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LT3693
1
3693f
36V, 3.5A, 2.4MHzStep-Down Switching Regulator
The LT3693 is an adjustable frequency (200kHz to2.4MHz) monolithic buck switching regulator that acceptsinput voltages up to 36V. A high efficiency 95m switchis included on the die along with a boost Schottky diodeand the necessary oscillator, control, and logic circuitry.Current mode topology is used for fast transient responseand good loop stability. Shutdown reduces input supplycurrent to less than 1A while a resistor and capacitor onthe RUN/SS pin provide a controlled output voltage ramp
(soft-start). A power good flag signals when VOUT reaches91% of the programmed output voltage. The LT3693 isavailable in 10-Pin MSOP and 3mm 3mm DFN packageswith exposed pads for low thermal resistance.
Automotive Battery Regulation Power for Portable Products Distributed Supply Regulation
Industrial Supplies Wall Transformer Regulation
Wide Input Range: 3.6V to 36V 3.5A Maximum Output Current Adjustable Switching Frequency: 200kHz to 2.4MHz Low Shutdown Current: IQ < 1A Integrated Boost Diode Synchronizable Between 250kHz to 2MHz Power Good Flag Saturating Switch Design: 95m On-Resistance 0.790V Feedback Reference Voltage
Output Voltage: 0.79V to 30V Thermal Protection Soft-Start Capability Small 10-Pin Thermally Enhanced MSOP and
(3mm 3mm) DFN Packages
5V Step-Down Converter
APPLICATIO SU
FEATURES DESCRIPTIOU
TYPICAL APPLICATIOU
SW
FB
VC
PG
RT
VIN BD
VIN6.3V TO 36V
VOUT5V3.5A
10 m F
0.47 m F
680pF
47m F100k
15k
63.4k
4.7m H
536k
GND
OFF ON
LT3693
3693 TA01a
RUN/SS BOOST
SYNC
Efficiency
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.All other trademarks are the property of their respective owners.
OUTPUT CURRENT (A)
0 0.550
EFFICIENCY(%)
70
100
1 2 2.5
3693 G01
60
90
80
1.5 3 3.5
VIN = 12V
VIN = 34V
VOUT = 5VL = 4.7Hf = 600kHz
VIN = 24V
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LT3693
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3693f
ELECTRICAL CHARACTERISTICS
VIN, RUN/SS Voltage .................................................36VBOOST Pin Voltage ...................................................56VBOOST Pin Above SW Pin .........................................30V
FB, RT, VC Voltage .......................................................5VPG, BD Voltage .........................................................30VSYNC Voltage ............................................................20V
(Note 1)
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Input Voltage 3 3.6 V
Quiescent Current from VIN VRUN/SS = 0.2V 0.01 0.5 A
VBD = 3V, Not Switching 0.45 1.2 mA
VBD = 0, Not Switching 1.3 2.3 mA
Quiescent Current from BD VRUN/SS = 0.2V 0.01 0.5 A
VBD = 3V, Not Switching 0.9 1.8 mA
The denotes the specifications which apply over the full operatingtemperature range, otherwise specifications are at TA = 25C. VIN = 10V, VRUN/SS = 10V, VBOOST = 15V, VBD = 3.3V unless otherwise
noted. (Note 2)
ABSOLUTE AXI U RATI GSW WW U
Operating Junction Temperature Range (Note 2)LT3693E ............................................. 40C to 125CLT3693I.............................................. 40C to 125C
Storage Temperature Range ................... 65C to 150CLead Temperature (Soldering, 10 sec)
(MSE Only) ....................................................... 300C
TOP VIEW
DD PACKAGE10-LEAD (3mm 3mm) PLASTIC DFN
10
9
6
7
8
4
5
3 11
2
1 RT
VC
FB
PG
SYNC
BD
BOOST
SW
VIN
RUN/SS
JA = 45C/W, JC = 10C/WEXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
123
45
BDBOOST
SW
VINRUN/SS
1098
76
RTVCFB
PGSYNC
TOP VIEW
MSE PACKAGE10-LEAD PLASTIC MSOP
11
JA = 45C/W, JC = 10C/WEXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
PIN CONFIGURATION
ORDER INFORMATION
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LT3693EDD#PBF LT3693EDD#TRPBF LDGB 10-Lead (3mm 3mm) Plastic DFN 40C to 125C
LT3693IDD#PBF LT3693IDD#TRPBF LDGB 10-Lead (3mm 3mm) Plastic DFN 40C to 125C
LT3693EMSE#PBF LT3693EMSE#TRPBF LTDFZ 10-Lead Plastic MSOP 40C to 125C
LT3693IMSE#PBF LT3693IMSE#TRPBF LTDFZ 10-Lead Plastic MSOP 40C to 125C
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/
http://www.linear.com/leadfree/http://www.linear.com/leadfree/http://www.linear.com/leadfree/http://www.linear.com/tapeandreel/http://www.linear.com/tapeandreel/http://www.linear.com/tapeandreel/http://www.linear.com/leadfree/ -
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LT3693
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3693f
PARAMETER CONDITIONS MIN TYP MAX UNITS
VBD
= 0, Not Switching 1 10 A
Minimum Bias Voltage (BD Pin) 2.7 3 V
Feedback Voltage
780775
790790
800805
mVmV
FB Pin Bias Current (Note 3) VFB = 0.8V, VC = 0.4V 10 40 nA
FB Voltage Line Regulation 4V < VIN < 36V 0.002 0.01 %/V
Error Amp gm 525 Mho
Error Amp Gain 2000
VC Source Current 60 A
VC Sink Current 60 A
VC Pin to Switch Current Gain 5.3 A/V
VC Clamp Voltage 2.0 V
Switching Frequency RT = 8.66kRT = 29.4kRT = 187k
2.21.0200
2.451.1230
2.71.25260
MHzMHzkHz
Minimum Switch Off-Time 60 150 nS
Switch Current Limit Duty Cycle = 5% 4.6 5.4 6.0 A
Switch VCESAT ISW = 3.5A 335 mV
Boost Schottky Reverse Leakage VSW = 10V, VBD = 0V 0.02 2 A
Minimum Boost Voltage (Note 4) 1.5 2.0 V
BOOST Pin Current ISW = 1A 35 60 mA
RUN/SS Pin Current VRUN/SS = 2.5V 5 8 A
RUN/SS Input Voltage High 2.5 V
RUN/SS Input Voltage Low 0.2 V
PG Threshold Offset from Feedback Voltage VFB Rising 65 mV
PG Hysteresis 10 mV
PG Leakage VPG = 5V 0.1 1 A
PG Sink Current VPG = 0.4V 200 800 A
SYNC Low Threshold 0.5 V
SYNC High Threshold 0.8 V
SYNC Pin Bias Current VSYNC = 0V 0.1 A
Note 1: Stresses beyond those listed under Absolute Maximum Ratingsmay cause permanent damage to the device. Exposure to any AbsoluteMaximum Rating condition for extended periods may affect devicereliability and lifetime.
Note 2: The LT3693E is guaranteed to meet performance specificationsfrom 0C to 125C. Specifications over the 40C to 125C operatingtemperature range are assured by design, characterization and correlationwith statistical process controls. The LT3693I specifications areguaranteed over the 40C to 125C temperature range.
Note 3: Bias current flows out of the FB pin.
Note 4: This is the minimum voltage across the boost capacitor needed toguarantee full saturation of the switch.
The denotes the specifications which apply over the full operatingtemperature range, otherwise specifications are at TA = 25C. VIN = 10V, VRUN/SS = 10V, VBOOST = 15V, VBD = 3.3V unless otherwisenoted. (Note 2)
ELECTRICAL CHARACTERISTICS
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LT3693
4
3693f
OUTPUT CURRENT (A)
0 0.550
EFFICIENCY(%)
70
100
1 2 2.5
3693 G01
60
90
80
1.5 3 3.5
VIN = 12V
VIN = 34V
VOUT = 5VL = 4.7Hf = 600kHz
VIN = 24V
OUTPUT CURRENT (A)
0 0.550
EFFICIENCY(%)
70
100
1 2 2.5
3693 G02
60
90
80
1.5 3 3.5
VIN = 12V
VIN = 34V
VOUT = 3.3VL = 3.3Hf = 600kHz
VIN = 24V
Efficiency Efficiency
INPUT VOLTAGE (V)
5
LOADCURRENT(A)
15
3693 G06
4.0
10 20
3.0
2.5
5.5
5.0
4.5
3.5
25 30
TYPICAL
MINIMUM
VOUT = 3.3VTA = 25CL = 4.7H
f = 600kHz
Maximum Load Current
DUTY CYCLE (%)
0
SWITCHCURRENTLIMIT(A)
40
3693 G08
4.5
20 60
3.5
3.0
6.0
5.5
5.0
4.0
80 100
Switch Current Limit
TYPICAL PERFOR A CE CHARACTERISTICSUW
SWITCH CURRENT (A)
0
BOOSTPINCURRENT(mA)
15
45
60
75
120
3693 G11
30
90
105
0 31 2 4 5
Boost Pin Current
TEMPERATURE (C)
SWITCH
CURRENTLIMIT(A)
4.0
4.5
5.5
5.0
3693 G09
3.5
3.0
2.0
2.5
6.5
6.0
DUTY CYCLE = 10 %
DUTY CYCLE = 90 %
50 2525 0 50 75 100 150125
Switch Current Limit
INPUT VOLTAGE (V)
5
LOADCURRENT(A)
15
3693 G07
4.5
10 20
3.5
3.0
5.5
5.0
4.0
25 30
TYPICAL
MINIMUM
VOUT = 5VTA = 25CL = 4.7H
f = 600kHz
Maximum Load Current
SWITCH CURRENT (A)
0
400
500
700
3
3693 G10
300
200
1 2 4 5
100
0
600
VOLTAGEDROP(mV)
Switch Voltage Drop
Efficiency
TA = 25C unless otherwise noted.
OUTPUT CURRENT (A)
0 0.550
EFFICIENCY(%)
TOTALPOWERLOSS(W)
70
100
1 2 2.5
3693 G03
60
90
80
0.5
1.5
3.0
1.0
2.5
2.0
1.5 3 3.5
VIN = 12VVOUT = 5VL = 4.7Hf = 600kHz
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-
8/2/2019 lt3693
5/24
LT3693
5
3693f
TEMPERATURE (C)
FEEDBACKVOLTAGE(mV)
800
3693 G12
760
840
780
820
50 2525 0 50 75 100 150125
Feedback Voltage
TEMPERATURE (C)
FREQUENCY(MHz)
1.00
1.10
3693 G13
0.90
0.80
1.20
0.95
1.05
0.85
1.15
50 2525 0 50 75 100 150125
RT = 34.0k
Switching Frequency
FB PIN VOLTAGE (mV)
0
SWITCHINGFREQUENCY(kHz)
800
1000
1200
600
3693 G14
600
400
200 400 800500100 300 700 900
200
0
RT = 34.0k
Frequency Foldback
TEMPERATURE (C)
MINIMUM
SWITCHONTIME(ns)
80
100
120
3693 G15
60
40
20
0
140
50 2525 0 50 75 100 150125
Minimum Switch On-Time
RUN/SS PIN VOLTAGE (V)
0
SWITCHCURRENTLIMIT(A)
1.5
3693 G16
4
2
0.5 1 2
1
0
7
6
5
3
2.5 3 3.5
Soft-Start
RUN/SS PIN VOLTAGE (V)
0
RUN/SSPINCURRENT(A)
8
10
12
15 25
3693 G17
6
4
5 10 20 30 35
2
0
RUN/SS Pin Current
BOOST DIODE CURRENT (A)
0
BOOSTDIODEVF(V)
0.8
1.0
1.2
2.0
3693 G18
0.6
0.4
00.5 1.0 1.5
0.2
1.4
Boost Diode
TYPICAL PERFOR A CE CHARACTERISTICSUW
TA = 25C unless otherwise noted.
FB PIN ERROR VOLTAGE (mV)
20050
VCPINCURRENT(A)
20
0
20
0 200
50
3693 G19
40
100 100
40
10
10
30
30
Error Amp Output Current
LOAD CURRENT (mA)
1
INP
UT
VOLTAGE
(V)
3.0
3.5
10000
3693 G20
2.5
2.0
10 100 1000
5.0
4.5
4.0
VOUT = 3.3VTA = 25CL = 4.7Hf = 600kHz
Minimum Input Voltage
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8/2/2019 lt3693
6/24
LT3693
6
3693f
1 1000010 100 1000
LOAD CURRENT (mA)
INPUT
VOLTAGE
(V)
5.0
5.5
3693 G21
4.5
4.0
6.5
6.0
VOUT = 5VTA = 25 CL = 4.7Hf = 600kHz
Minimum Input Voltage
TEMPERATURE (C)
VCVOLTAGE(V)
1.50
2.00
2.50
3693 G22
1.00
0.50
0
CURRENT LIMIT CLAMP
SWITCHING THRESHOLD
50 2525 0 50 75 100 150125
VC Voltages
TEMPERATURE (C)
THRESHOLDVOLTAGE(%)
85
90
95
3693 G23
80
7550 2525 0 50 75 100 150125
Power Good Threshold
3693 G25
IL0.2A/DIV
VSW5V/DIV
VOUT10mV/DIV
VIN = 12VVOUT = 3.3VILOAD = 110mA
1s/DIV
Switching Waveforms;Discontinuous Operation
3693 G26
IL0.5A/DIV
VSW5V/DIV
VOUT10mV/DIV
VIN = 12VVOUT = 3.3VILOAD = 1A
1s/DIV
Switching Waveforms;Continuous Operation
TYPICAL PERFOR A CE CHARACTERISTICSUW
TA = 25C unless otherwise noted.
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tp://www.linear.com/tapeandreel/http://www.linear.com/tapeandreel/http://www.linear.com/tapeandreel/http://www.linear.com/tapeandreel/http://www.linear.com/tapeandreel/http://www.linear.com/tapeandreel/http://www.linear.com/tapeandreel/http://www.linear.com/tapeandreel/http://www.linear.com/tapeandreel/http://www.linear.com/tapeandreel/ 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8/2/2019 lt3693
7/24
LT3693
7
3693f
PI FU CTIO SUUU
BD (Pin 1): This pin connects to the anode of the boostSchottky diode. BD also supplies current to the internalregulator.
BOOST (Pin 2): This pin is used to provide a drivevoltage, higher than the input voltage, to the internal bipolarNPN power switch.
SW (Pin 3): The SW pin is the output of the internal powerswitch. Connect this pin to the inductor, catch diode andboost capacitor.
VIN (Pin 4): The VIN pin supplies current to the LT3693sinternal regulator and to the internal power switch. Thispin must be locally bypassed.
RUN/SS (Pin 5): The RUN/SS pin is used to put theLT3693 in shutdown mode. Tie to ground to shut downthe LT3693. Tie to 2.5V or more for normal operation. Ifthe shutdown feature is not used, tie this pin to the V INpin. RUN/SS also provides a soft-start function; see theApplications Information section.
SYNC (Pin 6): This is the external clock synchronizationinput. Ground this pin when not used. Tie to a clock sourcefor synchronization. Clock edges should have rise and
fall times faster than 1s. Do not leave pin floating. Seesynchronizing section in Applications Information.
PG (Pin 7): The PG pin is the open collector output of aninternal comparator. PG remains low until the FB pin iswithin 9% of the final regulation voltage. PG output is validwhen VIN is above 3.6V and RUN/SS is high.
FB (Pin 8): The LT3693 regulates the FB pin to 0.790V.Connect the feedback resistor divider tap to this pin.
VC (Pin 9): The VC pin is the output of the internal error
amplifier. The voltage on this pin controls the peak switchcurrent. Tie an RC network from this pin to ground tocompensate the control loop.
RT (Pin 10):Oscillator Resistor Input. Connecting a resistorto ground from this pin sets the switching frequency.
Exposed Pad (Pin 11): Ground. The Exposed Pad mustbe soldered to PCB.
BLOCK DIAGRAW
+
+
+
OSCILLATOR200kHzTO2.4MHz
VC CLAMP
SOFT-START
SLOPE COMP
R
VINVIN
RUN/SS
BOOST
SW
SWITCHLATCH
VC
VOUT
C2
C3
CF
L1
D1
CC
RC
BD
RT
R2
GND
ERROR AMP
R1
FB
RT
C1
PG
0.7V
SQ
5
3693 BD
4
5
10
7
1
2
3
9
11 8
6
INTERNAL 0.79V REF
SYNC
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8/2/2019 lt3693
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LT3693
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3693f
The LT3693 is a constant frequency, current mode step-down regulator. An oscillator, with frequency set by RT,enables an RS flip-flop, turning on the internal power
switch. An amplifier and comparator monitor the currentflowing between the VIN and SW pins, turning the switchoff when this current reaches a level determined by thevoltage at VC. An error amplifier measures the outputvoltage through an external resistor divider tied to the FBpin and servos the VC pin. If the error amplifiers outputincreases, more current is delivered to the output; if itdecreases, less current is delivered. An active clamp on theVC pin provides current limit. The VC pin is also clamped tothe voltage on the RUN/SS pin; soft-start is implementedby generating a voltage ramp at the RUN/SS pin using an
external resistor and capacitor.
An internal regulator provides power to the control circuitry.The bias regulator normally draws power from the VIN pin,but if the BD pin is connected to an external voltage higherthan 3V bias power will be drawn from the external source(typically the regulated output voltage). This improves
efficiency. The RUN/SS pin is used to place the LT3693in shutdown, disconnecting the output and reducing theinput current to less than 0.5A.
The switch driver operates from either the input or fromthe BOOST pin. An external capacitor and diode are usedto generate a voltage at the BOOST pin that is higher thanthe input supply. This allows the driver to fully saturatethe internal bipolar NPN power switch for efficient opera-tion.
The oscillator reduces the LT3693s operating frequencywhen the voltage at the FB pin is low. This frequencyfoldback helps to control the output current during startupand overload.
The LT3693 contains a power good comparator which tripswhen the FB pin is at 91% of its regulated value. The PGoutput is an open-collector transistor that is off when theoutput is in regulation, allowing an external resistor to pullthe PG pin high. Power good is valid when the LT3693 isenabled and VIN is above 3.6V.
OPERATION
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LT3693
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3693f
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis-tors according to:
R RV
VOUT1 2
0 791=
.
Reference designators refer to the Block Diagram.
Setting the Switching Frequency
The LT3693 uses a constant frequency PWM architecturethat can be programmed to switch from 200kHz to 2.4MHz
by using a resistor tied from the RT pin to ground. A tableshowing the necessary RT value for a desired switchingfrequency is in Figure 1.
SWITCHING FREQUENCY (MHz) RT VALUE (k)
0.20.30.40.50.60.70.80.91.0
1.21.41.61.82.02.22.4
21514010078.763.453.645.339.234
26.722.118.215
12.710.79.09
Figure 1. Switching Frequency vs. RT Value
Operating Frequency Tradeoffs
Selection of the operating frequency is a tradeoff betweenefficiency, component size, minimum dropout voltage, and
maximum input voltage. The advantage of high frequencyoperation is that smaller inductor and capacitor values maybe used. The disadvantages are lower efficiency, lowermaximum input voltage, and higher dropout voltage. Thehighest acceptable switching frequency (fSW(MAX)) for agiven application can be calculated as follows:
fV V
t V V VSW MAXD OUT
ON MIN D IN SW( )
( )=
++( )
where VIN is the typical input voltage, VOUT is the outputvoltage, VD is the catch diode drop (~0.5V) and VSW is theinternal switch drop (~0.5V at max load). This equation
shows that slower switching frequency is necessary tosafely accommodate high VIN/VOUT ratio. Also, as shownin the next section, lower frequency allows a lower dropoutvoltage. The reason input voltage range depends on theswitching frequency is because the LT3693 switch has finiteminimum on and off times. The switch can turn on for aminimum of ~150ns and turn off for a minimum of ~150ns.Typical minimum on time at 25C is 80ns. This means thatthe minimum and maximum duty cycles are:
DC f t
DC f t
MIN SW ON MIN
MAX SW OFF MIN
=
=
( )
( )1
where fSW is the switching frequency, the tON(MIN) is theminimum switch on time (~150ns), and the tOFF(MIN) isthe minimum switch off time (~150ns). These equationsshow that duty cycle range increases when switchingfrequency is decreased.
A good choice of switching frequency should allow ad-equate input voltage range (see next section) and keepthe inductor and capacitor values small.
Input Voltage Range
The maximum input voltage for LT3693 applicationsdepends on switching frequency and Absolute Maxi-mum Ratings of the VIN and BOOST pins (36V and 56Vrespectively).
While the output is in start-up, short-circuit, or otheroverload conditions, the switching frequency should bechosen according to the following equation:
V
V V
f t V VIN MAXOUT D
SW ON MIND SW( )
( )=+
+
where VIN(MAX) is the maximum operating input voltage,VOUT is the output voltage, VD is the catch diode drop(~0.5V), VSW is the internal switch drop (~0.5V at maxload), fSW is the switching frequency (set by RT), andtON(MIN) is the minimum switch on time (~100ns). Note thata higher switching frequency will depress the maximum
APPLICATIONS INFORMATION
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8/2/2019 lt3693
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LT3693
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3693f
operating input voltage. Conversely, a lower switchingfrequency will be necessary to achieve safe operation athigh input voltages.
If the output is in regulation and no short-circuit, start-up, or overload events are expected, then input voltagetransients of up to 36V are acceptable regardless of theswitching frequency. In this mode, the LT3693 may enterpulse skipping operation where some switching pulsesare skipped to maintain output regulation. In this modethe output voltage ripple and inductor current ripple willbe higher than in normal operation.
The minimum input voltage is determined by either theLT3693s minimum operating voltage of ~3.6V or by its
maximum duty cycle (see equation in previous section).The minimum input voltage due to duty cycle is:
VV V
f tV VIN MIN
OUT D
SW OFF MIND SW( )
( )=
++
1
where VIN(MIN) is the minimum input voltage, and tOFF(MIN)is the minimum switch off time (150ns). Note that higherswitching frequency will increase the minimum inputvoltage. If a lower dropout voltage is desired, a lowerswitching frequency should be used.
Inductor Selection
For a given input and output voltage, the inductor valueand switching frequency will determine the ripple current.The ripple current IL increases with higher VIN or VOUTand decreases with higher inductance and faster switch-ing frequency. A reasonable starting point for selectingthe ripple current is:
IL = 0.4(IOUT(MAX))
where IOUT(MAX) is the maximum output load current. To
guarantee sufficient output current, peak inductor currentmust be lower than the LT3693s switch current limit (ILIM).The peak inductor current is:
IL(PEAK) = IOUT(MAX) + IL/2
where IL(PEAK) is the peak inductor current, IOUT(MAX) isthe maximum output load current, and IL is the inductor
ripple current. The LT3693s switch current limit (ILIM) is5.5A at low duty cycles and decreases linearly to 4.5A atDC = 0.8. The maximum output current is a function of
the inductor ripple current:
IOUT(MAX) = ILIM IL/2
Be sure to pick an inductor ripple current that providessufficient maximum output current (IOUT(MAX)).
The largest inductor ripple current occurs at the highestVIN. To guarantee that the ripple current stays below thespecified maximum, the inductor value should be chosenaccording to the following equation:
LV V
f I
V V
VOUT DSW LOUT DIN MAX
=+
+
1 ( )
where VD is the voltage drop of the catch diode (~0.4V),VIN(MAX) is the maximum input voltage, VOUT is the outputvoltage, fSW is the switching frequency (set by RT), andL is in the inductor value.
The inductors RMS current rating must be greater thanthe maximum load current and its saturation currentshould be about 30% higher. For robust operation in faultconditions (start-up or short circuit) and high input volt-
age (>30V), the saturation current should be above 5A.To keep the efficiency high, the series resistance (DCR)should be less than 0.05 , and the core material shouldbe intended for high frequency applications. Table 1 listsseveral vendors and suitable types.
Table 1. Inductor Vendors
VENDOR URL PART SERIES TYPE
Murata www.murata.com LQH55D Open
TDK www.componenttdk.com SLF10145 Shielded
Toko www.toko.com D75CD75F
ShieldedOpen
Sumida www.sumida.com CDRH74CR75CDRH8D43
ShieldedOpenShielded
NEC www.nec.com MPLC073MPBI0755
ShieldedShielded
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Of course, such a simple design guide will not always re-sult in the optimum inductor for your application. A largervalue inductor provides a slightly higher maximum load
current and will reduce the output voltage ripple. If yourload is lower than 3.5A, then you can decrease the valueof the inductor and operate with higher ripple current. Thisallows you to use a physically smaller inductor, or onewith a lower DCR resulting in higher efficiency. There areseveral graphs in the Typical Performance Characteristicssection of this data sheet that show the maximum loadcurrent as a function of input voltage and inductor valuefor several popular output voltages. Low inductance mayresult in discontinuous mode operation, which is okaybut further reduces maximum load current. For details of
maximum output current and discontinuous mode opera-tion, see Linear Technology Application Note 44. Finally,for duty cycles greater than 50% (VOUT/VIN > 0.5), thereis a minimum inductance required to avoid subharmonicoscillations. See AN19.
Input Capacitor
Bypass the input of the LT3693 circuit with a ceramiccapacitor of X7R or X5R type. Y5V types have poorperformance over temperature and applied voltage, and
should not be used. A 10F to 22F ceramic capacitor isadequate to bypass the LT3693 and will easily handle theripple current. Note that larger input capacitance is requiredwhen a lower switching frequency is used. If the inputpower source has high impedance, or there is significantinductance due to long wires or cables, additional bulkcapacitance may be necessary. This can be provided witha lower performance electrolytic capacitor.
Step-down regulators draw current from the input sup-ply in pulses with very fast rise and fall times. The inputcapacitor is required to reduce the resulting voltage
ripple at the LT3693 and to force this very high frequencyswitching current into a tight local loop, minimizing EMI.A 10F capacitor is capable of this task, but only if it isplaced close to the LT3693 and the catch diode (see thePCB Layout section). A second precaution regarding the
ceramic input capacitor concerns the maximum inputvoltage rating of the LT3693. A ceramic input capacitorcombined with trace or cable inductance forms a high
quality (under damped) tank circuit. If the LT3693 circuitis plugged into a live supply, the input voltage can ring totwice its nominal value, possibly exceeding the LT3693svoltage rating. This situation is easily avoided (see the HotPlugging Safety section).
For space sensitive applications, a 4.7F ceramic capaci-tor can be used for local bypassing of the LT3693 input.However, the lower input capacitance will result in in-creased input current ripple and input voltage ripple, andmay couple noise into other circuitry. Also, the increased
voltage ripple will raise the minimum operating voltageof the LT3693 to ~3.7V.
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Alongwith the inductor, it filters the square wave generated by theLT3693 to produce the DC output. In this role it determinesthe output ripple, and low impedance at the switchingfrequency is important. The second function is to storeenergy in order to satisfy transient loads and stabilize theLT3693s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the bestripple performance. A good starting value is:
CV fOUT OUT SW
=100
where fSW is in MHz, and COUT is the recommendedoutput capacitance in F. Use X5R or X7R types. Thischoice will provide low output ripple and good transientresponse. Transient performance can be improved witha higher value capacitor if the compensation network is
also adjusted to maintain the loop bandwidth. A lowervalue of output capacitor can be used to save space andcost but transient performance will suffer. See the Fre-quency Compensation section to choose an appropriatecompensation network.
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When choosing a capacitor, look carefully through thedata sheet to find out what the actual capacitance is underoperating conditions (applied voltage and temperature).A physically larger capacitor, or one with a higher volt-age rating, may be required. High performance tantalumor electrolytic capacitors can be used for the outputcapacitor. Low ESR is important, so choose one that isintended for use in switching regulators. The ESR shouldbe specified by the supplier, and should be 0.05 or less.
Such a capacitor will be larger than a ceramic capacitorand will have a larger capacitance, because the capacitormust be large to achieve low ESR. Table 2 lists severalcapacitor vendors.
Catch Diode
The catch diode conducts current only during switch offtime. Average forward current in normal operation can becalculated from:
ID(AVG) = IOUT (VIN VOUT)/VIN
where IOUT is the output load current. The only reason toconsider a diode with a larger current rating than necessaryfor nominal operation is for the worst-case condition ofshorted output. The diode current will then increase to thetypical peak switch current. Peak reverse voltage is equalto the regulator input voltage. Use a schottky diode with areverse voltage rating greater than the input voltage. Table3 lists several Schottky diodes and their manufacturers.
Table 3. Diode Vendors
PART NUMBERVR(V)
IAVE(A)
VF AT 3A(mV)
On SemiconductorMBRA340 40 3 500
Diodes Inc.PDS340B340AB340LA
404040
333
500500450
VENDOR PHONE URL PART SERIES COMMANDS
Panasonic (714) 373-7366 www.panasonic.com Ceramic,Polymer,
Tantalum
EEF Series
Kemet (864) 963-6300 www.kemet.com Ceramic,
Tantalum T494, T495
Sanyo (408) 749-9714 www.sanyovideo.com Ceramic,
Polymer,
Tantalum
POSCAP
Murata (408) 436-1300 www.murata.com Ceramic
AVX www.avxcorp.com Ceramic,
Tantalum TPS Series
Taiyo Yuden (864) 963-6300 www.taiyo-yuden.com Ceramic
Table 2. Capacitor Vendors
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