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LT1616 600mA, 1.4MHz Step-Down Switching Regulator in SOT-23
FEATURES
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DESCRIPTIO
Wide Input Range: 3.6V to 25V 5V at 400mA from 7V to 25V Input 3.3V at 400mA from 4.7V to 25V Input Fixed Frequency 1.4MHz Operation Uses Tiny Capacitors and Inductors Internally Compensated Low Shutdown Current: <1A Low VCESAT Switch: 220mV at 300mA Tiny 6-Lead SOT-23 Package
APPLICATIO S
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The LT (R)1616 is a current mode PWM step-down DC/DC converter with internal 0.6A power switch, packaged in a tiny 6-lead SOT-23. The wide input range of 3.6V to 25V makes the LT1616 suitable for regulating a wide variety of power sources, from 4-cell batteries and 5V logic rails to unregulated wall transformers and lead-acid batteries. Its high operating frequency allows the use of tiny, low cost inductors and ceramic capacitors. With its internal compensation eliminating additional components, a complete 400mA step-down regulator fits onto 0.15 square inches of PC board area. The constant frequency current mode PWM architecture and stable operation with ceramic capacitors results in low, predictable output ripple. Current limiting provides protection against shorted outputs. The low current (<1A) shutdown provides complete output disconnect, enabling easy power management in battery-powered systems.
, LTC and LT are registered trademarks of Linear Technology Corporation.
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Wall Transformer Regulation Local Logic Supply Conversion: 12V to 5V 12V or 5V to 3.3V, 2.5V or 1.8V Distributed Supply Regulation Digital Cameras Battery-Powered Equipment
TYPICAL APPLICATIO
3.3V Step-Down Converter
VIN 4.5V TO 25V 5 VIN BOOST LT1616 OFF ON 4 SHDN GND 2 C1 1F SW FB 3 R2 10k 6 R1 16.5k 1 D2 C3 0.01F L1 10H
100 90
EFFICIENCY (%)
D1
VOUT 3.3V 300mA: VIN = 4.5V TO 25V 400mA: VIN = 4.7V TO 25V C2 10F
C1: TAIYO-YUDEN TMK316BJ105 C2: TAIYO-YUDEN JMK316BJ106ML D1: ON SEMICONDUCTOR MBR0530 D2: 1N4148 L1: SUMIDA CR43-100
1616 TA01
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Efficiency
VIN = 5V VIN = 12V 80 70 60 50 40 30 0 100 200 300 400 LOAD CURRENT (mA) 500
1616 G02
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VIN = 20V
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LT1616
ABSOLUTE
(Note 1)
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RATI GS
PACKAGE/ORDER I FOR ATIO
ORDER PART NUMBER
TOP VIEW BOOST 1 GND 2 FB 3 6 SW 5 VIN 4 SHDN
Input Voltage (VIN) ................................................. 25V BOOST Pin Voltage ................................................. 35V BOOST Pin Above SW Pin ...................................... 25V SHDN Pin ............................................................... 25V FB Voltage ................................................................ 6V Current Into FB Pin ............................................... 1mA Operating Temperature Range (Note 2) .. - 40C to 85C Maximum Junction Temperature .......................... 125C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
LT1616ES6
S6 PACKAGE 6-LEAD PLASTIC SOT-23
S6 PART MARKING LTNB
TJMAX = 125C, JA = 250C/ W
Consult factory for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 10V, VBOOST = 15V, unless otherwise noted. (Note 2)
PARAMETER Undervoltage Lockout Feedback Voltage FB Pin Bias Current Quiescent Current Quiescent Current in Shutdown Reference Line Regulation Switching Frequency Frequency Shift Threshold on FB Pin Maximum Duty Cycle Switch Current Limit Switch VCESAT Switch Leakage Current Minimum Boost Voltage Above Switch BOOST Pin Current SHDN Input Voltage High SHDN Input Voltage Low SHDN Bias Current VSHDN = 3V VSHDN = 0V 8 0.01 ISW = 300mA ISW = 300mA 1.8 0.4 15 0.1 1.6 7 (Note 3) ISW = 300mA VFB = Measured VREF + 10mV Not Switching VSHDN = 0V VIN = 5V to 25V VFB = 1.1V fSW = 700kHz
q q q q
CONDITIONS
MIN 1.225
TYP 3.35 1.25 150 1.9 0.01 0.005
MAX 3.6 1.275 600 2.5 2 1.8
UNITS V V nA mA A %/V MHz V % mA
1 80 630
1.4 0.44 87 850 220
350 10 2.5 12
Note 1: Absolute Maximum Ratings are those values beyond which the life of the device may be impaired. Note 2: The LT1616E is guaranteed to meet performance specifications from 0C to 70C. Specifications over the -40C to 85C operating temperature range are assured by design, characterization and correlation with statistical process controls.
Note 3: Current limit guaranteed by design and/or correlation to static test. Slope compensation reduces current limit at higher duty cycle.
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mV A V mA V V A A
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LT1616 TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency, VOUT = 5V
100 90 80
EFFICIENCY (%)
VIN = 8V VIN = 12V
EFFICIENCY (%)
80 70 60 50 40 30 VIN = 20V
SWITCH VOLTAGE (mV)
70 60 50 40 30 0 100
VIN = 24V
300 400 LOAD CURRENT (mA)
200
Maximum Load Current at VOUT = 5V
500 L = 15H 400 400 500
BOOST PIN CURRENT (mA)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
L = 10H L = 6.8H
300 OUTPUT LIMITED BY DISSIPATION 200
100
0
5
15 10 INPUT VOLTAGE (V)
Switch Current Limit
1000
1.27
SWITCH CURRENT LIMIT (mA)
TYPICAL 600 MINIMUM 400
FEEDBACK PIN VOLTAGE (V)
800
1.26
UNDERVOLTAGE LOCKOUT (V)
200
0
0
20
60 40 DUTY CYCLE (%)
UW
1616 G01
Efficiency, VOUT = 3.3V
100 90 VIN = 5V VIN = 12V
500
Switch Voltage Drop
400
300
200
100
0 0 100 300 400 LOAD CURRENT (mA) 200 500
1616 G02
500
0
200 400 SWITCH CURRENT (mA)
600
1616 G03
Maximum Load Current at VOUT = 3.3V
16 L = 10H 14 12 10 8 6 4 2 100 20 25
1616 G04
BOOST Pin Current
L = 4.7H OUTPUT LIMITED BY DISSIPATION
300
200
0
5
15 10 INPUT VOLTAGE (V)
20
25
1616 G05
0
0
200
400
600
1616 G06
SWITCH CURRENT (mA)
Feedback Pin Voltage
3.7 3.6 3.5 3.4 3.3 3.2
Undervoltage Lockout
1.25
1.24
1.23
80
100
1616 G07
1.22 -50
-25
0 25 50 TEMPERATURE (C)
75
100
1616 G08
3.1 -50
-25
0 25 50 TEMPERATURE (C)
75
100
1616 G11
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LT1616 TYPICAL PERFOR A CE CHARACTERISTICS
Oscillator Frequency
2.00 1.75
SWITCHING FREQUENCY (MHz) 120 100
1.50 1.25 1.00 0.75 0.50 0.25 0 -50 -25 50 25 TEMPERATURE (C) 0 75 100
1616 G09
SHDN PIN CURRENT (A)
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BOOST (Pin 1): The BOOST pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar NPN power switch. GND (Pin 2): Tie the GND pin to a local ground plane below the LT1616 and the circuit components. Return the feedback divider to this pin. FB (Pin 3): The LT1616 regulates its feedback pin to 1.25V. Connect the feedback resistor divider tap to this pin. Set the output voltage according to VOUT = 1.25V (1 + R1/R2). A good value for R2 is 10k. SHDN (Pin 4): The SHDN pin is used to put the LT1616 in shutdown mode. Tie to ground to shut down the LT1616. Tie to 2V or more for normal operation. If the shutdown feature is not used, tie this pin to the VIN pin. VIN (Pin 5): The VIN pin supplies current to the LT1616's internal regulator and to the internal power switch. This pin must be locally bypassed. SW (Pin 6): The SW pin is the output of the internal power switch. Connect this pin to the inductor, catch diode and boost capacitor.
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SHDN Pin Current
80 60 40 20 0 0 5 10 15 20 SHDN PIN VOLTAGE 25
1616 G10
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LT1616
BLOCK DIAGRA
VIN 5
SHDN
4
INT REG AND UVLO
OSC
OPERATIO
(Refer to Block Diagram)
The LT1616 is a constant frequency, current mode Buck regulator. The 1.4MHz oscillator enables an RS flip-flop, turning on the internal 600mA power switch Q1. 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. An error amplifier measures the output voltage through an external resistor divider tied to the FB pin. This amplifier servos the switch current to regulate the FB pin voltage to 1.25V. An active clamp on the VC node provides current limit. An internal regulator provides power to the control circuitry. This regulator includes an undervoltage lockout to prevent switching when VIN is less than ~ 3.5V. The
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1 BOOST SLOPE COMP
R S Q Q
DRIVER
Q1 6 SW
FREQUENCY FOLDBACK
VC
gm
1.25V
2 GND
3 FB
1616BD
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SHDN pin is used to place the LT1616 in shutdown, disconnecting the output and reducing the input current to less than 1A. The switch driver operates from either the input or from the BOOST pin. An external capacitor and diode are 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 oscillator reduces the LT1616'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.
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LT1616
APPLICATIO S I FOR ATIO
The LT1616 efficiently converts power from an input voltage source to a lower output voltage using an inductor for energy storage. The LT1616 uses its internal power switch and an external catch diode (D1 of the application circuit on the first page of this data sheet) to produce a pulsewidth modulated square wave. Inductor L1 and output capacitor C2 filter this square wave to produce a DC output voltage. An error amplifier regulates the output by comparing the output (divided by the feedback resistor string R1 and R2) to an internal reference. The LT1616 uses current mode control; instead of directly modulating the pulse width, the error amplifier controls the peak current in the switch and inductor. Current mode control has several advantages, including simplified loop compensation and cycle-by-cycle current limiting. Figure 1 shows several waveforms of the application circuit on the front page of this data sheet. The circuit is converting a 12V input to 3.3V at 300mA. The first trace is the voltage at the SW pin. When the internal switch is on, the SW pin voltage is near the 12V input. This applies a voltage across inductor L1, and the current in the switch
VSW 5V/DIV
ISW 0.2A/DIV
200ns/DIV
1616 F01a
IL1 0.2A/DIV
VOUT 5mV/DIV
200ns/DIV
1616 F01b
Figure 1. Operating Waveforms of the LT1616 Converting 12V to 3.3V at 300mA
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(second trace) and the inductor (third trace) increases. When the switch turns off, the switch current immediately drops to zero and the inductor current flows through the catch diode D1, which clamps the switch node 0.4V below ground. The voltage across the inductor in this state has the opposite sense and is equal to the output voltage plus the catch diode drop, so the inductor current begins to decrease. The fourth trace shows the output voltage ripple. At light loads, the inductor current may reach zero on each pulse. The diode will turn off, and the switch voltage will ring, as shown in Figure 2. This is discontinuous mode operation, and is normal behavior for the switching regulator. The LT1616 will also skip pulses when the load is light.
VSW 5V/DIV IL1 0.2A/DIV VIN = 12V VOUT = 5V IOUT = 18mA 500ns/DIV
1616 F02
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Figure 2. Discontinuous Mode Operation
If the output is shorted to ground, the output voltage will collapse and there will be very little voltage to reset the current in the inductor. The LT1616 can sense this condition at its FB pin. In order to control the current, the LT1616 reduces its operating frequency, allowing more time for the catch diode to reset the inductor current. The input and output voltages determine the duty cycle of the switch. The inductor value combined with these voltages determines the ripple current in the inductor. Along with the switch current limit, the inductor ripple current determines the maximum load current that the circuit can supply. At minimum, the input and output capacitors are required for stable operation. Specific values are chosen based on allowable ripple and desired transient performance. The rest of the applications information is mainly concerned with choosing these and the other components in an LT1616 application.
LT1616
APPLICATIO S I FOR ATIO
The duty cycle of the internal switch is: DC = (VOUT + VD)/(VIN - VSW + VD) where VD is the forward voltage drop of the catch diode (D1) and VSW is the voltage drop of the internal switch. Usually one is interested in DC at full load current, so you can use VD = VSW = 0.4V. Note that the LT1616 has a maximum guaranteed duty cycle of 0.8. This will limit the minimum input voltage for a particular output voltage. When the switch is off, the inductor sees the output voltage plus the catch diode drop. This gives the peak-topeak ripple current in the inductor: IL = (1 - DC)(VOUT + VD)/(L * f) where f is the switching frequency of the LT1616 and L is the value of the inductor. The average inductor current is equal to the output current, so the peak inductor current will be the output current plus one half of the ripple current: ILPK = IOUT + IL /2. To maintain output regulation, this peak current must be less than the LT1616's switch current limit ILIM. ILIM is at least 630mA at low duty cycles, decreasing to 430mA at 80% duty cycle. The maximum output current is a function of the chosen inductor value: IOUT(MAX) = ILIM - IL /2. If the inductor value is chosen so that the ripple current is small, then the available output current will be near the switch current limit. A good approach is to choose the inductor so that the peak-to-peak inductor ripple is equal to one third of the switch current limit. This leads to: L = 3(1 - DC)(VOUT + VD)/(ILIM * f) and IOUT(MAX) = (5/6)ILIM. These expressions depend on duty cycle and therefore on input voltage. Pick a nominal input voltage to calculate L, then check the maximum available output current at the minimum and maximum input voltages. Inductor Selection and Maximum Output Current
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If your application calls for output current less than 400mA, you may be able to relax the value of the inductor and operate with higher ripple current. This may allow you to pick a physically smaller inductor or one with a lower DC resistance. Be aware that these equations assume continuous inductor current. If the inductor value is low or the load current is light, then the inductor current may become discontinuous. This occurs when IL = 2IOUT. For details of discontinuous mode operation, see Linear Technology Application Note AN44. Also, high duty cycle operation may require slightly higher inductor values to avoid subharmonic oscillations. See AN19. The maximum load current as a function of input voltage is plotted in the Typical Performance Characteristics section of this data sheet. Maximum load current for 3.3V and 5V outputs is shown for several values of L. At the highest input voltages, the load current is limited by power dissipation in the LT1616. Choose an inductor that is intended for power applications. Table 1 lists several manufacturers and inductor series. The saturation current of the inductor should be above 0.5A. The RMS current rating should be equal to or greater than output current. For indefinite operation into a short circuit, the RMS current rating should be greater than 0.7A. The DC resistance should be less than 0.5 in order maintain circuit efficiency. Capacitor Selection A Buck regulator draws from its input a square wave of current with peak-to-peak amplitude as high as the switch current limit. The input capacitor (C1) must supply the AC component of this current. An RMS current rating of 250mA is adequate for LT1616 circuits. The input capacitor must bypass the LT1616 internal control circuitry and any other circuitry that operates from the input source. A 1F ceramic capacitor will satisfy both of these requirements. If the impedance of the input source is high (due to long wires or filter components), additional bulk input capacitance may be required. In high duty cycle applications (5VIN to 3.3VOUT, for example), increase the input capacitor to 2.2F. It may be possible to achieve lower cost by using an electrolytic capacitor (tantalum or aluminum)
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LT1616
APPLICATIO S I FOR ATIO
Table 1. Inductor Vendors
Vendor Murata Sumida Phone (404) 426-1300 (847) 956-0666
URL www.murata.com www.sumida.com
Coilcraft
(847) 639-6400
www.coilcraft.com
Coiltronics Toko
(407) 241-7876
www.coiltronics.com www.tokoam.com
Table 2. Capacitor Vendors
Vendor Taiyo-Yuden AVX Murata Phone (408) 573-4150 (803) 448-9411 (404) 436-1300 URL www.t-yuden.com www.avxcorp.com www.murata.com Part Series Ceramic Caps Ceramic Caps Tantalum Caps Ceramic Caps Comments X5R Dielectric
in combination with a 0.1F ceramic capacitor. However, input voltage ripple will be higher, and you may want to include an additional 0.1F ceramic a short distance away from the LT1616 circuit in order to filter the high frequency ripple. The input capacitor should be rated for the maximum input voltage. The output capacitor has two essential functions. Along with the inductor, it filters the square wave generated by the LT1616 to produce the DC output. In this role it determines the output ripple. The second function is to store energy in order to satisfy transient loads and stabilize the LT1616's control loop. In most switching regulators the output ripple is determined by the equivalent series resistance (ESR) of the output capacitor. Because the LT1616's control loop doesn't depend on the output capacitor's ESR for stable operation, you are free to use ceramic capacitors to achieve very low output ripple and small circuit size. You can estimate output ripple with the following equations: VRIPPLE = IL * ESR for electrolytic capacitors (tantalum and aluminum) VRIPPLE = IL/(2 * f * COUT) for ceramic capacitors
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Part Series LQH3C CR43 CLS62 CLQ61 DO1607C DO1608C DT1608C CTXxx-1 TP1 3DF D52LC 1:1 Coupled Toroid 1.8mm Height Comments Small, Low Cost, 2mm Height 1:1 Coupled 1.5mm Height
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Another constraint on the output capacitor is that it must have greater energy storage than the inductor; if the stored energy in the inductor is transferred to the output, you would like the resulting voltage step to be small compared to the regulation voltage. For a 5% overshoot, this requirement becomes COUT > 10 * L(ILIM/VOUT)2 Finally, there must be enough capacitance for good transient performance. The last equation gives a good starting point. Alternatively, you can start with one of the designs in this data sheet and experiment to get the desired performance. Figure 3 illustrates some of the trade-off between different output capacitors. Figure 4 shows the test circuit. The lowest trace shows total output current, which jumps from 100mA to 250mA. The other traces show the output voltage ripple and transient response with different output capacitors. The capacitor value, size and type are listed. Note that the time scale at 50s per divison is much larger than the switching period, so you can't see the output ripple at the switching frequency. The output ripple appears as vertical broadening of the trace. The first trace (COUT = 4.7F) has peak-to-peak output ripple of ~ 6mV, while the third trace shows peak-to-peak ripple of ~15mV.
LT1616
APPLICATIO S I FOR ATIO
COUT = 4.7F CERAMIC, CASE SIZE 0805
COUT = 10F CERAMIC, CASE SIZE 1206
COUT = 47F, ESR 0.080 (SANYO POSCAP 6TPA47M) C CASE
COUT = 100F, ESR 0.150 (TANTALUM AVX TPSC107M006R0150) C CASE
VOUT 20mV/DIV
ILOAD 100mA/DIV
0 COUT = 100F TANTALUM AND 2.2F CERAMIC
Figure 3. Transient Load Response of the LT1616
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VIN 10V 5 VIN BOOST LT1616 4 SHDN GND 2 SW FB 3 COUT
1616 F04
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6
10H
VOUT 3.3V 22 33
Figure 4. Circuit Used for Transient Load Test Shown in Figure 3
Regardless of which capacitor or combination of capacitors you choose, you should do transient load tests to evaluate the circuit's stability. Avoid capacitors or combinations that result in a ringing response. Problems may occur if the output capacitance is very low or if a high value inductor is used in combination with a large value, low ESR capacitor. The high performance (low ESR), small size and robustness of ceramic capacitors make them the preferred type for LT1616 applications. However, all ceramic capacitors are not the same. Many of the higher value capacitors use poor dielectrics with high temperature and voltage coefficients. In particular, Y5V types should be regarded with suspicion. Stick with X7R and X5R types. Don't be afraid to run them at their rated voltage. Table 2 lists several capacitor manufacturers. Catch Diode A 0.5A Schottky diode is recommended for the catch diode D1. The ON Semiconductor MBR0530 is a good choice; it is rated for 0.5A forward current and a maximum reverse voltage of 30V. For circuits with VIN less than 20V, the MBR0520L can be used. Other suitable diodes are the Zetex ZHCS500TR and ZHCS750TR, and various versions of the 1N5818.
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LT1616
APPLICATIO S I FOR ATIO
BOOST Pin Considerations
Capacitor C3 and diode D2 are used to generate a boost voltage that is higher than the input voltage. In most cases a 0.01F capacitor and fast switching diode (such as the 1N4148 or 1N914) will work well. Figure 5 shows two ways to arrange the boost circuit. The BOOST pin must be more than 2.5V above the SW pin for best efficiency. For outputs of 3.3V and above, the standard circuit (Figure 5a) is best. For outputs between 2.8V and 3.3V, use a 0.033F capacitor and a small Schottky diode (such as the BAT-54). For lower output voltages the boost diode can be tied to the input (Figure 5b). The circuit in Figure 5a is more efficient because the BOOST pin current comes from a lower voltage source. You must also be sure that the maximum voltage rating of the BOOST pin is not exceeded. The minimum operating voltage of an LT1616 application is limited by the undervoltage lockout (< 3.6V) and by the maximum duty cycle as outlined above. For proper startup, the minimum input voltage is also limited by the boost circuit. If the input voltage is ramped slowly, or the LT1616 is turned on with its SHDN pin when the output is already in regulation, then the boost capacitor may not be fully
D2
INPUT VOLTAGE (V)
BOOST LT1616 VIN VIN GND SW
C3 VOUT
1616 F05a
VBOOST - VSW VOUT MAX VBOOST VIN + VOUT D2
INPUT VOLTAGE (V)
(5a)
BOOST LT1616 VIN VIN GND SW
C3 VOUT
1616 F05b
VBOOST - VSW VIN MAX VBOOST 2VIN (5b)
Figure 5. Two Circuits for Generating the Boost Voltage
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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 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. Use a Schottky diode (such as the BAT-54) for the lowest start-up voltage.
Minimum Input Voltage VOUT = 3.3V
7 BOOST DIODE TIED TO OUTPUT 6 VOUT = 3.3V DBOOST = BAT54 BOOST DIODE TIED TO INPUT V TO START 5 4 V TO RUN 3 1 10 100 LOAD CURRENT (mA) 500
1616 F06a
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Minimum Input Voltage VOUT = 5V
9 BOOST DIODE TIED TO OUTPUT 8 VOUT = 5V DBOOST = BAT54
BOOST DIODE TIED TO INPUT V TO START
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6
5
V TO RUN
4 1 10 100 LOAD CURRENT (mA) 500
1616 F06b
Figure 6. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit
LT1616
APPLICATIO S I FOR ATIO
Shorted Input Protection
If the inductor is chosen so that it won't saturate excessively, an LT1616 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 LT1616 is absent. This may occur in battery charging applications or in battery backup systems where a battery or some other supply is diode OR-ed with the LT1616's output. If the VIN pin is allowed to float and the SHDN pin is held high (either by a logic signal or because it is tied to VIN), then the LT1616's internal circuitry will pull its quiescent current through its SW pin. This is fine if your system can tolerate a few mA in this state. If you ground
D4 VIN 5 VIN BOOST LT1616 100k 4 SHDN GND 100k 2 SW FB 3 6 1
D4: MBR0530
Figure 7. 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 LT1616 Runs Only When the Input is Present
VIN
SW
GND
(a)
IC1 VIN SW L1
C1
Figure 8. Subtracting the Current When the Switch is On (a) from the Current When the Switch is Off (b) Reveals the Path of the High Frequency Switching Current (c). Keep This Loop Small. The Voltage on the SW and BOOST Nodes Will Also be Switched; Keep These Nodes as Small as Possible. Finally, Make Sure the Circuit is Shielded with a Local Ground Plane
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the SHDN pin, 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 LT1616 can pull large currents from the output through the SW pin and the VIN pin. Figure 7 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input. PCB Layout For proper operation and minimum EMI, care must be taken during printed circuit board layout. Figure 8 shows the high current paths in the buck regulator circuit. Note that large, switched currents flow in the power switch, the
VOUT BACKUP
1616 F07
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VIN
SW
GND
(b)
VSW
GND
D1
C2
1616 F08
(c)
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LT1616
APPLICATIO S I FOR ATIO
catch diode (D1) and the input capacitor (C1). The loop formed by these components should be as small as possible. Furthermore, the system ground should be tied to the regulator ground in only one place; this prevents the switched current from injecting noise into the system ground. 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, and tie this ground plane to system ground at one location, ideally at the ground terminal of the output capacitor C2. Additionally, the SW and BOOST nodes should be kept as small as possible. Finally, keep the FB node as small as possible so that the ground pin and ground traces will shield it from the SW and BOOST nodes. Figure 9 shows component placement with trace, ground plane and via locations. Include two vias near the GND pin of the LT1616 to help remove heat from the LT1616 to the ground plane.
SHUTDOWN VIN VOUT SYSTEM GROUND
VIAS TO LOCAL GROUND PLANE OUTLINE OF LOCAL GROUND PLANE
Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation
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Outputs Greater than 6V For outputs greater than 6V, connect a diode (such as a 1N4148) from the SW pin to VIN to prevent the SW pin from ringing above VIN during discontinuous mode operation. The 12V output circuit below shows the location of this diode. Also note that for outputs above 10V, the input voltage range will be limited by the maximum rating of the BOOST pin. The 12V circuit shows how to overcome this limitation using an additional Zener diode. Other Linear Technology Publications Application notes AN19, AN35 and AN44 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 DN100 shows how to generate a bipolar output supply using a Buck regulator.
1616 F09
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LT1616
TYPICAL APPLICATIO S
12V Output
D4 5 1 D2 C3 0.01F D3
VIN 16V TO 25V
OFF ON
VIN 3.6V TO 12V
OFF ON
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VIN
BOOST LT1616
4
SHDN GND 2
SW FB 3 R2 10k
6 R1 86.6k
L1 33H VOUT 12V 300mA C2 2.2F 16V
1616 TA03
D1
C1 1F 25V
GND
C1: TAIYO-YUDEN TMK316BJ105ML C2: TAIYO-YUDEN EMK316BJ225ML D1: ON SEMICONDUCTOR MBR0530 D2, D4: 1N4148 D3: CMPZ5234B 6.2V ZENER. D3 LIMITS BOOST PIN VOLTAGE TO VIN + 6V L1: COILCRAFT DO1608C-333
1.8V Output
D2 5 1 C3 0.01F L1 4.7H
VIN
BOOST LT1616
4
SHDN GND 2
SW FB 3 R2 20k
6 R1 8.87k
VOUT 400mA
D1 C2 10F 6.3V
1616 TA04
C1 1F 16V
GND
C1: TAIYO-YUDEN EMK212BJ105MG C2: TAIYO-YUDEN JMK316BJ106ML D1: ON SEMICONDUCTOR MBR0520L D2: 1N4148 OR EQUIVALENT L1: MURATA LQH3C4R7M24
13
LT1616
TYPICAL APPLICATIO S
2.5V Output
VIN 3.6V TO 16V
OFF ON
VIN 7V TO 25V
OFF ON
14
U
D2 5 1 C3 0.01F L1 4.7H
VIN
BOOST LT1616
4
SHDN GND 2
SW FB 3 R2 10k
6 R1 10k
VOUT 2.5V 350mA
D1 C2 4.7F 6.3V
1616 TA05
C1 1F 16V
GND
C1: TAIYO-YUDEN EMK212BJ105MG C2: TAIYO-YUDEN JMK212BJ475MG D1: ON SEMICONDUCTOR MBR0520 D2: 1N4148 L1: MURATA LQH3C4R7M24
5V Output
5
VIN
BOOST LT1616
1
D2 C3 0.01F L1 15H
4
SHDN GND 2
SW FB 3 R2 10k
6 R1 30.1k
D1
VOUT 5V 300mA: VIN = 7V TO 25V 400mA: VIN = 8V TO 25V C2 10F 6.3V
1616 TA07
C1 1F 25V
C1: TAIYO-YUDEN TMK316BJ105ML C2: TAIYO-YUDEN JMK316BJ106MG D1: ON SEMICONDUCTOR MBR0530 D2: 1N4148 L1: TOKO A914BYW-150M
LT1616
PACKAGE DESCRIPTION
0.35 - 0.55 (0.014 - 0.022)
NOTE: 1. DIMENSIONS ARE IN MILLIMETERS 2. DIMENSIONS ARE INCLUSIVE OF PLATING 3. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR 4. MOLD FLASH SHALL NOT EXCEED 0.254mm 5. PACKAGE EIAJ REFERENCE IS SC-74A (EIAJ)
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.
U
Dimensions in inches (millimeters) unless otherwise noted.
S6 Package 6-Lead Plastic SOT-23
(LTC DWG # 05-08-1634)
2.80 - 3.00 (0.110 - 0.118) (NOTE 3)
2.6 - 3.0 (0.110 - 0.118) 1.50 - 1.75 (0.059 - 0.069)
1.90 (0.074) REF 0.00 - 0.15 (0.00 - 0.006)
0.95 (0.037) REF
0.90 - 1.45 (0.035 - 0.057)
0.09 - 0.20 (0.004 - 0.008) (NOTE 2)
0.35 - 0.50 0.90 - 1.30 (0.014 - 0.020) (0.035 - 0.051) SIX PLACES (NOTE 2) S6 SOT-23 0898
15
LT1616
TYPICAL APPLICATION
Bipolar Output DC/DC Converter
VIN 7.5V TO 25V 5 1 C3 0.01F D2 L1A 22H VIN BOOST LT1616 OFF ON 4 SHDN GND 2 C1 1F 25V SW FB 3 R2 10k C5 1F 6.3V L1B 22H 6 R1 30.1k
RELATED PARTS
PART NUMBER
(R)
DESCRIPTION Wide Input Range Step-Down Switching Regulator 0.5A Micropower Step-Down in SOT-23 500mA, Low Noise, LDO Micropower Regulator 1.5A, 1.4MHz Step-Down DC/DC Converter Constant Frequency Step-Down Controller in SOT-23 0.25A Micropower Step-Down in SOT-23 0.6A Synchronous Step-Down Regulator 300mA, Low Noise, LDO Micropower Regulator
LTC 1474/LTC1475 0.5A Micropower Step-Down Regulator LT1676/LT1776 LTC1701 LT1763 LT1767 LTC1772 LTC1779 LTC1877 LT1962
16
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 q FAX: (408) 434-0507 q www.linear-tech.com
U
*
D1 C2 10F 6.3V
5V 200mA
GND
C1: TAIYO-YUDEN TMK316BJ105ML C2, C4: TAIYO-YUDEN JMK316BJ106ML C5: TAIYO-TUDEN JMK107BJ105MA D1, D3: ON SEMICONDUCTOR MBR0530 D2: 1N4148 L1: 22H 1:1 SUMIDA CLS62-220 OR COILTRONICS CTX20-1 -5V LOAD SHOULD BE LESS THAN 1/2 5V LOAD, SEE DESIGN NOTE 100
*
D3
C4 10F 6.3V
1616 TA06
-5V 100mA
COMMENTS 10A IQ, 100% Duty Cycle, 8-Lead MSOP Package 60V Input, 0.7A Internal Switch 100% Duty Cycle, High Efficiency: Up to 94% 30A IQ, 20VRMS Noise Higher Current, 8-Lead MSOP Package Higher Current, High Efficiency: Up to 94% Lower Current, 100% Duty Cycle High Efficiency, No Diode Required, 8-Lead MSOP 30A IQ, 20VRMS Noise, 8-Lead MSOP Package
sn1616 1616fs LT/TP 0201 4K * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 2000

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