3508FE

LT3508 LINER | Alldatasheet

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APPLICATIONS

DESCRIPTION

Dual Monolithic 1.4A Step-Down Switching Regulator The L T®3508 is a dual current mode PWM step-down DC/DC converter with internal power switches capable of generating two 1.4A outputs. The wide input voltage range of 3.7V to 36V makes the L T3508 suitable for regulating power from a wide variety of sources, including automo- tive batteries, 24V industrial supplies and unregulated wall adapters. Both converters are synchronized to a single os- cillator programmable up to 2.5MHz and run with opposite phases, reducing input ripple current. Its high operating frequency allows the use of small, low cost inductors and ceramic capacitors, resulting in low, predictable output ripple. Each regulator has independent tracking and soft- start circuits and generates a power good signal when its output is in regulation, easing power supply sequencing and interfacing with microcontrollers and DSPs. Cycle-by-cycle current limit, frequency foldback and ther- mal shutdown provide protection against shorted outputs, and soft-start eliminates input current surge during start- up. The low current (<2μA) shutdown mode enables easy power management in battery-powered systems. 3.3V and 5V Dual Output Step-Down Converter with Output Sequencing n Wide Input Voltage Range: 3.7V to 36V n T wo 1.4A Output Switching Regulators with Internal Power Switches n Adjustable 250kHz to 2.5MHz Switching Frequency n Synchronizable over the Full Frequency Range n Anti-Phase Switching Reduces Ripple n Uses Small Inductors and Ceramic Capacitors n Accurate Programmable Undervoltage Lockout n Independent T racking, Soft-Start and Power Good Circuits Ease Supply Sequencing n Output Adjustable Down to 800mV n Small 4mm × 4mm 24-Pin QFN or 16-Pin Thermally Enhanced TSSOP Surface Mount Packages n Automotive n DSP Power Supplies n Wall T ransformer Regulation n DSL and Cable Modems n PCI Express VIN 5.6V TO 36V BOOST2 L T3508 VIN SHDN GND

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10μF 22μF 4.7μF 0.22μF 0.22μF ON OFF 43k 150pF 51k 10.7k 52.3k fSW = 700kHz 35.7k 100k 56.2k 11.5k 10μH6.8μH POWER GOOD OUT2 1.4A OUT1 3.3V 1.4A 1nF BOOST1 SW2SW1 FB2FB1 VC2 PG1 VC1 TRACK/SS1 PG2TRACK/SS2 , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. LOAD CURRENT (A) EFFICIENCY (%) 0.5 1

3508 TA01b

1.5 VIN = 12V VOUT2 = 5V VOUT1 = 3.3V Effi ciency

TRACK/SS, FB, R Operating Junction Temperature Range (Note 2) (Note 1) FE PACKAGE 16-LEAD PLASTIC TSSOP TOP VIEW TRACK/SS1 BOOST1 SW1 V IN1 VIN2 SW2 BOOST2 TRACK/SS2 FB1 V PG1 R T/SYNC SHDN PG2 V FB2 θJA = 40°C/W , θJC = 10°C/W EXPOSED PAD (PIN 17) IS GND AND MUST BE SOLDERED TO PCB 24 23 22 21 20 19 7 8 9 TOP VIEW UF PACKAGE 24-LEAD (4mm s 4mm) PLASTIC QFN 10 11 12 18FB1 TRACK/SS1 GND GND GND GND FB2 TRACK/SS2 GND GND GND GND VC1 PG1 RT/SYNC SHDN PG2 VC2 BOOST1 SW1 VIN1 VIN2 SW2 BOOST2 θJA = 40°C/W , θJC = 10°C/W EXPOSED PAD (PIN 25) IS GND AND MUST BE SOLDERED TO PCB Storage Temperature Range Lead Temperature (Soldering, 10 sec) ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE L T3508EFE#PBF L T3508EFE#TRPBF 3508FE 16-Lead Plastic TSSOP –40°C to 125°C L T3508IFE#PBF L T3508IFE#TRPBF 3508FE 16-Lead Plastic TSSOP –40°C to 125°C L T3508HFE#PBF L T3508HFE#TRPBF 3508HFE 16-Lead Plastic TSSOP –40°C to 150°C L T3508EUF#PBF L T3508EUF#TRPBF 3508 24-Lead (4mm × 4mm) Plastic QFN –40°C to 125°C L T3508IUF#PBF L T3508IUF#TRPBF 3508 24-Lead (4mm × 4mm) Plastic QFN –40°C to 125°C L T3508HUF#PBF L T3508HUF#TRPBF 3508H 24-Lead (4mm × 4mm) Plastic QFN –40°C to 150°C Consult L TC Marketing for parts specifi ed with wider operating temperature ranges.*Temperature grades are identifi ed by a label on the shipping container . Consult L TC Marketing for information on non-standard lead based fi nish parts. For more information on lead free part marking, go to: http://www.linear .com/leadfree/ For more information on tape and reel specifi cations, go to: http://www.linear .com/tapeandreel/

ELECTRICAL CHARACTERISTICS The l denotes the specifi cations which apply over the full operating temperature range, otherwise specifi cations are at TA = 25°C. VIN = 12V , VBOOST = 17V unless otherwise noted. (Note 2) Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The L T3508E is guaranteed to meet performance specifi cations from 0°C to 125°C junction temperature. Specifi cations over the –40°C to 125°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The L T3508I is guaranteed over the full –40°C to 125°C operating junction temperature range. The L T3508H is guaranteed over the full –40°C to 150°C operating junction temperature range. High junction temperatures degrade operating lifetimes. Operating lifetime is derated at junction temperatures greater than 125°C. Note 3: Current fl ows out of pin. Note 4: VBOOST =12V . Circuitry increases the maximum duty cycle of the L T3508 when VBOOST > VIN + 2.5V . See “Minimum Operating Voltage” in the Applications Information section for details. Note 5: Current limit is guaranteed by design and/or correlation to static test. Slope compensation reduces current limit at higher duty cycles. Note 6: Current fl ows into pin. Note 7: 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 range when overtemperature protection is active. Continuous operation above the specifi ed maximum operating junction temperature may impair device reliability. PARAMETER CONDITIONS MIN TYP MAX UNITS Minimum Operating Voltage, V IN1 l 3.4 3.7 V Minimum Operating Voltage, VIN2 VIN1 = 12V l 2.5 3.0 V VIN1 Quiescent Current Not Switching 4.3 5.2 mA VIN2 Quiescent Current Not Switching 320 500 μA Shutdown Current (VIN1 + VIN2)V SHDN = 0.3V 0.1 2 μA FB Voltage l 0.790 0.784 0.800 0.814 0.816 V V FB Pin Bias Current (Note 3) V FB = 0.800V , VC = 0.4V l 50 300 nA FB Voltage Line Regulation 5V < V IN < 40V 0.01 %/V Error Amp T ransconductance 300 μS Error Amp Voltage Gain 600 V/V VC to Switch Current Gain 2.5 A/V Switching Frequency R T = 33.2k l 0.92 1 1.06 MHz Switching Phase R T = 33.2k 150 180 210 Deg Maximum Duty Cycle (Note 4) R T = 33.2k RT = 7.50k RT = 169k l 84 90 Foldback Frequency R T = 33.2k, VFB = 0V 120 kHz Switch Current Limit (Note 5) Duty Cycle = 15% l 2.0 2.6 3.2 A Switch VCESAT ISW = 1.5A 300 mV Switch Leakage Current 0.01 1 μA Minimum Boost Voltage 1.7 2.5 V Boost Pin Current I SW = 1.5A 35 50 mA TRACK/SS Pin Current V TRACK/SS = 0V 0.8 1.2 2.2 μA PG Threshold Offset V FB Rising 56 75 110 mV PG Voltage Output Low V FB = 0.6V , IPG = 250μA 0.13 0.4 V PG Pin Leakage V PG = 2V 0.01 1 μA SHDN Threshold Voltage 2.53 2.63 2.73 V SHDN Input Current (Note 6) V SHDN = 60mV Above Threshold Voltage 6 8 10 μA SHDN Threshold Current Hysteresis 5.5 7.5 9.5 μA SYNC Threshold Voltage 1 1.25 1.5 V SYNC Input Frequency 0.25 2.5 MHz

TEMPERATURE (°C) –50 0.790 FEEDBACK VOL TAGE (V)0.795 0.800 0.805 0.810 –25 0 25 50

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TEMPERATURE (°C) CURRENT LIMIT (A) 2.0 2.5 3.0

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1.5 1.0 0.5 –50 –25 0 25 50 75 100 125 150 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) SWITCHING FREQUENCY (MHz) 0.8 1.0 1.2

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0.6 0.4 0.2 RT = 33.2k FEEDBACK VOL TAGE (mV) SWITCHING FREQUENCY (MHz)0.5 1.0 1.5 2.0 200 400 600 800

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2.5 3.0 100 300 500 700 TA = 25°C RT = 7.50k RT = 33.2k RT = 169k TYPICAL PERFORMANCE CHARACTERISTICS Effi ciency, VOUT = 5V Effi ciency, VOUT = 3.3V Effi ciency, V OUT = 1.8V Feedback Voltage Switch Current Limit vs Temperature Switch Current Limit vs Duty Cycle Switching Frequency vs RT Switching Frequency vs Temperature Switching Frequency Foldback LOAD CURRENT (A) EFFICIENCY (%) 0.5 1

3508 G01

1.5 TA = 25°C f = 700kHz VIN = 12V VIN = 24V VIN = 32V LOAD CURRENT (A) EFFICIENCY (%)70 0.5 1

3508 G02

1.5 TA = 25°C f = 700kHz VIN = 12V VIN = 24V VIN = 32V LOAD CURRENT (A) EFFICIENCY (%) 0.5 1

3508 G03

1.5 TA = 25°C f = 1MHz VIN = 3.3V VIN = 5V VIN = 12V DUTY CYCLE (%) CURRENT LIMIT (A) 0.5 1.0 1.5 2.0 2.5 3.0 20 40 60 80

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TA = 25°C TYPICAL MINIMUM FREQUENCY (MHz) 0.1 RT (kΩ) 100 1000 11 0

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TA = 25°C

–50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C)

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OUTPUT CURRENT (μA) SINKING SOURCING –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) INPUT VOL TAGE (V) 3.5

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2.0 1.0 0.5 4.0 3.0 2.5 1.5 VIN1 VIN2 TYPICAL PERFORMANCE CHARACTERISTICS Quiescent Current V C Voltages Error Amp Output Current Switch Voltage Drop Boost Pin Current Undervoltage LockoutSHDN Pin Current INPUT VOL TAGE (V) INPUT CURRENT (mA) 0.5 1.5 2.0 2.5 5.0 3.5 10 20 25

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1.0 4.0 4.5 3.0 51 5 30 35 40 TA = 25°C VIN1 VIN2 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) VC VOL TAGE (V)1.0 2.5

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0.5 2.0 1.5 CLAMP VOL TAGE TO SWITCH SWITCH CURRENT (A) 350 300 250 200 150 100

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0.5 1 1.5 SWITCH VOL TAGE (mV) TA = 25°C SWITCH CURRENT (A)

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0.5 1 1.5 BOOST PIN CURRENT (mA) TA = 25°C SHDN PIN VOL TAGE (V) SHDN PIN CURRENT (μA) 10 20 30 40

3508 G15

TA = –45°C TA = 125°C TA = 25°C

BOOST1, BOOST2: The BOOST pins are used to provide drive voltages, higher than the input voltage, to the internal NPN power switches. Tie through a diode to a 2.8V or higher supply, such as V OUT or VIN. Exposed Pad: The Exposed Pad metal of the package pro- vides both electrical contact to ground and good thermal contact to the printed circuit board. The Exposed Pad must be soldered to the circuit board for proper operation. FB1, FB2: The L T3508 regulates each feedback pin to 0.800V . Connect the feedback resistor divider taps to these pins. GND: Tie the GND pins directly to the Exposed Pad and ground plane. PG1, PG2: The power good pins are the open-collector outputs of an internal comparator . PG remains low until the FB pin is within 10% of the fi nal regulation voltage. As well as indicating output regulation, the PG pins can be used to sequence the two switching regulators. These pins can be left unconnected. The PG outputs are valid when V IN1 is greater than 2.4V and SHDN is high. The PG comparators are disabled in shutdown. RT/SYNC: The R T/SYNC pin is used to set the internal oscillator frequency. Tie a 33.2k resistor from R T/SYNC to GND for a 1MHz switching frequency. To synchronize the part to an external frequency, drive the R T/SYNC pin with a logic-level signal with positive and negative pulse widths of at least 80ns. SHDN: The shutdown pin is used to put the L T3508 in shutdown mode. Pull the pin below 0.3V to shut down the L T3508. The 2.63V threshold can function as an accurate undervoltage lockout (UVLO), preventing the regulator from operating until the input voltage has reached the programmed level. Do not drive SHDN more than 6V above V IN1. SW1, SW2: The SW pins are the outputs of the internal power switches. Connect these pins to the inductors, catch diodes and boost capacitors. TRACK/SS1, TRACK/SS2: The TRACK/SS pins are used to soft-start the two channels, to allow one channel to track the other output, or to allow both channels to track another output. For tracking, tie a resistor divider to this pin from the tracked output. For soft-start, tie a capacitor to this pin. An internal 1.2μA soft-start current charges the capacitor to create a voltage ramp at the pin. Leave these pins disconnected if unused. V C1, VC2: The VC pins are the outputs of the internal error amps. The voltages on these pins control the peak switch currents. These pins are normally used to compensate the control loops, but can also be used to override the loops. Pull these pins to ground with an open drain to shut down each switching regulator . V IN1: The VIN1 pin supplies current to the L T3508 internal circuitry and to the internal power switch connected to SW1 and must be locally bypassed. V IN1 must be greater than 3.7V for channel 1 or channel 2 to operate. VIN2: The VIN2 pin supplies current to the internal power switch connected to SW2 and must be locally bypassed. Connect this pin directly to V IN1 unless power for chan- nel 2 is coming from a different source. V IN2 must be greater than 3V and V IN1 must be greater than 3.7V for channel 2 to operate.

3508 F01

Figure 1. Block Diagram of the L T3508 with Associated External Components (One of T wo Switching Regulators Shown)

The L T3508 is a dual constant frequency, current mode regulator with internal power switches. Operation can be best understood by referring to the Block Diagram. If the SHDN pin is tied to ground, the L T3508 is shut down and draws minimal current from the input source tied to the V IN pins. If the SHDN pin exceeds 1V , the internal bias circuits turn on, including the internal regulator , reference and oscillator . The switching regulators will only begin to operate when the SHDN pin exceeds 2.63V . The switcher is a current mode regulator . Instead of directly modulating the duty cycle of the power switch, the feedback loop controls the peak current in the switch during each cycle. Compared to voltage mode control, current mode control improves loop dynamics and provides cycle-by- cycle current limit. A pulse from the oscillator sets the RS fl ip-fl op and turns on the internal NPN power switch. Current in the switch and the external inductor begins to increase. When this current exceeds a level determined by the voltage at V C, current comparator C1 resets the fl ip-fl op, turning off the switch. The current in the inductor fl ows through the external Schottky diode and begins to decrease. The cycle begins again at the next pulse from the oscillator . In this way, the voltage on the V C pin controls the current through the inductor to the output. The internal error amplifi er regulates the output current by continually adjusting the V C pin voltage. The threshold for switching on the VC pin is 0.8V , and an active clamp of 1.75V limits the output current. The switching frequency is set either by the resistance to GND at the RT/SYNC pin or the frequency of the logic-level signal driving the RT/SYNC pin. A detection circuit monitors for the presence of a SYNC signal on the pin and switches between the two modes. Unique circuitry generates the appropriate slope compensation ramps and generates the 180° out-of-phase clocks for the two channels. The switching regulator performs frequency foldback during overload conditions. An amplifi er senses when V FB is less than 0.625V and begins decreasing the oscil- lator frequency down from full frequency to 12% of the nominal frequency when V FB = 0V . The FB pin is less than 0.8V during start-up, short-circuit and overload conditions. Frequency foldback helps limit switch current under these conditions. The switch driver operates either from V IN or from the BOOST pin. An external capacitor and Schottky diode are used to generate a voltage at the BOOST pin that is higher than the input supply. This allows the driver to saturate the internal bipolar NPN power switch for ef- fi cient operation. The TRACK/SS pin serves as an alternative input to the error amplifi er . The amplifi er will use the lowest voltage of either the reference of 0.8V or the voltage on the TRACK/SS pin as the positive input of error amplifi er . Since the TRACK/SS pin is driven by a constant current source, a single capacitor on the pin will generate a linear ramp on the output voltage. Tying the TRACK/SS pin to a resistor divider from the output of one of the switching regulators allows one output to track another . 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 L T3508 is enabled (SHDN is high) and V IN1 is greater than ~2.4V .

Setting the Output Voltage The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the 1% resis- tors according to: RR V V OUT12 08 1= ⎛ ⎠⎟. – R2 should be 20k or less to avoid bias current errors. Reference designators refer to the Block Diagram. Minimum Operating Voltage The minimum operating voltage is determined either by the L T3508’s undervoltage lockout or by its maximum duty cycle. If V IN1 and VIN2 are tied together , the undervoltage lockout is at 3.7V or below. If the two inputs are used separately, then V IN1 has an undervoltage lockout of 3.7V or below and VIN2 has an undervoltage lockout of 3V or below. Because the internal supply runs off V IN1 , chan- nel 2 will not operate unless V IN1 > 3.7V . The duty cycle is the fraction of time that the internal switch is on and is determined by the input and output voltages: DC VV VV V OUT F IN SW F = + Unlike many fi xed frequency regulators, the L T3508 can extend its duty cycle by turning on for multiple cycles. The L T3508 will not switch off at the end of each clock cycle if there is suffi cient voltage across the boost capacitor (C3 in Figure 1). Eventually, the voltage on the boost capacitor falls and requires refreshing. Circuitry detects this condi- tion and forces the switch to turn off, allowing the inductor current to charge up the boost capacitor . This places a limitation on the maximum duty cycle as follows: DCMAX SW 1 1 β where βSW is equal to the SW pin current divided by the BOOST pin current as shown in the Typical Performance Characteristics section. This leads to a minimum input voltage of: V VV DC VVIN MIN OUT F MAX FS W() –= + + where VF is the forward voltage drop of the catch diode (~0.4V) and VSW is the voltage drop of the internal switch (~0.4V at maximum load). Example: ISW = 1.5A and I BOOST = 50mA, V OUT = 3.3V , V VV VVVIN MIN() 96 04 04 38 Maximum Operating Voltage The maximum operating voltage is determined by the Absolute Maximum Ratings of the V IN and BOOST pins, and by the minimum duty cycle: DC MIN = tON(MIN) • f where tON(MIN) is equal to 130ns (for TJ > 125°C tON(MIN) is equal to 150ns) and f is the switching frequency. Running at a lower switching frequency allows a lower minimum duty cycle. The maximum input voltage before pulse skipping occurs depends on the output voltage and the minimum duty cycle: V VV DC VVIN PS OUT F MIN FS W() –= + + Example: f = 790kHz, VOUT = 3.3V , DCMIN = 130ns • 790kHz = 0.103: V VV VV VIN PS() . –. .= + +=33 04 0 103 04 04 3 6 The L T3508 will regulate the output current at input voltages greater than VIN(PS). For example, an application with an output voltage of 1.8V and switching frequency of 1.5MHz has a V IN(PS) of 11.3V , as shown in Figure 2. Figure 3 shows operation at 18V . Output ripple and peak inductor current have signifi cantly increased. Exceeding V IN(PS) is safe if the output is in regulation, if the external components have adequate ratings to handle the peak conditions and if the peak inductor current does not exceed 3.2A. A saturating inductor may further reduce performance. Do not exceed V IN(PS) during start-up or overload conditions (for outputs greater than 5V , use VOUT = 5V to calculate V IN(PS)). For operation above 20V in pulse skipping mode, program the switching frequency to 1.1MHz or less.

Table 1. Programming the Switching Frequency lists several vendors and types that are suitable. Table 2. Inductor Vendors Figure 2. Operation Below VIN(PS). VIN = 10V , VOUT = 1.8V and Figure 3. Operation Above VIN(PS). VIN = 18V , VOUT = 1.8V

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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 cur- rent. This allows you to use a physically smaller inductor , or one with a lower DCR resulting in higher effi ciency. 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 Linear Technology’s Application Note 44. Finally, for duty cycles greater than 50% (V OUT/VIN > 0.5), a minimum inductance is required to avoid sub-harmonic oscillations: LV V kHz fMIN OUT F=+() • 800 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 L T3508 limits its switch cur- rent in order to protect itself and the system from overload faults. Therefore, the maximum output current that the L T3508 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 catch diode drop. This gives the peak-to-peak ripple current in the inductor: ΔI DC V V LfL OUT F= () +()1– where f is the switching frequency of the L T3508 and L is the value of the inductor . The peak inductor and switch current is: II I I SW PK L PK OUT L () () == + Δ To maintain output regulation, this peak current must be less than the L T3508’s switch current limit I LIM. ILIM is at least 2A for at low duty cycles and decreases linearly to 1.55A at DC = 90%. The maximum output current is a function of the chosen inductor value: II I AD C I OUT MAX LIM LL 2 21 0 2 5 2 Choosing an inductor value so that the ripple current is small will allow a maximum output current near the switch current limit. 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 L T3508 will be able to deliver the required output current. Note again that these equations assume that the inductor current is continuous. Discontinu- ous operation occurs when I OUT is less than ΔIL/2. Input Capacitor Selection Bypass the VIN pins of the L T3508 circuit with a ceramic capacitor of X7R or X5R type. For switching frequencies above 500kHz, use a 4.7μF capacitor or greater . For switch- ing frequencies below 500kHz, use a 10μF or higher capaci- tor . If the V IN pins are tied together only a single capacitor is necessary. If the VIN pins are separated, each pin will need its own bypass. The following paragraphs describe the input capacitor considerations in more detail. Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. The input ca- pacitor is required to reduce the resulting voltage ripple at the L T3508 input and to force this switching current into a tight local loop, minimizing EMI. The input capacitor must have low impedance at the switching frequency to do this effectively, and it must have an adequate ripple current rating. With two switchers operating at the same frequency but with different phases and duty cycles, calculating the input capacitor RMS current is not simple. However , a conservative value is the RMS input current for the channel that is delivering most power (V OUT times IOUT): CI VV V V I IN RMS OUT OUT IN OUT IN OUT () • = () < 2 and is largest when V IN = 2V OUT (50% duty cycle). As the second, lower power channel draws input current, APPLICATIONS INFORMATION

current will always be less than 0.7A. value. Use X5R and X7R types. in order to meet the ESR and ripple current requirements. rated voltage of the capacitor . ceramic capacitor . For details see Application Note 88. will provide low output ripple and good transient response. Table 3. Capacitor Vendors

will then increase to the typical peak switch current. Peak reverse voltage is equal to the regulator input voltage. consider using a Schottky with low reverse leakage. Table 4. Schottky Diodes applications 1MHz or faster , a 0.1μF capacitor is suffi cient. and SW pins is less than 30V .

3508 F04

Figure 4. Generating the Boost Voltage

achieve low output ripple and small circuit size. addition, there may be a lower value capacitor in parallel. output capacitor has high ESR. Figure 5. The Minimum Input Voltage Depends on Output

3508 F05a

3508 G05b

limit or latch low under low source voltage conditions. voltages where the problems might occur . 4.75V and is to stop if the input falls below 4V . to prevent coupling problems from the switch node. both TRACK/SS pins. The pins clamp at 1.3V .

3508 F06

Figure 6. Model for Loop Response Figure 7. Undervoltage Lockout

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than the 800mV reference voltage. TRACK/SS pins as shown in Figure 8e. IN2 when VOUT2 is at its maximum load. can accommodate input voltages as low as 3V on V IN2. Figure 9. 1MHz, Wide Input Range 5V and 1.8V Outputs

3508 F09

against a shorted or reversed input. Figure 11. A Good PCB Layout Ensures Proper Low EMI Operation SW pins, the catch diode (D1) and the input capacitor (CIN). within the circuit board and on the bottom side.

3508 F10

Figure 10. Diode D4 Prevents a Shorted Input from Discharging

1MHz, 3.3V and 1.8V Outputs with Sequencing VIN 3.9V TO 16V BOOST2 L T3508 VIN2VIN1 SHDN GND

3508 TA02

10μF 47μF 4.7μF 0.1μF 0.1μF ON OFF 39k 330pF 47k 11.5k 33.2k f SW = 1MHz 18.7k 100k 35.7k 15.0k L2 4.7μH L1 3.3μH D1 D2 D3 D4 POWER GOOD OUT2 3.3V 1.4A OUT1 1.8V 1.4A OUT2 1nF BOOST1 SW2SW1 FB2FB1 VC2 PG1 VC1 TRACK/SS1 PG2TRACK/SS2 C1 TO C5: X5R OR X7R D1, D2: MMSD4148 D3: DIODES INC. B140 D4: DIODES INC. B240A High Temperature Considerations The die temperature of the L T3508 must be lower than the maximum rating of 125°C (150°C for the H grade). This is generally not a concern unless the ambient temperature is above 85°C. For higher temperatures, care should be taken in the layout of the circuit to ensure good heat sinking of the L T3508. The maximum load current should be derated as the ambient temperature approaches 125°C (150°C for the H grade). The die temperature is calculated by multiplying the L T3508 power dissipation by the thermal resistance from junction to ambient. Power dissipation within the L T3508 can be estimated by calculating the total power loss from an effi ciency measurement and subtract- ing the catch diode loss. Thermal resistance depends on the layout of the circuit board, but values from 30°C/W to 60°C/W are typical. Die temperature rise was measured on a 4-layer , 6.5cm × 7.5cm circuit board in still air at a load current of 1.4A (f SW = 700kHz). For a 12V input to 3.3V output the die temperature elevation above ambient was 13°C; for 24V IN to 3.3V OUT the rise was 18°C; for 12VIN to 5VOUT the rise was 14°C and for 24VIN to 5VOUT the rise was 19°C. Outputs Greater Than 6V For outputs greater than 6V , add a resistor of 1k to 2.5k across the inductor to damp the discontinuous ringing of the SW node, preventing unintended SW current. The 12V output circuit in the Typical Applications section shows the location of this resistor . Other Linear Technology Publications Application Notes 19, 35 and 44 contain more detailed descriptions and design information for step-down regu- lators and other switching regulators. The L T1376 data sheet has a more extensive discussion of output ripple, loop compensation and stability testing. Design Note 318 shows how to generate a dual polarity output supply using a step-down regulator . APPLICATIONS INFORMATION

1MHz, Wide Input Range 5V and 1.8V Outputs VIN 5.7V TO 36V BOOST2 L T3508 VIN1 VIN2 GND

3508 TA04

47μF 10μF 4.7μF 0.1μF 0.1μF 47k 100pF 39k 15.0k 33.2k f SW = 1MHz 56.2k 100k 18.7k 10.7k 3.3μH L1 6.8μH OUT1 D3 D4 POWER GOOD OUT2 1.8V OUT1 0.9A 1nF 3.3nF BOOST1 SHDN SW2SW1 FB2FB1 VC2 PG1 VC1 TRACK/SS1 PG2TRACK/SS2 ON OFF C1 TO C5: X5R OR X7R D1, D2: MMSD4148 D3: DIODES INC. B240A D4: DIODES INC. B120 3.3V and 5V Dual Output Step-Down Converter with Output Sequencing VIN 5.7V TO 36V BOOST2 L T3508 VIN2VIN1 SHDN GND

3508 TA03

10μF 22μF 4.7μF 0.22μF 0.22μF ON OFF 43k 150pF 51k 10.7k 52.3k f SW = 700kHz 35.7k 100k 56.2k 11.5k L2 10μH L1 6.8μH D1 D2 D3 D4 POWER GOOD OUT2 1.4A OUT1 3.3V 1.4A 1nF BOOST1 SW2SW1 FB2FB1 VC2 PG1 VC1 TRACK/SS1 PG2TRACK/SS2 C1 TO C5: X5R OR X7R D1, D2: MMSD4148 D3: DIODES INC. B140 D4: DIODES INC. B240A TYPICAL APPLICATIONS

1MHz, 5V and 12V Outputs VIN 14V TO 36V BOOST2 L T3508 VIN2VIN1 SHDN GND

3508 TA06

10μF 4.7μF 4.7μF 0.1μF 0.1μF ON OFF 39k 100pF 43k 10.7k 33.2k f SW = 1MHz R1 154k R2 1k 100k C1 TO C5: X5R OR X7R D1, D2: MMSD4148 D3: DIODES INC. B240A D4: DIODES INC. B140 R2: USE 0.25W RESISTOR. FOR CONTINUOUS OPERATION ABOVE 30V , USE TWO 2k, 0.25W RESISTORS IN PARALLEL *DERATE OUTPUT CURRENT AT HIGHER AMBIENT TEMPERATURES AND INPUT VOL TAGES TO MAINTAIN JUNCTION TEMPERATURE BELOW THE ABSOLUTE MAXIMUM 56.2k 11.0k L2 6.8μH L1 15μH POWER GOOD OUT2 1.4A* OUT1 12V 1.4A* OUT2 1nF BOOST1 SW2SW1 FB2FB1 VC2 PG1 VC1 TRACK/SS1 PG2TRACK/SS2

16-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663) Exposed Pad Variation BA PACKAGE DESCRIPTION FE16 (BA) TSSOP 0204 0.09 – 0.20 (.0035 – .0079) 0° – 8° 0.25 REF 0.50 – 0.75 (.020 – .030) 4.30 – 4.50* (.169 – .177) 13 4 5 6 7 8 10 9 4.90 – 5.10* (.193 – .201) 16 1514 13 12 11 1.10 (.0433) MAX 0.05 – 0.15 (.002 – .006) 0.65 (.0256) BSC 2.74 (.108) 2.74 (.108) 0.195 – 0.30 (.0077 – .0118) TYP MILLIMETERS (INCHES) *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.150mm (.006") PER SIDE NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS 2. DIMENSIONS ARE IN RECOMMENDED SOLDER PAD LAYOUT 3. DRAWING NOT TO SCALE 0.45 ±0.05

0.65 BSC

4.50 ±0.10 6.60 ±0.10 1.05 ±0.10 2.74 (.108) 2.74 (.108) SEE NOTE 4 4. RECOMMENDED MINIMUM PCB METAL SIZE FOR EXPOSED PAD ATTACHMENT 6.40 (.252) BSC

Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However , no responsibility is assumed for its use. Linear Technology Corporation makes no representa- tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. PACKAGE DESCRIPTION 24-Lead Plastic QFN (4mm × 4mm) (Reference LTC DWG # 05-08-1697) 4.00 ± 0.10 (4 SIDES) NOTE: 1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGD-X)—TO BE APPROVED 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE, IF PRESENT 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 PIN 1 TOP MARK (NOTE 6) 0.40 ± 0.10 2423 BOTTOM VIEW—EXPOSED PAD 2.45 ± 0.10 (4-SIDES) 0.75 ± 0.05 R = 0.115 TYP 0.25 ± 0.05

0.50 BSC

0.200 REF

0.00 – 0.05 (UF24) QFN 0105 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 0.70 ±0.05 0.25 ±0.05 2.45 ± 0.05 (4 SIDES)3.10 ± 0.05 4.50 ± 0.05 PACKAGE OUTLINE PIN 1 NOTCH R = 0.20 TYP OR 0.35 × 45° CHAMFER

Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2007 LT 0208 REV B • PRINTED IN USA RELATED PARTS PART NUMBER DESCRIPTION COMMENTS L T1765 25V , 2.75A (I OUT), 1.25MHz, High Effi ciency Step-Down DC/DC Converter VIN: 3V to 25V , VOUT(MIN) = 1.2V , IQ = 1mA, S8, TSSOP16E Packages L T1766 60V , 1.2A (I OUT), 200kHz, High Effi ciency Step-Down DC/DC Converter VIN: 5.5V to 60V , VOUT(MIN) = 1.2V , IQ = 2.5mA, TSSOP16/TSSOP16E Packages L T1767 25V , 1.2A (I OUT), 1.25MHz, High Effi ciency Step-Down DC/DC Converter VIN: 3V to 25V , VOUT(MIN) = 1.2V , IQ = 1mA, MS8, MS8E Packages L T1940/L T1940L Dual Monolithic 1.4A, 1.1MHz Step-Down Switching Regulators VIN: 3.6V to 25V , VOUT(MIN) = 1.25V , IQ = 3.8mA, TSSOP16E Packages L TC3407 Dual 600mA, 1.5MHz, Synchronous Step-Down Regulator VIN: 2.5V to 5.5V , VOUT(MIN) = 0.6V , IQ = 40μA, MSE Package L T3493 1.2A, 750kHz Step-Down Switching Regulator in 2mm × 3mm DFN VIN: 3.6V to 36V , VOUT(MIN) = 0.78V , IQ = 1.9mA, 2mm × 3mm DFN Package L T3501/L T3510 Dual 3A/2A, 1.5MHz High Effi ciency Step-Down Switching Regulators VIN: 3.6V to 25V , VOUT(MIN) = 0.8V , IQ = 3.7mA, ISD < 10μA, L T3506/L T3506A Dual Monolithic 1.6A, 1.1MHz Step-Down Switching Regulators VIN: 3.6V to 25V , VOUT(MIN) = 0.8V , IQ = 3.8mA, 16-Lead DFN and 16-Lead TSSOPE Packages L TC3701 T wo Phase, Dual, 500kHz, Constant Frequency, Current Mode, High Effi ciency Step-Down DC/DC Controller VIN: 2.5V to 10V , VOUT(MIN) = 0.8V , IQ = 460μA, SSOP-16 Package L TC3736 Dual T wo Phase, No R SENSE™, Synchronous Controller with Output T racking VIN: 2.75V to 9.8V , VOUT(MIN) = 0.6V , IQ = 300μA, 4mm × 4mm QFN or SSOP-24 Packages L TC3737 Dual T wo Phase, No R SENSE DC/DC Controller with Output T racking VIN: 2.75V to 9.8V , VOUT(MIN) = 0.6V , IQ = 220μA, 4mm × 4mm QFN or SSOP-24 Packages No RSENSE is a trademark of Linear Technology Corporation. TYPICAL APPLICATION VIN 12V BOOST2 L T3508 VIN1 VIN2 GND

3508 TA05

47μF 10μF 4.7μF 0.1μF 40.2k R10 14.7k 0.1μF 47k 100pF 43k 15.0k 33.2k C1 TO C6: X5R OR X7R D1, D2: MMSD4148 D3: DIODES INC. B140 D4: DIODES INC. B120 f SW = 1MHz 52.3k 100k 18.7k 10k 3.3μH L1 6.8μH VIN2 3.3V D3 D4 POWER GOOD OUT2 1.8V 1.4A OUT1 0.9A 4.7μF 0.047μF C10 0.047μF BOOST1 SHDN SW2SW1 FB2FB1 VC2 PG1 VC1 TRACK/SS1 PG2TRACK/SS2 5V , 1.8V Output from PCI Express