
25
INPUT VOLTAGE (V)
30
35
3690 G07
3690f
5
LT3690
TYPICAL PERFORMANCE CHARACTERISTICS
Switch Current Limit
vs Duty Cycle
TA = 25°C, unless otherwise noted.
Switch Current Limit
vs Temperature
8
0.6
8.0
VFB > 0.75V
6
VFB = 0V
5
7.0
HS SWITCH VOLTAGE DROP (V)
7
HS SWITCH CURRENT LIMIT (A)
HS SWITCH CURRENT LIMIT (A)
7.5
DUTY CYCLE = 10%
6.5
6.0
DUTY CYCLE = 90%
5.5
5.0
4.5
4.0
Switch Voltage Drop vs ISW
0.5
0.4
0.3
0.2
0.1
3.5
4
0
20
40
60
DUTY CYCLE (%)
80
3.0
–50
100
–25
0
25
50
75
TEMPERATURE (°C)
3690 G08
1.4
BST DIODE VOLTAGE DROP (V)
BST PIN CURRENT (mA)
80
60
40
20
0
0
1
2
3
4
SWITCH CURRENT (A)
5
0.8
0.6
0.4
0.2
0
0.5
1
1.5
2
BST DIODE CURRENT (A)
4
3
2
0
2.5
40
1
10
100
1000
LOAD CURRENT (mA)
10000
3690 G14
0.4
0.6 0.8 1.0 1.2
VOLTAGE DROP (V)
INPUT VOLTAGE (V)
30
4.0
3.5
2.5
10
100
1000
LOAD CURRENT (mA)
1.6
LIMITED BY
TJ = 125°C
25
20
VOUT = 3.3V
L = 3.3µH
ƒ = 600kHz
15
10
1
1.4
Maximum VIN for Fixed Frequency
VSYNC > 0.8V, TA = 25°C
VSYNC > 0.8V, TA = 85°C
VSYNC < 0.4V, TA = 25°C
VSYNC < 0.4V, TA = 85°C
5
4.0
0.2
35
3.0
4.5
0
3690 G13
VOUT = 3.3V
L = 4.7µH
ƒ = 600kHz
4.5
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
5
1
5.0
5.0
Catch Diode Voltage Drop
(VGND – VSW) vs ISW
3690 G12
VOUT = 5V
L = 4.7µH
ƒ = 600kHz
6
5
6
Minimum Input Voltage
vs Load Current
5.5
2
3
4
SWITCH CURRENT (A)
7
1.0
Minimum Input Voltage
vs Load Current
6.0
8
1.2
0
6
Boost Diode Drop (VBIAS – VBST)
vs IBST
3690 G11
6.5
1
3690 G09
BST Pin Current vs ISW
100
0
3690 G10
SWITCH CURRENT (A)
120
0
125
100
10000
3690 G15
0
0
1
2
3
SWITCH CURRENT (A)
4
3690 G16
3690f
6
LT3690
TYPICAL PERFORMANCE CHARACTERISTICS
2.0
EN THRESHOLD VOLTAGE (V)
EN PIN CURRENT (µA)
6
4
2
0
5
10
15
20
25
EN PIN VOLTAGE (V)
2.5
2.5
8
0
UVLO Threshold Voltage
vs Temperature
EN Threshold Voltage
30
UVLO THRESHOLD VOLTAGE (V)
10
EN Pin Current vs VEN
TA = 25°C, unless otherwise noted.
1.5
1.0
0.5
0
–50
35
–25
0
25
50
75
TEMPERATURE (°C)
100
1.5
1.0
0.5
0
–50
125
–25
0
25
50
75
TEMPERATURE (°C)
UVLO Pin Current vs Temperature
(VUVLO = 1.33V)
100
125
3690 G19
3690 G18
3690 G17
–1.4
2.0
Power Good Threshold
vs Temperature
Feedback Voltage vs Temperature
0.82
95
0.81
90
–2.0
–2.2
–2.4
–2.6
PG THRESHOLD (%)
–1.8
FB PIN VOLTAGE (V)
UVLO PIN CURRENT (µA)
–1.6
0.80
0.79
85
80
–2.8
–25
0
25
50
75
TEMPERATURE (°C)
100
125
0.78
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
3690 G20
60
2.5
VC Voltages vs Temperature
800
–20
125
Frequency Foldback vs VFB
RT = 32.4k
600
1.5
1.0
SWITCHING THRESHOLD
500
400
300
200
0.5
–40
100
700
2.0
VC PIN VOLTAGE (V)
VC PIN CURRENT (µA)
40
0
0
25
50
75
TEMPERATURE (°C)
3690 G22
CURRENT LIMIT CLAMP
20
–25
3690 G21
Error Amp Output Current vs VFB
–60
0.6
75
–50
125
FREQUENCY (kHz)
–3.0
–50
100
0.7
0.8
0.9
FB PIN VOLTAGE (V)
1.0
3690 G23
0
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
125
3690 G24
0
0
0.1
0.2
0.3 0.4 0.5 0.6
FB PIN VOLTAGE (V)
0.7
0.8
3690 G25
3690f
7
LT3690
TYPICAL PERFORMANCE CHARACTERISTICS
Minimum Switch On-Time
vs Temperature
Switching Frequency
vs Temperature
MINIMUM SWITCH ON-TIME (ns)
RT = 32.4k
FREQUENCY (kHz)
630
610
590
570
550
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
–1.4
–1.6
200
160
120
60
40
0
–50
125
–1.8
–2.0
–2.2
–2.4
–2.6
–2.8
–25
0
25
50
75
TEMPERATURE (°C)
100
125
–3.0
–50
VCCINT vs VIN
VVCCINT Current Limit
–10
4
0
0
1
2
3 4 5 6 7
VIN PIN VOLTAGE (V)
8
9
10
–20
–30
–50
3690 G29
3
2
0
1
2
3
4
VCCINT PIN VOLTAGE (V)
5
0
–50 –25
IL
0.5A/DIV
VOUT
10mV/DIV
3690 G32
25 50 75 100 125 150
TEMPERATURE (°C)
3690 G31
Switching Waveforms, Full
Frequency Continuous Operation
VSW
5V/DIV
VSW
5V/DIV
IL
0.5A/DIV
IL
1A/DIV
VOUT
10mV/DIV
VOUT
10mV/DIV
1µs/DIV
VIN = 12V, ILOAD = 200mA
FRONT PAGE APPLICATION
0
3690 G30
Switching Waveforms, Transition
from Burst Mode Operation to Full
Frequency
VSW
5V/DIV
125
1
–40
Switching Waveforms,
Burst Mode Operation
5µs/DIV
VIN = 12V, ILOAD = 20mA
FRONT PAGE APPLICATION
VCCINT PIN VOLTAGE (V)
4
VCCINT PIN CURRENT (mA)
5
1
100
VCCINT Pin Voltage
0
2
0
25
50
75
TEMPERATURE (°C)
3690 G28
5
3
–25
3690 G27
3690 G26
VCCINT PIN VOLTAGE (V)
Soft-Start Pin Current
240
SOFT-START PIN CURRENT (µA)
650
TA = 25°C, unless otherwise noted.
3690 G33
1µs/DIV
3690 G34
VIN = 12V, ILOAD = 2A
FRONT PAGE APPLICATION
3690f
8
LT3690
PIN FUNCTIONS
SW (Pins 1-4, 23-26, Exposed Pad Pin 27): The SW pin
is the emitter output of the internal highside NPN power
switch (HS) and the drain output of the internal lowside
power N-channel switch (LS). Connect this pin to the
inductor and boost capacitor. This pin is driven up to the
VIN voltage by the HS switch during the on-time of the
PWM duty cycle. The inductor current drives the SW pin
negative during the off-time. The on-resistance of the LS
switch and the internal Schottky diode fixes the negative
voltage.
The exposed pad is connected internally with SW pins
1-4, 23-26 and should be soldered to a large copper area
to reduce thermal resistance.
SYNC (Pin 5): The SYNC pin is used to synchronize the
internal oscillator to an external signal. It is directly logic
compatible and can be driven with any signal between
20% and 80% duty cycle. The synchronizing range is
from 170kHz to 1.5MHz. See the Synchronization section
in the Applications Information section for details. When
not used for synchronization, the SYNC pin can be tied to
ground to select low ripple Burst Mode operation or tied
to the output voltage to select standard PWM mode.
RT (Pin 7): Oscillator Resistor Input. Connecting a resistor to ground (Pin 10) from this pin sets the switching
frequency.
VC (Pin 8): The VC pin is the output of the internal error
amplifier. The voltage on this pin controls the peak switch
current. Tie an RC network from this pin to ground to
compensate the control loop.
FB (Pin 9): The LT3690 regulates the FB pin to 0.8V.
Connect the feedback resistor divider tap to this pin. The
adjacent ground pin (Pin 10) is recommended for the
resistor divider.
SS (Pin 11): The SS pin is used to provide a soft-start
or tracking function. The internal 2µA pull-up current
ISS in combination with an external capacitor tied to this
pin creates a voltage ramp. The output voltage tracks to
this voltage. For tracking, tie a resistor divider to this pin
from the tracked output. In undervoltage, overvoltage
and thermal shutdown, the SS pin pulls low if the output
voltage is below the power good threshold to restart the
output voltage with soft-start behavior. Leave this pin
disconnected if unused.
UVLO (Pin 12): Tie a resistor divider between VIN, UVLO,
and GND to program an undervoltage lockout threshold.
The UVLO pin has an accurate 1.25V threshold. Above the
threshold, the part operates normally. Below the threshold,
the part drops into a low quiescent current state. See the
Undervoltage Lockout section in the Applications Information section for more details.
VIN (Pins 13, 14, 15): The VIN pin supplies current to
the LT3690’s internal regulator and to the internal power
switch. This pin must be locally bypassed.
EN (Pin 17): The EN input is used to put the LT3690 in
shutdown mode. Pull to GND to shut down the LT3690.
Tie to 1.5V or more for normal operation.
PG (Pin 18): The PG pin is the open collector output of
an internal comparator. PG remains low until the FB pin is
within 10% of the final regulation voltage. The PG output
is valid when VIN is above 3.9V and EN is high.
BIAS (Pin 19): This pin connects to the anode of the internal
boost Schottky diode. BIAS also supplies the current to
the LT3690’s internal regulator. Tie this pin to the lowest
available voltage source above 3V (typically VOUT).
VCCINT (Pin 20): VCCINT is an output of the internally generated supply voltage for the synchronous power DMOS
transistor driver. An external capacitor CVCC must be connected between this pin and ground (Pin 21) to buffer the
internal supply voltage of the LS switch.
BST (Pin 22): This pin is used to provide, with the external
boost capacitor, a drive voltage higher than the input voltage VIN to the internal bipolar NPN power switch.
GND (Exposed Pad Pin 28, Pin 6, Pin 10, Pin 16, Pin 21):
Ground. The exposed pad is connected internally to GND
Pins 6, 10, 16 and 21, and should be soldered to a large
copper area to reduce thermal resistance.
3690f
9
LT3690
BLOCK DIAGRAM
VIN
14
CIN
15
16
17
VIN
CURRENT SENSE
VIN
VIN
+
–
13
GND
19
EN
2.5V
REFERENCE
2µA
12
5
7
BST
0.8V
22
0.72V
SLEEP
VIN MONITOR
OVLO
TEMPERATURE
MONITOR
UVLO
CBST
TSD
HS
SWITCH
UVLO
SYNC
OSCILLATOR
0.17MHz
TO 1.5MHz
SYNC
–
+
SLOPE
COMP
R
S
FF
Q
SWITCH
CONTROL
+
–
CSS
TSD
OVLO
UVLO
R1
9
R2
10
POWER GOOD
+
–
0.72V
VC CLAMP
OVLO
UVLO
TSD
L
VOUT
COUT
VCCINT
20
LS
SWITCH
+
–
ERROR AMP
+
gm
–
SS
PG
23, 24,
25, 26
ZERO
2µA
18
SW
CVCC
+
–
0.8V
1, 2
3, 4
REG
SOFT-START/TRACKING
2.5V
SW
VIN
BURST MODE
DETECT
RT
RT
11
BIAS
6
GND
21
OVERLOAD
VC
8
RC
CF
CC
FB
GND
3690 BD
3690f
10
LT3690
OPERATION
The LT3690 is a constant frequency, current mode stepdown regulator. An oscillator, with frequency set by RT,
enables an RS flip-flop, turning on the internal high side
(HS) power switch. An amplifier and comparator monitor
the current flowing between the VIN and SW pins, turning the RS flip-flop and HS switch off when this current
reaches a level determined by the voltage at VC.
While the high side switch is off, the inductor current conducts through the catch diode and the turned on low side
(LS) switch until either the next clock pulse of the oscillator
starts the next cycle, or the inductor current becomes too
low, as indicated by the zero crossing comparator. This
prevents the inductor from running reverse current.
An error amplifier measures the output voltage through
an external resistor divider tied to the FB pin and servos
the VC pin. 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 pin provides
current limit.
The SS node acts as an auxiliary input to the error amplifier. The voltage at FB will servo to the SS voltage until SS
goes above 0.8V. Soft-start is implemented by generating
a voltage ramp at the SS pin using an external capacitor
CSS which is charged by an internal constant current.
Alternatively, connecting the SS pin to a resistive divider
between the voltage to be tracked and ground provides a
tracking function.
An internal regulator provides power to the control circuitry. The bias regulator normally draws power from the
VIN pin, but if the BIAS pin is connected to an external
voltage higher than 3V, bias power will be drawn from the
external source (typically the regulated output voltage).
This improves efficiency.
The EN pin is used to place the LT3690 in shutdown,
disconnecting the output and reducing the input current
to less than 1µA. A comparator monitors the voltage at
the UVLO input. A external resistive divider connected to
VIN programs the wake up threshold and hysteresis. If
unused, connect the input to VIN or above 1.5V.
The HS switch driver operates from either the input 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.
The synchronously driven N-channel transistor (LS switch)
in parallel with the catch diode reduces the overall solution
size and improves efficiency. Internal overload comparator
circuitry monitors the current through the LS switch and
delays the generation of new switch pulses if this current is
too high (above 5A nominal). This mechanism also protects
the part during short-circuit and overload conditions by
keeping the current through the inductor under control.
A short-circuit protected regulator at VCCINT supplies the
LS driver. The LS switch only operates at VCCINT voltages
greater than 3.8V.
To further optimize efficiency, the LT3690 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 70µA in a typical application. Pulling the
SYNC pin above 0.8V prevents Burst Mode operation. The
positive edge of an external clock signal at the SYNC pin
synchronizes the internal oscillator and therefore switching.
The oscillator reduces the LT3690’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 conditions.
The LT3690 contains a power good comparator which trips
when the FB pin is at 90% of its regulated value. The PG
output is an open-collector 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 LT3690 is
enabled and VIN is above 3.9V.
The LT3690 has an overvoltage protection feature which
disables switching action when VIN goes above 38V (typical)
during transients. When switching is disabled, the LT3690
can safely sustain transient input voltages up to 60V.
3690f
11
LT3690
APPLICATIONS INFORMATION
FB Resistor Network
Operating Frequency Trade-Offs
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the resistor
values according to:
Selection of the operating frequency is a trade-off 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:
⎛V
⎞
R1 = R2 ⎜ OUT − 1⎟
⎝ 0.8V ⎠
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
Setting the Switching Frequency
The LT3690 uses a constant frequency PWM architecture
that can be programmed to switch from 150kHz to 1.5MHz
by using a resistor tied from the RT pin to ground. Table 1
shows the necessary RT value for a desired switching
frequency.
Table 1. Switching Frequency vs RT Value
SWITCHING FREQUENCY (MHz)
RT VALUE (kΩ)
0.15
164
0.2
117
0.3
72.9
0.4
52.2
0.5
40.2
0.6
32.4
0.7
26.8
0.8
22.7
0.9
19.6
1.0
17.0
1.1
15.0
1.2
13.3
1.3
11.8
1.4
10.6
1.5
9.59
ƒSW(MAX) =
VOUT + VLS
tON(MIN) • ( VIN − VSW + VLS )
where VIN is the typical input voltage, VOUT is the output
voltage, VLS is the LS switch drop (0.12V at maximum load)
and VSW is the internal switch drop (0.37V at maximum
load). This equation shows that slower switching frequency
is necessary to accommodate high VIN /VOUT ratio. Also, as
shown in the Input Voltage Range section, lower frequency
allows a lower dropout voltage. Input voltage range depends
on the switching frequency because the LT3690 switch has
finite minimum on and off times. An internal timer forces
the switch to be off for at least tOFF(MIN) per cycle; this timer
has a maximum value of 210ns over temperature. On the
other hand, delays associated with turning off the power
switch dictate the minimum on-time tON(MIN) before the
switch can be turned off; tON(MIN) has a maximum value
of 210ns over temperature. The minimum and maximum
duty cycles that can be achieved taking minimum on and
off times into account are:
DCMIN = ƒSW • tON(MIN)
DCMAX = 1 – ƒSW • tOFF(MIN)
where ƒSW is the switching frequency, the tON(MIN) is the
minimum switch on-time (210ns), and the tOFF(MIN) is the
minimum switch off-time (210ns). These equations show
that duty cycle range increases when switching frequency
is decreased.
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.
3690f
12
LT3690
APPLICATIONS INFORMATION
Input Voltage Range
The minimum input voltage is determined by either the
LT3690’s minimum operating voltage of 3.9V (VBIAS >
3V) or by its maximum duty cycle (see equation in the
Operating Frequency Trade-offs section). The minimum
input voltage due to duty cycle limitation is:
VIN(MIN) =
VOUT + VLS
− VLS + VSW
1− ƒSW • tOFF(MIN)
where VIN(MIN) is the minimum input voltage, and tOFF(MIN)
is the minimum switch off-time (210ns). 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 LT3690 applications
depends on switching frequency, the absolute maximum
ratings of the VIN and BST pins, and the operating mode.
The LT3690 can operate from continuous input voltages
up to 36V. Input voltage transients of up to 60V are also
safely withstood. However, note that while VIN > VOVLO
(typically 38V), the LT3690 will stop switching, allowing
the output to fall out of regulation.
For a given application where the switching frequency
and the output voltage are already fixed, the maximum
input voltage that guarantees optimum output voltage
ripple for that application can be found by applying the
following expression:
V
+V
VIN(MAX) = OUT LS − VLS + VSW
ƒSW • tON(MIN)
where VIN(MAX) is the maximum operating input voltage,
VOUT is the output voltage, VLS is the LS switch drop
(0.12V at maximum load), VSW is the internal switch drop
(0.37V at maximum load), fSW is the switching frequency
(set by RT), and tON(MIN) is the minimum switch on-time
(210ns). Note that a higher switching frequency will reduce the maximum operating input voltage. Conversely,
a lower switching frequency will be necessary to achieve
optimum operation at high input voltages.
Special attention must be paid when the output is in startup, short-circuit, or other overload conditions. In these
cases, the LT3690 tries to bring the output in regulation by
driving current into the output load. During these events,
the inductor peak current might easily reach and even
exceed the maximum current limit of the LT3690, especially in those cases where the switch already operates at
minimum on-time. The circuitry monitoring the current
through the LS switch prevents the HS switch from turning on again if the inductor valley current is above IPSDLIM
(5A nominal). In these cases, the inductor peak current is
therefore the maximum current limit of the LT3690 plus
the additional current overshoot during the turn-off delay
due to minimum on-time:
IL(PEAK) = 8A +
VIN(MAX) − VOUTOL
L
• tON(MIN)
where IL(PEAK) is the peak inductor current, VIN(MAX) is
the maximum expected input voltage, L is the inductor
value, tON(MIN) is the minimum on-time and VOUTOL is
the output voltage under the overload condition. The part
is robust enough to survive prolonged operation under
these conditions as long as the peak inductor current does
not exceed 9A. Inductor current saturation and excessive
junction temperature may further limit performance.
If the output is in regulation and no short-circuit, startup, or overload events are expected, then input voltage
transients of up to VOVLO are acceptable regardless of the
switching frequency. In this case, the LT3690 may enter
pulse-skipping operation where some switching pulses
are skipped to maintain output regulation. In this mode,
the output voltage ripple and inductor current ripple will
be higher than in normal operation.
3690f
13
LT3690
APPLICATIONS INFORMATION
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
(
)
L = VOUT + VLS •
0.67MHz
ƒSW
where VLS is the voltage drop of the low side switch (0.12V),
ƒSW is in MHz, and L is in μH. The inductor’s RMS current
rating must be greater than the maximum load current and
its saturation current should be at least 30% higher. For
highest efficiency, the series resistance (DCR) should be
less than 0.03Ω. Table 2 lists several vendors and types
that are suitable.
Table 2. Inductor Vendors
VENDOR
URL
PART SERIES
Murata
www.murata.com
LQH6P
TDK
www.tdk.com
CLF10040T
SLF10165T
Toko
www.toko.com
DEM8045C
FDVE1040
Coilcraft
www.coilcraft.com
MSS1048
Sumida
www.sumida.com
CDRH8D43
CDRH105R
Vishay
www.vishay.com
IHLP-2525EZ
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, then 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 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 mode operation, see Application Note 44.
Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5),
a minimum inductance is required to avoid sub-harmonic
oscillations:
0.42MHz
LMIN = ( VOUT + VLS ) •
ƒSW
where VLS is the voltage drop of the low side switch (0.12V
at maximum load), ƒSW is in MHz, and LMIN is in μH.
The current in the inductor is a triangle wave with an average
value equal to the load current. The peak switch current
is equal to the output current plus half the peak-to-peak
inductor ripple current. The LT3690 limits its switch current in order to protect itself and the system from overload
faults. Therefore, the maximum output current that the
LT3690 will deliver depends on the switch current limit,
the inductor value, and the input and output voltages.
When the switch is off, the potential across the inductor
is the output voltage plus the low side switch drop. This
gives the peak-to-peak ripple current in the inductor:
ΔIL =
(1− DC) ( VOUT + VLS )
(L • ƒSW )
where ƒSW is the switching frequency of the LT3690 and L
is the value of the inductor. The peak inductor and switch
current is:
ΔIL
ISW(PK) = IL(PK) = IOUT +
2
To maintain output regulation, this peak current must be
less than the LT3690’s switch current limit ILIM. See the
Typical Performance graphs for the change in current
limit vs duty cycle.
Choosing an inductor value so that the ripple current is
small will allow a maximum output current near the switch
current limit.
3690f
14
LT3690
APPLICATIONS INFORMATION
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
these equations to check that the LT3690 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.
Input Capacitor
Bypass the input of the LT3690 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 10µF ceramic capacitor is adequate to bypass
the LT3690, and easily handles the ripple current. Note
that larger input capacitance is required when a lower
switching frequency is used. 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 lower 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 LT3690 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 10µF capacitor is capable of this task, but only if it is
placed close to the LT3690 (see the PCB Layout section).
A second precaution regarding the ceramic input capacitor
concerns the maximum input voltage rating of the LT3690.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank
circuit. If the LT3690 circuit is plugged into a live supply,
the input voltage can ring to twice its nominal value, possibly exceeding the LT3690’s maximum voltage rating. See
Application Note 88 for more details.
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by the
LT3690 to produce the DC output. In this role it determines
the output ripple, and low impedance at the switching
frequency is important. The second function is to store
energy in order to satisfy transient loads and stabilize the
LT3690’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
150
COUT =
VOUT • ƒSW
where ƒSW is in MHz, and COUT is the recommended output capacitance in µF. Use X5R or X7R types, which will
provide low output ripple and good transient response.
Using a high value capacitor on the output can improve
transient performance, but a phase lead capacitor across
the feedback resistor R1 may be required to get the full
benefit (see the Frequency Compensation section).
High performance electrolytic capacitors can be used
for the output capacitor. If using an electrolytic capacitor,
choose one intended for use in switching regulators, and
with a specified ESR of 0.03Ω or less. Such a capacitor will
be larger than a ceramic capacitor and will have a larger
capacitance because the capacitor must be large to achieve
low ESR. Table 3 lists several capacitor vendors.
Table 3. Capacitor Vendors
VENDOR
PART SERIES
COMMENTS
Panasonic
Ceramic, Polymer, Tantalum
EEF Series
Kemet
Ceramic, Tantalum
T494, T495
Sanyo
Ceramic, Polymer, Tantalum
POSCAP
Murata
Ceramic
AVX
Ceramic, Tantalum
Taiyo Yuden
Ceramic
TPS Series
Ceramic Capacitors
Ceramic capacitors are small, robust and have very
low ESR. However, ceramic capacitors can sometimes
cause problems when used with the LT3690 due to their
piezoelectric nature. When in Burst Mode operation, the
LT3690’s switching frequency depends on the load current,
and at very light loads the LT3690 can excite the ceramic
capacitor at audio frequencies, generating audible noise.
3690f
15
LT3690
APPLICATIONS INFORMATION
Since the LT3690 operates at a lower current limit during
Burst Mode operation, the noise is typically very quiet. If
this is unacceptable, use a high performance tantalum or
electrolytic capacitor at the output.
Frequency Compensation
The LT3690 uses current mode control to regulate the
output. This simplifies loop compensation. In particular, the
LT3690 does not require the ESR of the output capacitor
for stability, so you are free to use ceramic capacitors to
achieve low output ripple and small circuit size.
Frequency compensation is provided by the components
tied to the VC pin, as shown in Figure 1. Generally a capacitor
(CC) and a resistor (RC) in series to ground are used. In
addition, there may be a lower value capacitor in parallel.
This capacitor (CF) is not part of the loop compensation
but is used to filter noise at the switching frequency, and
is required only if a phase-lead capacitor is used or if the
output capacitor has high ESR.
Loop compensation determines the stability and transient
performance. The best values for the compensation network depend on the application and, in particular, the type
of output capacitor. A practical approach is to start with
one of the circuits in this data sheet that is similar to your
application and tune the compensation network to optimize
the performance. Stability should then be checked across all
operating conditions, including load current, input voltage
and temperature. The LT1375 data sheet contains a more
thorough discussion of loop compensation and describes
how to test the stability using a transient load.
Figure 1 shows an equivalent circuit for the LT3690 control
loop. The error amplifier is a transconductance amplifier
with finite output impedance. The power section, consisting
of the modulator, power switch and inductor, is modeled
as a transconductance amplifier generating an output
current proportional to the voltage at the VC pin. Note that
the output capacitor integrates this current, and that the
capacitor on the VC pin (CC) integrates the error amplifier
output current, resulting in two poles in the loop. In most
cases, a zero is required and comes either from the output
capacitor ESR or from a resistor RC in series with CC.
This simple model works well as long as the value of the
inductor is not too high and the loop crossover frequency
is much lower than the switching frequency. A phase lead
capacitor (CPL) across the feedback divider may improve
the transient response.
LT3690
CURRENT MODE
POWER STAGE
gm = 4.6S
SW
FB
R1
CPL
–
gm = 400µS
0.8V
+
3M
VC
CF
GND
RC
CC
VOUT
100mV/DIV
OUTPUT
ESR
+
C1
C1
20µs/DIV
CERAMIC
POLYMER
OR
TANTALUM
R2
OR
ELECTROLITIC
IL
2A/DIV
3690 F02
VIN = 12V, ILOAD STEPPED BETWEEN 0.6A AND 3.5A
FRONT PAGE APPLICATION
Figure 2. Transient Load Response
3690 F01
Figure 1. Model for Loop Response
3690f
16
LT3690
APPLICATIONS INFORMATION
Low-Ripple Burst Mode and Pulse-Skipping Mode
The LT3690 is capable of operating in either low ripple
Burst Mode operation or pulse-skipping mode, which is
selected using the SYNC pin. See the Synchronization and
Mode section for details.
To enhance efficiency at light loads, the LT3690 can be
operated in low ripple Burst Mode operation that keeps
the output capacitor charged to the proper voltage while
minimizing the input quiescent current. During Burst Mode
operation, the LT3690 delivers single cycle bursts of current
to the output capacitor followed by sleep periods where
the output capacitor is delivers output power to the load.
Because the LT3690 delivers power to the output with
single, low current pulses, the output ripple stays below
15mV for a typical application. In addition, VIN and BIAS
quiescent currents are reduced to 35µA and 70µA (typical),
respectively, during the sleep time. As the load current
decreases towards a no-load condition, the percentage
of time that the LT3690 operates in sleep mode increases
and the average input current is greatly reduced, resulting
in high efficiency even at very low loads (see Figure 3).
At higher output loads (above about 385mA at VIN = 12V
for the front page application) the LT3690 will run at the
frequency programmed by the RT resistor, and operate in
standard PWM mode. The transition between PWM and
low ripple Burst Mode operation is seamless, and does
not disturb the output voltage.
If low quiescent current is not required, the LT3690 can
operate in pulse-skipping mode. The benefit of this mode
is that the LT3690 will enter full frequency standard PWM
operation at a lower output load current than when in
Burst Mode operation. The front page application circuit
will switch at full frequency at output loads higher than
about 64mA at VIN = 12V.
Low Side Switch Considerations
The operation of the internal low side switch is optimized
for reliable, high efficiency operation. The low side switch
is connected in parallel with a catch diode. When the top
side switch turns off, the inductor current pulls the SW
pin low, and forward biases the internal catch diode. In
order to prevent shoot through currents, the internal low
side switch only turns on after detecting the SW pin going
low. Once the low side switch turns on, the voltage drop
between SW and GND is very small, minimizing power
loss and improving efficiency. At the end of the switching
cycle, the low side switch turns off, and after a delay, the
top side switch can turn on again. The switching sequence
is shown in Figure 4.
The overload comparator monitors the current flowing
through the low side switch and helps protect the circuit.
This comparator delays switching if the low side switch
current goes higher than 5A (typical) during a fault condition such as a shorted output with high input voltage.
VIN = 12V
VOUT = 3.3V
L = 3.3µH
VSW
5V/DIV
VSW
2V/DIV
IL
0.5A/DIV
VOUT
10mV/DIV
0V
5µs/DIV
3690 F03
200ns/DIV
3690 F04
VIN = 12V: ILOAD = 20mA
FRONT PAGE APPLICATION
Figure 3. Burst Mode Operation
Figure 4. Switching Sequence of High Side,
Catch Diode and Low Side Switch
3690f
17
LT3690
APPLICATIONS INFORMATION
The switching will only resume once the low side switch
current has fallen below the 5A limit. This way, the comparator regulates the valley current of the inductor to
5A during short-circuit. With properly chosen external
components, this will ensure that the part will survive a
short-circuit event.
VCCINT Considerations
The linear voltage regulator requires a capacitor of 0.47µF
to deliver the peak current for the gate driver of the low
side N-channel transistor. The output voltage is monitored
by a comparator. To ensure proper operation, the low side
driver only turns on if VCCINT is above 3.8V (typ).
BST and BIAS Pin Considerations
Capacitor CBST and the internal boost Schottky diode (see
the Block Diagram) are used to generate boost voltages
that are higher than the input voltage. In most cases a
0.68µF capacitor will work well. Figure 5 shows three
ways to arrange the boost circuit. The BST pin must be
more than 2.3V above the SW pin for best efficiency. For
outputs of 3V and above, the standard circuit (Figure 5a)
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
while using the internal boost diode. For reliable BST pin
operation with 2.5V outputs, use a good external Schottky
diode (such as the ON Semi MBR0540), and a 1µF boost
capacitor (see Figure 5b). For lower output voltages, the
boost diode can be tied to the input (Figure 5c), or to
another supply greater than 2.8V. The circuit in Figure 5a
is more efficient because the BST pin current and BIAS
pin quiescent current comes from a lower voltage source.
However, the full benefit of the BIAS pin is not realized
unless it is at least 3V. Ensure that the maximum voltage
ratings of the BST and BIAS pins are not exceeded.
The minimum operating voltage of an LT3690 application
is limited by the minimum input voltage (3.9V) and by the
maximum duty cycle as outlined in the Input Voltage Range
section. For proper start-up, the minimum input voltage
VIN
BIAS
VIN
LT3690
CBST
VOUT
BST
SW
GND
3690 F05a
(5a) VOUT > 2.8
VIN
VIN
BIAS
VIN
LT3690
D2
BIAS
VIN
CBST
VOUT
BST
LT3690
SW
CBST
VOUT
BST
SW
GND
GND
3690 F05b
(5b) 2.5V < VOUT < 2.8V
3690 F05c
(5c) VOUT < 2.5V, VIN(MAX) = 27V
Figure 5. Three Circuits for Generating the Boost Voltage
3690f
18
LT3690
APPLICATIONS INFORMATION
6.0
TO START
5.0
4.5
4.0
VOUT = 5V
L = 4.7µH
ƒ = 600kHz
TO START
7.0
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
5.5
7.5
VOUT = 3.3V
L = 4.7µH
ƒ = 600kHz
6.5
6.0
5.5
TO RUN
TO RUN
3.5
3.0
5.0
1
10
100
1000
LOAD CURRENT (mA)
10000
4.5
1
10
100
1000
LOAD CURRENT (mA)
3690 F06a
10000
3690 F06b
Figure 6. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit
is also limited by the boost circuit. If the input voltage is
ramped slowly, or the LT3690 is turned on with its EN pin
when the output is already in regulation, then the boost
capacitor may not be fully charged. Because the boost
capacitor charges with the energy stored in the inductor,
the circuit relies on some minimum load current to get the
boost circuit running properly. This minimum load depends
on the 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 6 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 BST pin. At light loads, the inductor current becomes
discontinuous and the effective duty cycle can be very high.
This reduces the minimum input voltage to approximately
300mV above VOUT. At higher load currents, the inductor
current is continuous and the duty cycle is limited by the
maximum duty cycle of the LT3690, requiring a higher
input voltage to maintain regulation.
Soft-Start
The SS (soft-start) pin provides a soft-start function. If a
capacitor CSS is tied from the SS pin to ground, then the
internal pull-up current will generate a voltage ramp on this
pin. A good value for the soft-start capacitor is COUT /10000,
where COUT is the value of the output capacitor.
The soft-start function limits peak input current to the circuit
during start-up. The output of the LT3690 regulates to the
lowest voltage present at either the SS pin or an internal
0.8V reference. A capacitor from the SS pin to ground is
charged by an internal 2μA current source resulting in a
linear output ramp from 0V to the regulated output voltage.
The ramp duration is given by:
C • 0.8V
t RAMP = SS
2µA
At power-up, an internal open-collector output discharges
the SS pin. The SS pin can be left floating if the soft-start
feature is not used. The internal current sources will charge
this pin to ~2V as shown in Figure 7.
VEN
2V/DIV
VSS
1V/DIV
VOUT
2V/DIV
IL
2A/DIV
CSS = 0.22µF
50ms/DIV
3690 F07
Figure 7. Soft-Start Ramp
3690f
19
LT3690
APPLICATIONS INFORMATION
LT3690
SS
OUT1
5V
VEN
2V/DIV
0.1µF
VOUT1
2V/DIV
LT3690
SS
OUT2
3.3V
0.047µF
VOUT2
2V/DIV
3690 F08a
5ms/DIV
3690 F08b
5ms/DIV
3690 F08d
(8a) Independent Start-Up
LT3690
SS
OUT1
5V
0.22µF
VOUT1
2V/DIV
LT3690
SS
OUT2
VEN
2V/DIV
3.3V
VOUT2
2V/DIV
3690 F08c
(8b) Ratiometric Start-Up
Figure 8. Output Tracking and Sequencing
3690f
20
LT3690
APPLICATIONS INFORMATION
LT3690
SS
5V
OUT1
VEN
2V/DIV
0.1µF
R1
28.7k
VOUT1
2V/DIV
LT3690
R2
10k
SS
OUT2
VOUT2
2V/DIV
3.3V
5ms/DIV
3690 F09b
5ms/DIV
3690 F09d
3690 F09a
(9a) Coincident Start-Up
SS
LT3690
OUT1
5V
VEN
2V/DIV
PG1
0.1µF
VOUT1
2V/DIV
LT3690
SS
OUT2
VOUT2
2V/DIV
3.3V
0.047µF
3690 F09b
(9b) Output Sequencing
Figure 9. Output Tracking and Sequencing
3690f
21
LT3690
APPLICATIONS INFORMATION
Output Tracking and Sequencing
Output tracking and sequencing between voltage regulators
can be implemented using the LT3690’s SS and PG pins.
Figures 8 and 9 show several configurations for output
tracking and sequencing of the LT3690 and an additional
regulator. Independent soft-start for each channel is shown
in Figure 8a. The output ramp time for each output is set
by the soft-start capacitor as described in the soft-start
section.
Ratiometric tracking is achieved in Figure 8b by connecting
SS pins of two regulators together. In this configuration,
the SS pin current is set by the sum of the SS pin currents
of the two regulators, which must be taken into account
when calculating the output rise time.
By connecting a feedback network from OUT1 to the
SS pin with the same ratio that set the OUT2 voltage,
absolute tracking shown in Figure 9a is implemented. A
small OUT2 voltage offset will be present due to the SS
pin’s 2µA source current. This offset can be corrected by
slightly reducing the value of R2.
Figure 9b illustrates output sequencing. When VOUT1 is
within 10% of its regulated voltage, PG releases the SS
soft-start pin, allowing VOUT2 to soft-start.
Synchronization
To select low-ripple Burst Mode operation, tie the SYNC
pin below 0.4V (this can be ground or a logic output).
Synchronize the LT3690 oscillator to an external frequency
by connecting a square wave (with positive and negative
pulse width > 100ns) to the SYNC pin. The square wave
amplitude should have valleys that are below 0.4V and
peaks that are above 1V (up to 6V).
and safe operation, the LT3690 will synchronize when the
output voltage is above 90% of its regulated voltage. 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). It is
also important to note that slope compensation is set by
the RT value. When the synchronization frequency is much
higher than the one set by RT, the slope compensation is
significantly reduced, which may require a larger inductor
value to prevent sub-harmonic oscillation.
For duty cycles greater than 50% (VOUT /VIN > 0.5), a
minimum inductance is required to avoid sub-harmonic
oscillations:
0.42MHz
LMIN = ( VOUT + VLS ) •
ƒSW
where VLS is the voltage drop of the low side switch
(0.12V at maximum load), ƒSW is in MHz, and LMIN is in
μH. For ƒSW in the above calculation, use the frequency
programmed by RT, not the synchronization frequency.
Undervoltage Lockout
Figure 10 shows how to add undervoltage lockout (UVLO)
to the LT3690. Typically, UVLO 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.
VIN
VIN
2µA
The LT3690 will not enter Burst Mode operation at low
output loads while synchronized to an external clock, but
instead will skip pulses to maintain regulation.
The LT3690 may be synchronized over a 170kHz to 1.5MHz
range. The RT resistor should be chosen to set the LT3690
switching frequency 20% below the lowest synchronization
input. For example, if the synchronization signal will be
350kHz and higher, choose RT for 280kHz. To assure reliable
LT3690
R3
UVLO
C4
R4
1.25V
–
+
SLEEP
3690 F10
Figure 10. Undervoltage Lockout
3690f
22
LT3690
APPLICATIONS INFORMATION
The UVLO circuitry prevents the regulator from operating
at source voltages where the problems might occur. An
internal comparator will force the part into shutdown below
the fixed VIN UVLO threshold of 3.0V. This feature can be
used to prevent excessive discharge of battery-operated
systems. If an adjustable UVLO threshold is required,
the UVLO pin can be used. The threshold voltage of the
UVLO pin comparator is 1.25V. Current hysteresis is added
above the UVLO threshold. This can be used to set voltage
hysteresis of the UVLO using the following equations:
V − VL
R3 = H
2µA
1
VH
R4 = R3 •
1.25V
−1
Example: switching should not start until the input is above
4.4V, and is to stop if the input falls below 4V.
R3 =
4.4V − 4.0V
2µA
R4 = 200kΩ •
= 200kΩ
1
4.4V
1.25V
= 79.4kΩ
−1
Keep the connection from the resistor to the UVLO pin
short and minimize the interplane or surface capacitance
to switching nodes. If high resistor values are used, the
UVLO pin should be bypassed with a 1nF capacitor to
prevent coupling problems from the switch node.
Shorted and Reversed Input Protection
If the inductor is chosen to prevent excessive saturation,
the LT3690 will tolerate a shorted output. When operating in short-circuit condition, the LT3690 will reduce its
frequency until the valley current is at a typical value of 5A
(see Figure 11). There is another situation to consider in
systems where the output is held high when the input to
the LT3690 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 LT3690’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 LT3690’s internal circuitry will pull its
quiescent current through its SW pin. This is acceptable
if the system can tolerate a few mA in this state. If the EN
pin is grounded, the SW pin current will drop to essentially
zero. However, if the VIN pin is grounded while the output
is held high, then parasitic diodes inside the LT3690 can
pull large currents from the output through the SW pin
and the VIN pin. Figure 12 shows a circuit that will run
only when the input voltage is present and that protects
against a shorted or reversed input.
D4
MBRS540
VIN
VIN
VSW
10V/DIV
BIAS
UVLO
0V
EN
IL1
2A/DIV
VOUT
BST
LT3690
GND
SW
FB
BACKUP
3690 F12
2µs/DIV
3690 F11
Figure 11. The LT3690 Reduces its Frequency to Below
250kHz to Protect Against Shorted Output with 36V Input
Figure 12. 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 LT3690
Runs Only When the Input Is Present
3690f
23
LT3690
APPLICATIONS INFORMATION
PCB Layout
High Temperature Considerations
For proper operation and minimum EMI, care must be taken
during printed circuit board layout. Figure 13 shows the
recommended component placement with trace, ground
plane and via locations. Note that large, switched currents
flow in the LT3690’s VIN, SW and GND pins and the input
capacitor (CIN). 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 BST
nodes should be small as possible. If synchronizing the
part externally using the SYNC pin, avoid routing this signal
near sensitive nodes, especially VC and FB. Finally, keep
the FB and VC nodes small so that the ground traces will
shield them from the SW and BST nodes. The exposed
GND 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 LT3690
to additional ground planes within the circuit board and
on the bottom side. In addition, the exposed SW pad on
the bottom of the package must be soldered to the PCB
to act as a heat sink for the low side switch. Add thermal
vias under the SW pad and to the bottom side.
The PCB must provide heat sinking to keep the LT3690 cool.
The GND exposed pad on the bottom of the package must
be soldered to a ground plane and the SW exposed pad
must be soldered to a SW plane. Tie the ground plane and
SW plane to large copper layers below with thermal vias;
these layers will spread the heat dissipated by the LT3690.
Placing additional vias can reduce thermal resistance
further. With these steps, the thermal resistance from die
(or junction) to ambient can be reduced to θJA = 40°C/W
or less. With 100 LFPM airflow, this resistance can fall by
another 25%. Further increases in airflow will lead to lower
thermal resistance. Because of the large output current
capability of the LT3690, it is possible to dissipate enough
heat to raise the junction temperature beyond the absolute
maximum of 125°C. When operating at high ambient temperatures, the maximum load current should be derated
as the ambient temperature approaches 125°C. Power
dissipation within the LT3690 can be estimated by calculating the total power loss from an efficiency measurement.
The die temperature is calculated by multiplying the LT3690
power dissipation by the thermal resistance from junctionto-ambient. Thermal resistance depends on the layout of
the circuit board, but values from 20°C/W to 60°C/W are
typical. Die temperature rise was measured on a 4-layer,
6cm • 6cm circuit board in still air at a load current of 4A
(ƒSW = 600kHz). For a 12V input to 3.3V output the die
temperature elevation above ambient was 43°C; for 24VIN
to 3.3VOUT the rise was 52°C; for 12VIN to 5VOUT the rise
was 55°C and for 24VIN to 5VOUT the rise was 62°C.
CC
CF
R1
R2
L
RC
RT
Other Linear Technology Publications
CSS
VOUT
VIN
CIN
COUT
CBST
GND
CVCC
Application Notes 19, 35 and 44 contain 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.
Figure 13. Top Layer PCB Layout and Component
Placement in the LT3690 Demonstration Board
3690f
24
LT3690
TYPICAL APPLICATIONS
5V Step-Down Converter
VIN
6.3V TO 36V
VIN
10µF
BIAS
UVLO
ON OFF
PG
EN
LT3690
SS
VC
1nF
0.47µF
L
4.7µH
SW
536k
FB
VCCINT
15k
0.68µF
BST
RT
SYNC
47µF
GND
680pF
VOUT
5V
4A
32.4k
102k
ƒ = 600kHz
3690 TA02
3.3V Step-Down Converter
VIN
4.5V TO 36V
VIN
10µF
BIAS
UVLO
ON OFF
PG
EN
LT3690
SS
VC
1nF
0.47µF
BST
L
3.3µH
SW
316k
FB
VCCINT
22k
0.68µF
RT
SYNC
GND
680pF
VOUT
3.3V
4A
100µF
32.4k
102k
ƒ = 600kHz
3690 TA03
(FIXED FREQUENCY AT VIN < 26V)
2.5V Step-Down Converter
VIN
3.9V TO 36V
VIN
10µF
BIAS
UVLO
ON OFF
EN
SS
BST
1nF
160k
FB
RT
SYNC
GND
1nF
L
3.3µH
SW
VCCINT
0.47µF
1µF
LT3690
VC
15k
MBR0540
PG
VOUT
2.5V
4A
100µF
32.4k
75k
ƒ = 600kHz
3690 TA04
(FIXED FREQUENCY AT VIN < 21V)
3690f
25
LT3690
TYPICAL APPLICATIONS
1.8V Step-Down Converter
AUXILIARY SUPPLY
3.3V OR 5V
1µF
VIN
3.9V TO 36V
100k
VIN
10µF
BIAS
UVLO
ON OFF
PG
EN
LT3690
SS
VC
1nF
0.47µF
0.68µF
L
4.7µH
VOUT
1.8V
4A
BST
SW
18.7k
FB
VCCINT
16k
POWER GOOD
RT
SYNC
100µF
GND
1nF
40.2k
15k
ƒ = 500kHz
3690 TA05
(FIXED FREQUENCY AT VIN < 18.5V)
1.2V Step-Down Converter
AUXILIARY SUPPLY
3.3V OR 5V
1µF
VIN
3.9V TO 36V
100k
2×
10µF
VIN
BIAS
UVLO
ON OFF
PG
EN
LT3690
SS
VC
14k
0.47µF
0.68µF
L
8.2µH
BST
VOUT
1.2V
4A
SW
23.2k
FB
VCCINT
10nF
POWER GOOD
RT
SYNC
GND
2.2nF
100µF
140k
+
46.4k
680µF
LOW ESR
ƒ = 170kHz
3690 TA06
5V Step-Down Converter with Undervoltage Lockout
VIN
14V TO 36V
10µF
200k
VIN
BIAS
UVLO
21k
ON OFF
PG
EN
SS
LT3690
VC
1nF
15k
536k
FB
RT
SYNC
GND
1nF
SLEEP: VIN < 12.3V
WAKE UP: VIN > 13.4V
L
4.7µH
SW
VCCINT
0.47µF
0.68µF
BST
VOUT
5V
4A
100µF
40.2k
102k
ƒ = 500kHz
3690 TA07
3690f
26
LT3690
PACKAGE DESCRIPTION
UFE Package
UFEMA
Package
26-Lead
Plastic
QFN (4mm × 6mm)
26-Lead
PlasticLTC
QFN
(4mm
× 6mm)Rev A)
(Reference
DWG
# 05-08-1770
(Reference LTC DWG # 05-08-1770 Rev A)
0.70 ±0.05
2.64 ± 0.05
4.50 ± 0.05
2.64 ± 0.05
2.31 ± 0.05
3.10 ± 0.05
0.41 ± 0.05
2.18 ± 0.05
2.50 REF
2.36 ± 0.05
PACKAGE OUTLINE
0.25 ±0.05
0.50 BSC
2.55 ± 0.05
3.25 ± 0.05
4.50 REF
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
4.00 ± 0.10
0.75 ± 0.05
PIN 1
TOP MARK
(NOTE 6)
PIN 1 NOTCH
R = 0.30 OR
0.35 × 45°
CHAMFER
R = 0.10
TYP
26
25
1
2.64 ± 0.10
2.18 ± 0.10
0.41 ± 0.10
4.50 REF
6.00 ± 0.10
2
R = 0.125
TYP
0.50 BSC
2.36 ± 0.10
2.31 ± 0.10
2.64 ± 0.10
0.25 ± 0.05
0.40 ± 0.10
0.200 REF
0.00 – 0.05
2.50 REF
(UFE26MA) QFN 0608 REV A
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE
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 0.20mm 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
3690f
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.
27
LT3690
TYPICAL APPLICATION
3.3V Step-Down Converter
VIN
4.5V TO 36V
BIAS
VIN
10µF
UVLO
ON OFF
PG
EN
SS
LT3690
VC
1nF
0.47µF
316k
FB
RT
SYNC
GND
680pF
L
3.3µH
SW
VCCINT
22k
0.68µF
BST
VOUT
3.3V
4A
100µF
40.2k
102k
ƒ = 500kHz
3690 TA08
(FIXED FREQUENCY AT VIN < 31V)
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT3680
36V, 3.5A, 2.4MHz High Efficiency MicroPower Step-Down
DC/DC Converter
VIN(MIN) = 3.6V, VIN(MAX) = 36V, VOUT(MIN) = 0.8V, IQ = 75µA,
ISD <1µA, 3mm × 3mm DFN-10 and MSOP-10E Packages
LT3972
Transients to 60V, 3.5A, 2.4MHz High Efficiency Step-Down
DC/DC Converter
VIN(MIN) = 3.6V, VIN(MAX) = 33V, VOUT(MIN) = 0.8V, IQ = 75µA,
ISD <1µA, 3mm × 3mm DFN-10 and MSOP-10E Packages
LT3971
38V, 1.2A (IOUT), 2MHz, High Efficiency Step-Down DC/DC
Converter with Only 2.8µA of Quiescent Current
VIN(MIN) = 4.3V, VIN(MAX) = 38V, VOUT(MIN) = 1.19V, IQ = 2.8µA,
ISD <1µA, 3mm × 3mm DFN-10 and MSOP-10E Packages
LT3991
55V, 1.2A (IOUT), 2MHz, High Efficiency Step-Down DC/DC
Converter with Only 2.8µA of Quiescent Current
VIN(MIN) = 4.3V, VIN(MAX) = 38V, VOUT(MIN) = 1.19V, IQ = 2.8µA,
ISD <1µA, 3mm × 3mm DFN-10 and MSOP-10E Packages
LT3480
36V with Transient Protection to 60V, 2A (IOUT), 2.4MHz, High
Efficiency Step-Down DC/DC Converter with Burst Mode Operation
VIN(MIN) = 3.6V, VIN(MAX) = 38V, VOUT(MIN) = 0.78V, IQ = 70µA,
ISD <1µA, 3mm × 3mm DFN-10 and MSOP-10E Packages
LT3685
36V with Transient Protection to 60V, 2A (IOUT), 2.4MHz,
High Efficiency Step-Down DC/DC Converter
VIN(MIN) = 3.6V, VIN(MAX) = 38V, VOUT(MIN) = 0.78V, IQ = 70µA,
ISD <1µA, 3mm × 3mm DFN-10 and MSOP-10E Packages
LT3500
36V, 40VMAX, 2A, 2.5MHz High Efficiency Step-Down DC/DC
Converter and LDO Controller
VIN(MIN) = 3.6V, VIN(MAX) = 36V, VOUT(MIN) = 0.8V, IQ = 2.5mA,
ISD <10µA, 3mm × 3mm DFN-10 Package
LT3507
36V 2.5MHz, Triple (2.4A + 1.5A + 1.5A (IOUT)) with LDO Controller
High Efficiency Step-Down DC/DC Converter
VIN(MIN) = 4.0V, VIN(MAX) = 36V, VOUT(MIN) = 0.8V, IQ = 7mA,
ISD = 1µA, 5mm × 7mm QFN-38 Package
LT3682
36V, 60VMAX, 1A, 2.2MHz High Efficiency Micropower Step-Down
DC/DC Converter
VIN(MIN) = 3.6V, VIN(MAX) = 36V, VOUT(MIN) = 0.8V, IQ = 75µA,
ISD <1µA, 3mm × 3mm DFN-12 Package
3690f
28 Linear Technology Corporation
LT 0211 • PRINTED IN USA
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