LT8616 Dual 42V Synchronous Monolithic Step-Down Regulator with 6.5µA Quiescent Current Description Features Wide Input Voltage Range: 3.4V to 42V 2.5A and 1.5A Buck Regulators with Separate Inputs Fast Minimum Switch On-Time: 35ns Ultralow Quiescent Current Burst Mode® Operation: n 6.5µA I Regulating 12V to 5V Q IN OUT and 3.3VOUT n Output Ripple < 15mV n180° Out of Phase Switching n Adjustable and Synchronizable: 200kHz to 3MHz n Accurate 1V Enable Pin Thresholds n Internal Compensation n Output Soft-Start and Tracking n TSSOP Package: Output Stays at or Below Regulation Voltage During Adjacent Pin Short or When a Pin Is Left Floating n Thermally Enhanced 28-Lead TSSOP Package n n n Applications The LT®8616 is a high efficiency, high speed, dual synchronous monolithic step-down switching regulator that consumes only 6.5µA of quiescent current with both channels enabled. Both channels contain all switches and necessary circuitry to minimize the need for external components. Low ripple Burst Mode operation enables high efficiency down to very low output currents while minimizing output ripple. A SYNC pin allows synchronization to an external clock. Internal compensation with peak current mode topology allows the use of small inductors and results in fast transient response and good loop stability. The enable pins have accurate 1V thresholds and can be used to program undervoltage lockout. Capacitors on the TR/SS pins programs the output voltage ramp rate during startup while the PG pins signal when each output is within 10% of the programmed output voltage. The LT8616 is available in a TSSOP package for high reliability. Automotive and Industrial Supplies n General Purpose Step-Down L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. n n Typical Application 5V, 3.3V, 700kHz Step-Down Converter 4.7µF 4.7µF VIN1 BOOST1 0.1µF EN/UV1 10nF 1µF 56.2k 90 10µH SW1 VIN2 1M EN/UV2 FB1 PG1 PG2 SYNC/MODE BOOST2 70 60 40 4.7µH SW2 1M GND VIN1 = VIN2 = 12V fSW = 700kHz 80 50 BIAS 0.1µF TR/SS1 TR/SS2 INTVCC RT 5.6pF VOUT1 5V, 1.5A 47µF 187k LT8616 1M Efficiency 100 EFFICIENCY (%) VIN 12V 10pF VOUT2 3.3V, 2.5A 30 0.01 CH1, 5VOUT CH2, 3.3VOUT 0.1 1 10 LOAD (mA) 100 1000 8616 TA01b FB2 2 x 47µF 316k 8616 TA01a 8616f For more information www.linear.com/LT8616 1 LT8616 Absolute Maximum Ratings Pin Configuration (Note 1) TOP VIEW VIN1, VIN2, EN/UV1, EN/UV2, PG1, PG2......................42V BIAS...........................................................................30V BST1 Above SW1, BST2 Above SW2, FB1, FB2, TR/SS1, TR/SS2......................................................4V SYNC/MODE................................................................6V Operating Junction Temperature Range (Note 2) LT8616E.............................................. –40°C to 125°C LT8616I............................................... –40°C to 125°C LT8616H............................................. –40°C to 150°C Storage Temperature Range................... –60°C to 150°C EN/UV2 1 28 TR/SS2 PG2 2 27 FB2 SW2 3 26 FB2 SW2 4 25 NC SW2 5 24 VIN2 BOOST2 6 NC 7 BOOST1 8 SW1 9 23 NC 29 GND 22 BIAS 21 INTVCC 20 NC SW1 10 19 VIN1 PG1 11 18 NC TR/SS1 12 17 SYNC/MODE FB1 13 16 EN/UV1 FB1 14 15 RT FE PACKAGE 28-LEAD PLASTIC TSSOP TJMAX = 150°C, θJA = 30°C/W, θJC = 10°C/W EXPOSED PAD (PIN 29) IS GND, MUST BE SOLDERED TO PCB Order Information LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT8616EFE#PBF LT8616EFE#TRPBF LT8616FE 28-Lead Plastic TSSOP –40 to 125°C LT8616IFE#PBF LT8616IFE#TRPBF LT8616FE 28-Lead Plastic TSSOP –40 to 125°C LT8616HFE#PBF LT8616HFE#TRPBF LT8616FE 28-Lead Plastic TSSOP –40 to 150°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. Consult LTC Marketing for information on nonstandard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ 2 8616f For more information www.linear.com/LT8616 LT8616 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. PARAMETER CONDITIONS MIN TYP MAX UNITS l 1.7 1.7 4.0 8.0 µA µA l 3.0 3.0 5.0 12.0 µA µA l 0.5 1.0 mA 790 790 798 802 mV mV Common Quiescent Current EN/UV1 = EN/UV2 = 0V, Current from VIN1 EN/UV1 = EN/UV2 = 2V, SYNC = 0V (Burst Mode), Not Switching, Current from VIN1 EN/UV1 = EN/UV2 = 2V, SYNC = 3V (Pulse-Skipping Mode), Not Switching, Current from BIAS or VIN1 FB Voltage VIN = 6V, Load = 0.5A l FB Voltage Line Regulation VIN = 4V to 25V, Load = 0.5A FB Pin Input Current FB = 0.79V EN/UV Pin Threshold Rising 782 778 0.005 –20 l 0.97 EN/UV Pin Hysteresis %/V 20 1.03 1.09 50 –20 nA V mV EN/UV Pin Current EN/UV = 2V PG Upper Threshold from VFB FB Rising l 6 10 13 % PG Lower Threshold from VFB FB Falling l –6 –10 –13 % PG Hysteresis 20 1 PG Leakage PG = 3.3V PG Pull-Down Resistance PG = 0.1V –100 % 100 350 TR/SS Source Current 1 TR/SS Pull-Down Resistance TR/SS = 0.1V BIAS Pin Current Consumption VOUT1 = 3.3V, Load1 = 0.5A, VOUT2 = 3.3V, Load2 = 0.5A, fSW = 1MHz Oscillator Frequency RT = 14.7kΩ RT = 37.4kΩ RT = 221kΩ SYNC Threshold SYNC Falling SYNC Rising SYNC Pin Current SYNC = 3V l l l 1.85 900 160 2 nA nA Ω 3 µA 250 Ω 7 mA 2.05 1000 200 0.4 –100 2.25 1100 240 MHz kHz kHz 2.4 V V 100 nA Channel 1 Minimum VIN1 Voltage l Supply Current in Regulation VIN = 6V, VOUT1 = 3.3V, Load = 100µA VIN = 6V, VOUT1 = 3.3V Load = 1mA SW1 Minimum On-Time Load = 0.25A, Pulse-Skipping Mode l (Note 3) l 20 SW1 Top NMOS On-Resistance SW1 Peak Current Limit 3.4 V 80 620 110 910 µA µA 35 55 ns 310 3.2 SW1 Bottom NMOS On-Resistance 4.2 mΩ 5.2 190 SW1 Valley Current Limit SW1 Leakage Current 3.0 l VIN1 = 42V, VSW1 = 0V, 42V 1.5 –2 2.0 A mΩ 3.0 A 2 µA 8616f For more information www.linear.com/LT8616 3 LT8616 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. PARAMETER CONDITIONS MIN TYP MAX UNITS 3.0 3.4 V 80 620 110 910 µA µA 35 55 ns Channel 2 Minimum VIN1 Voltage to Use Channel2 l Supply Current in Regulation VIN = 6V, VOUT2 = 3.3V, Load2 = 100µA VIN = 6V, VOUT2 = 3.3V Load2 = 1mA SW2 Minimum On-Time Load = 0.25A, Pulse-Skipping Mode l 20 (Note 3) l 4.5 SW2 Top NMOS On-Resistance SW2 Peak Current Limit 145 SW2 Bottom NMOS On-Resistance l VIN2 = 42V, VSW2 = 0V, 42V 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 LT8616E is guaranteed to meet performance specifications from 0°C to 125°C junction temperature. Specifications over the –40°C to 125°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LT8616I is guaranteed over the full –40°C to 125°C operating junction temperature range. The LT8616H is guaranteed over the full –40°C to 150°C operating junction temperature range. 4 mΩ 6.5 120 SW2 Valley Current Limit SW2 Leakage Current 5.5 2.5 –2 3.5 A mΩ 4.5 A 2 µA Note 3: Current limit guaranteed by design and/or correlation to static test. Slope compensation reduces current limit at higher duty cycle. Note 4: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed the maximum operating junction temperature when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability. See High Temperature Considerations section. 8616f For more information www.linear.com/LT8616 LT8616 Typical Performance Characteristics Efficiency at 5VOUT fSW = 700kHz 94 EFFICIENCY (%) 92 90 88 86 CH1, 12VIN CH1, 24VIN CH2, 12VIN CH2, 24VIN 84 82 80 0 0.25 0.5 0.75 1.0 1.25 1.5 1.75 2.0 2.25 2.5 LOAD (A) 94 92 CH1, 12VIN CH1, 24VIN CH2, 12VIN CH2, 24VIN 100 98 fSW = 700kHz 96 EFFICIENCY (%) EFFICIENCY (%) 94 88 86 CH1, 12VIN CH1, 24VIN CH2, 12VIN CH2, 24VIN 84 82 80 0 0.25 0.5 0.75 1.0 1.25 1.5 1.75 2.0 2.25 2.5 LOAD (A) 30 VIN1 = VIN2 = 12V VOUT1 = 5V 25 VOUT2 = 3.3V IN REGULATION BIAS CURRENT (mA) SUPPLY CURRENT (µA) 85 0.2 15 10 CH1, 12VIN CH1, 24VIN CH2, 12VIN CH2, 24VIN 100 7 6 5 4 3 2 8616 G07 VOUT1 = 3.3V VOUT2 = 5V 1 0 1000 0 5 10 15 20 25 VIN (V) 1.06 12 10 8 VIN1 = VIN2 = 12V VOUT1 = 5V LOAD1 = 1A VOUT2 = 3.3V LOAD2 = 1A 0.5 1.0 1.5 2.0 FREQUENCY (MHz) 35 40 EN/UV Threshold 14 0 30 8616 G06 1.08 0 3.0 8 16 6 2.6 9 1.1 2 25 50 75 100 125 150 TEMPERATURE (°C) 1.0 1.4 1.8 2.2 FREQUENCY (MHz) 10 18 4 5 0.6 8616 G03 BIAS Current vs Switching Frequency 20 CH1, 5V, 1A CH2, 3.3V, 1.5A 8616 G05 No Load Supply Current 0 88 No Load Supply Current 100 95 fSW = 700kHz 90 85 80 75 70 65 60 55 50 45 40 35 30 0.01 0.1 1 10 LOAD (mA) 8616 G04 0 –50 –25 89 86 Efficiency at 3.3VOUT 90 90 8616 G02 Efficiency at 3.3VOUT 92 91 87 1000 8616 G01 100 VIN1 = VIN2 = 12V 93 INPUT CURRENT (µA) EFFICIENCY (%) 96 95 2.5 3.0 8616 G08 EN THRESHOLD (V) 98 Efficiency vs Frequency 100 f = 700kHz 95 SW 90 85 80 75 70 65 60 55 50 45 40 35 30 0.01 10 0.1 1 LOAD (mA) EFFICIENCY (%) Efficiency at 5VOUT 100 1.04 1.02 1.0 0.98 0.96 EN RISING EN FALLING 0.94 0.92 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 8616 G09 8616f For more information www.linear.com/LT8616 5 LT8616 Typical Performance Characteristics 0.1 Line Regulation vs VIN1 0.15 0.06 CHANGE IN VOUT (%) 0.04 0.02 0 –0.02 –0.06 –0.1 25 50 75 100 125 150 TEMPERATURE (°C) Power-Good Thresholds 0 5 10 15 20 25 VIN1 (V) 35 –0.2 40 PG LOW FALLING PG HIGH RISING PG LOW RISING PG HIGH FALLING 2.25 800 2.2 2.15 600 500 400 300 25 50 75 100 125 150 TEMPERATURE (°C) 0 100 200 300 400 500 600 700 800 900 1000 SS/TR VOLTAGE (mV) RT = 14.7kΩ 0 25 50 75 100 125 150 TEMPERATURE (°C) 8616 G14 8616 G15 Minimum On-Time Switching Period vs RT Minimum Off-Time 50 90 85 45 200 80 180 140 120 100 TIME (ns) 40 160 TIME (ns) RT RESISTOR (kΩ) 2 1.85 –50 –25 220 35 30 80 75 70 65 60 60 25 40 55 20 0 1 2 3 4 SWITCHING PERIOD (µs) 5 8616 G16 6 2.05 1.9 100 0 2.1 1.95 8616 G13 0 0 0.25 0.5 0.75 1.0 1.25 1.5 1.75 2.0 2.25 2.5 LOAD (A) Switching Frequency 900 200 –10 0 FB1 FB2 8616 G12 FREQUENCY (MHz) FB VOLTAGE (mV) PG THRESHOLD RELATIVE TO FB (%) 30 700 5 –15 –50 –25 –0.1 Soft-Start Tracking 10 –5 0 8616 G11 15 0 0.05 –0.15 –0.08 0 0.1 –0.05 –0.04 8616 G10 240 Load Regulation 0.2 VIN1 = VIN2 0.08 VOLTAGE (%) VOLTAGE (mV) FB Voltage vs Temperature 806 804 802 800 798 796 794 792 790 788 786 784 782 780 778 776 774 –50 –25 20 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 8616 G17 50 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 8616 G18 8616f For more information www.linear.com/LT8616 LT8616 Typical Performance Characteristics Burst Frequency vs Load 80 600 60 500 VIN1 = VIN2 =12V fSW = 700kHz SYNC = 0V 300 200 100 50 50 40 30 20 10 CH1, 5V CH2, 3.3V 0 5.0 0 100 150 200 250 300 350 400 LOAD CURRENT (mA) 0 5 10 15 20 25 VIN (V) 30 35 8616 G19 600 3.0 2.0 40 4 3 2 CH1, 0% DC PEAK CH1, VALLEY CH2, 0% DC PEAK CH2, VALLEY 25 50 75 100 125 150 TEMPERATURE (°C) 40 60 DUTY CYCLE (%) 80 200 100 fSW = 2MHz 700 600 500 400 300 200 CH1, 5V CH2, 5V 100 0 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 0 0 0.5 1 1.5 LOAD (A) 2 2.5 8616 G24 8616 G23 Start-Up Dropout (CH2, 3.3V) VIN1 1V/DIV VIN1 AND VIN2 1V/DIV VIN2 1V/DIV VOUT1 1V/DIV VOUT2 1V/DIV VOUT2 1V/DIV 100ms/DIV VIN1 = VIN2 RLOAD2 = 1.32Ω (2.5A) 100 800 Start-Up Dropout (CH2, 3.3V) 8616 G26 20 8616 G21 900 300 Start-Up Dropout (CH1, 5V) 100ms/DIV 0 Dropout Voltage vs Load 400 8616 G22 RLOAD1 = 3.33Ω (1.5A) CH1 CH2 1000 CH1, TOP CH1, BOTTOM CH2, TOP CH2, BOTTOM 500 RESISTANCE (mΩ) CURRENT LIMIT (A) 5 0 3.5 Switch Resistance vs Temperature 6 0 –50 –25 4.0 8616 G20 Current Limit vs Temperature 1 4.5 2.5 CH1, 5V CH2, 3.3V DROPOUT VOLTAGE (mV) 400 5.5 CURRENT LIMIT (A) 700 fSW = 700kHz 70 SYNC = 3V LOAD CURRENT (mA) SWITCHING FREQUENCY (kHz) 800 0 Top FET Current Limit vs Duty Cycle Minimum Load for Full Frequency 100ms/DIV 8616 G27 VIN1 = 6V RLOAD2 = 1.32Ω (2.5A) 8616 G28 8616f For more information www.linear.com/LT8616 7 LT8616 Typical Performance Characteristics 3.6 VIN1 UVLO Transient CH1, 5V Transient CH2, 3.3V 3.4 VOUT1 (AC) 200mV/DIV 3.2 VOUT2 (AC) 50mV/DIV VIN1 (V) 3.0 IL1 500mA/DIV 2.8 IL2 1A/DIV 2.6 2.4 20µs/DIV 2.2 2.0 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 8616 G25 CCM 8616 G29 20µs/DIV VIN1 = 12V VOUT1 = 5V L1 = 10µH COUT1 = 47µF CFF = 5.6pF VIN2 = 12V VOUT2 = 3.3V L2 = 4.7µH COUT2 = 2 x 47µF CFF2 = 10pF Burst Mode DCM VSW 5V/DIV VSW 5V/DIV VSW 5V/DIV IL 500mA/DIV IL 500mA/DIV IL 500mA/DIV 1µs/DIV 12VIN TO 5VOUT AT 500mA 8616 G31 1µs/DIV 12VIN TO 5VOUT AT 50mA SYNC = 0V CH1 CCM, CH2 CCM 8616 G32 1µs/DIV 12VIN TO 5VOUT AT 50mA SYNC = 3V CH1 CCM, CH2 Burst Mode SW1 5V/DIV SW1 5V/DIV SW2 5V/DIV SW2 5V/DIV SW2 5V/DIV VIN = 12V CH1 = 5V, 1A CH2 = 3.3V, 1A SYNC = 0V 8 8616 G34 1µs/DIV 8616 G33 CH1 Shorted, CH2 CCM SW1 5V/DIV 500ns/DIV 8616 G30 8616 G35 VIN = 12V CH1 = 5V, 1A CH2 = 3.3V, 0.1A SYNC = 0V 5µs/DIV 8616 G36 VIN = 12V CH1 = 0V SHORT CH2 = 3.3V, 1A SYNC = 0V 8616f For more information www.linear.com/LT8616 LT8616 Pin Functions BIAS: The BIAS pin supplies the internal regulator when tied to a voltage higher than 3.1V. For output voltages of 3.3V and above this pin should be tied to the appropriate VOUT. Connect a 1µF bypass capacitor to this pin if it is connected to a supply other than VOUT1 or VOUT2. Ground if unused. BOOST1, BOOST2: The BOOST pins are used to provide drive voltages, higher than the input voltage, to the internal topside power switches. Place 0.1µF capacitors between BOOST and its corresponding SW pin as close as possible to the IC. BOOST nodes should be kept small on the PCB for good performance. EN/UV1, EN/UV2: The EN/UV pins are used to independently disable each channel when pulled low and enable when pulled high. The hysteretic threshold voltage is 1.03V going up and 0.98V going down. Tie to VIN supply if the shutdown feature is not used. External resistor dividers from VIN can be used to program thresholds below which each channel is disabled. Don’t float these pins. FB1, FB2: The FB pins are regulated to 0.790V. Connect the feedback resistor divider taps to the FB pins. Also connect phase lead capacitors between FB pins and VOUT nodes. Typical phase lead capacitors are 1.5pF to 10pF. GND: The GND pins and exposed pad must be connected to the negative terminal of the input capacitors and soldered to the PCB in order to lower the thermal resistance. INTVCC: The INTVCC pin provides power to internal gate drivers and control circuits. INTVCC current will be supplied from BIAS if VBIAS > 3.1V, otherwise current will be drawn from VIN1. Decouple this pin to ground with at least a 1μF low ESR ceramic capacitor. Do not load the INTVCC pin with external circuitry. NC: The NC pins have no internal connection. Float NC pins to increase fault tolerance or connect to ground to facilitate PCB layout. PG1, PG2: The PG pins are the open-drain outputs of the internal power good comparators. Each channel's PG pin remains low until the respective FB pin is within ±10% of the final regulation voltage and there are no fault conditions. RT: A resistor is tied between RT and ground to set the switching frequency. SW1, SW2: The SW pins are the outputs of each channel's internal power switches. Connect these pins to the inductors and boost capacitors. SW nodes should be kept small on the PCB for good performance. SYNC/MODE: Ground the SYNC/MODE pin for low ripple Burst Mode operation at low output loads. Tie to a clock source for synchronization to an external frequency. Apply a DC voltage of 2.4V or higher or tie to INTVCC for pulseskipping mode. When in pulse-skipping mode, the IQ will increase to several hundred μA. Channel 1 will align its positive switching edge to the positive edge of the external clock and channel 2 will align its positive switching edge to the negative external clock edge. Do not float this pin. TR/SS1, TR/SS2: The TR/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 the TR/SS pin from the tracked output. For soft-start, tie a capacitor to TR/ SS. Internal 2μA pull-up currents from INTVCC charge soft-start capacitors to create voltage ramps. A TR/SS voltage below 0.79V forces the LT8616 to regulate the corresponding FB pins to equal the TR/SS pin voltage. When TR/SS voltages are above 0.79V, the tracking function is disabled and the internal reference resumes control of the error amplifiers. TR/SS pins are individually pulled to ground with internal 250Ω MOSFETs during shutdown and fault conditions; use series resistors if driving from a low impedance output. VIN1: VIN1 supplies current to the LT8616's internal circuitry and to channel 1's topside power switch. This pin must be locally bypassed. Be sure to place the positive terminal of the input capacitor as close as possible to the pin, and the negative capacitor terminal as close as possible to the GND pins. VIN1 must be connected to 3.4V or above even if only channel 2 is in use. VIN2: VIN2 supplies current to internal channel 2's topside power switch. This pin must be locally bypassed. Be sure to place the positive terminal of the input capacitor as close as possible to the pin, and the negative capacitor terminal as close as possible to the GND pins. Please note VIN1 must be 3.4V or above to operate channel 2. 8616f For more information www.linear.com/LT8616 9 LT8616 Block Diagram VIN1 R5 1.03V EN/UV1 + – SHDN1 BIAS R7 1.03V EN/UV2 VIN1 INTERNAL REFERENCE AND 3.3V REGULATOR R6 VIN2 + – INTVCC VIN1 INTVCC CIN1 SHDN2 R8 CVCC PG1 SHDN1 TSD VIN1 UVLO INTVCC UVLO BOOST1 0.79V VOUT1 CFF1 R1 VC1 ERROR AMP ±10% FB1 + – + BURST LOGIC SWITCH LOGIC AND ANTISHOOT THROUGH CBST1 DRIVER SW1 L1 VOUT1 COUT1 INTVCC R2 2µA GND SLOPE COMP TR/SS1 SHDN1 TSD VIN1 UVLO CSS1 RT RT OSCILLATOR 200kHz TO 3MHz SYNC/MODE VIN2 SLOPE COMP PG2 SHDN2 TSD VIN1 UVLO INTVCC UVLO 0.79V VOUT2 CFF2 R3 ±10% FB2 R4 + – + VC2 ERROR AMP BURST LOGIC VIN2 CIN2 INTVCC BOOST2 SWITCH LOGIC AND ANTISHOOT THROUGH CBST2 DRIVER SW2 L2 VOUT2 COUT2 INTVCC GND 2µA TR/SS2 CSS2 SHDN2 TSD VIN1 UVLO GND 8616 BD 10 8616f For more information www.linear.com/LT8616 LT8616 Operation Foreword The LT8616 is a dual monolithic step down regulator. The two channels differ in maximum current and input range. The following sections describe the operation of channel 1 and common circuits. They will highlight channel 2 differences and interactions only when relevant. To simplify the application, both VIN1 and VIN2 are assumed to be connected to the same input supply. However, note that VIN1 must be greater than 3.4V for either channel to operate. Operation The LT8616 is a dual monolithic, constant frequency, peak current mode step-down DC/DC converter. An oscillator, with frequency set using a resistor on the RT pin, turns on the internal top power switch at the beginning of each clock cycle. Current in the inductor then increases until the top switch current comparator trips and turns off the top power switch. The peak inductor current at which the top switch turns off is controlled by the voltage on the internal VC node. The error amplifier servos the VC node by comparing the voltage on the FB pin with an internal 0.790V reference. When the load current increases it causes a reduction in the feedback voltage relative to the reference, causing the error amplifier to raise the VC voltage until the average inductor current matches the new load current. When the top power switch turns off, the synchronous power switch turns on until the next clock cycle begins or inductor current falls to zero. If overload conditions result in more than the valley current limit flowing through the bottom switch, the next clock cycle will be delayed until current returns to a safe level. If either EN/UV pin is low, the corresponding channel is shut down. If both EN/UV pins are low, the LT8616 is fully shut down and draws 1.7µA from the input supply. When the EN/UV pins are above 1.03V, the corresponding switching regulators will become active. 1.3μA is supplied by VIN1 to common bias circuits for both channels. Each channel can independently enter Burst Mode operation to optimize efficiency at light load. Between bursts, all circuitry associated with controlling the output switch is shut down, reducing the channel's contribution to input supply current. In a typical application, 6.5μA will be consumed from the input supply when regulating both channels with no load. Ground the SYNC/MODE pin for Burst Mode operation or apply a DC voltage above 2.4V to use pulse-skipping mode. If a clock is applied to the SYNC/MODE pin, both channels will synchronize to the external clock frequency and operate in pulse-skipping mode. While in pulse-skipping mode the oscillator operates continuously and SW transitions are aligned to the clock. During light loads, switch pulses are skipped to regulate the output and the quiescent current per channel will be several hundred µA. To improve efficiency across all loads, supply current to internal circuitry can be sourced from the BIAS pin when biased at 3.1V or above. Otherwise, the internal circuitry will draw current exclusively from VIN1. The BIAS pin should be connected to the lowest VOUT programmed at 3.3V or above. Comparators monitoring the FB pin voltage will pull the corresponding PG pin low if the output voltage varies more than ±10% (typical) from the regulation voltage or if a fault condition is present. Tracking soft-start is implemented by providing constant current via the TR/SS pin to an external soft-start capacitor to generate a voltage ramp. FB voltage is regulated to the voltage at the TR/SS pin until it exceeds 0.790V; FB is then regulated to the 0.790V reference. Soft-start also reduces the valley current limit to avoid inrush current during start-up. The SS capacitor is reset during shutdown, VIN1 undervoltage, or thermal shutdown. Channel 1 is designed for 1.5A load, whereas channel 2 is designed for 2.5A load. Channel 1 has a minimum VIN1 requirement of 3.4V, but channel 2 can operate with no minimum VIN2 provided that the minimum VIN1 has been satisfied. 8616f For more information www.linear.com/LT8616 11 LT8616 Applications Information Achieving Ultralow Quiescent Current As the output load decreases, the frequency of single current pulses decreases (see Figure 1a) and the percentage of time that the LT8616 is in sleep mode increases, resulting in much higher light load efficiency than for typical converters. By maximizing the time between pulses, the converter quiescent current approaches 6.5µA for a typical application when there is no output load. Therefore, to optimize the quiescent current performance at light loads, the current in the feedback resistor divider must be minimized as it appears to the output as load current. While in Burst Mode operation the current limit of the top switch is approximately 400mA for channel 1 (600mA for channel 2) resulting in output voltage ripple shown in Figure 2. Increasing the output capacitance will decrease the output ripple. As load increases from zero the switching frequency will increase but only up to the switching frequency programmed by the resistor at the RT pin as shown in Figure 1a. The output load at which the LT8616 reaches the programmed frequency varies based on input voltage, output voltage, and inductor value. For some applications it is desirable to select pulse-skipping mode to maintain full switching frequency at lower output load (see Figure 1b). See Pulse-Skipping Mode section. VIN1 = VIN2 = 12V 700 fSW = 700kHz SYNC = 0V 600 500 400 300 200 100 0 CH1, 5V CH2, 3.3V 0 50 100 150 200 250 300 350 400 LOAD CURRENT (mA) 8616 F01a Figure 1a. Frequency vs Load in Burst Mode Operation 80 fSW = 700kHz 70 SYNC = 3V LOAD CURRENT (mA) To enhance efficiency at light loads, the LT8616 operates in low ripple Burst Mode operation, which keeps the output capacitor charged to the desired output voltage while minimizing the input quiescent current and output voltage ripple. 1.7μA is supplied by VIN1 to common bias circuits. In Burst Mode operation the LT8616 delivers single small pulses of current to the output capacitor followed by sleep periods where the output power is supplied by the output capacitor. While in sleep mode the LT8616 consumes 3μA. SWITCHING FREQUENCY (kHz) 800 60 50 40 30 20 10 0 CH1, 5V CH2, 3.3V 0 5 10 15 20 25 VIN (V) 30 35 40 8616 F01b Figure 1b. Minimum Load for Full Frequency in Pulse-Skipping Mode VOUT (AC) 5mV/DIV FB Resistor Network The output voltage is programmed with a resistor divider between the output and the FB pin (R1 to R2 for channel 1, R3 to R4 for channel 2). Choose the resistor values according to: R1= R2 ⎛ VOUT1 – 1⎞ (1) ⎜⎝ ⎟ 0.790V ⎠ 12 IL 200mA/DIV 5µs/DIV 8616 F02 CH1 = 5V, 25mA Figure 2. Burst Mode Operation 8616f For more information www.linear.com/LT8616 LT8616 Applications Information Reference designators refer to the Block Diagram. 1% resistors are recommended to maintain output voltage accuracy. If low input quiescent current and good light-load efficiency are desired, use large resistor values for the FB resistor divider. The current flowing in the divider acts as a load current and will increase the no-load input current to the converter, which is approximately: ⎞ ⎛ 1⎞ ⎛ V ⎞⎛ V IQ = 3µA + ⎜ OUT1 ⎟ ⎜ OUT1 ⎟ ⎜ ⎟ ⎝ R1+R2 ⎠ ⎝ VIN1 ⎠ ⎝ η ⎠ (2) where 3µA is the quiescent current, the second term is the current in the feedback divider reflected to the input of channel 1 operating at its light load efficiency η. For a 3.3V application with R1 = 1M and R2 = 316k, the feedback divider draws 2.5µA. With VIN = 12V and η = 70%, this adds 1µA to the 3µA quiescent current resulting in 4µA no-load current from the 12V supply. The two channels of the LT8616 operate 180° out of phase to avoid aligned switching edge noise and input current ripple. Table 1. SW Frequency vs RT Value fSW (MHz) RT kΩ) fSW (MHz) RT kΩ) 0.2 221 1.6 20.5 0.3 143 1.8 17.8 0.4 105 2.0 15.4 0.5 80.6 2.05 14.7 0.6 66.5 2.2 13.3 0.7 56.2 2.4 11.8 0.8 47.5 2.6 10.3 1.0 37.4 2.8 9.31 1.2 29.4 3.0 8.25 1.4 24.3 Substitute R1 and R2 with R3 and R4 in the above equation if VIN1 and VIN2 are connected to the same voltage. For a typical FB resistor of 1MΩ, a 1.5pF to 10pF phaselead capacitor should be connected from VOUT to FB. The RT resistor required for a desired switching frequency can be calculated using: fSW 2 + 42.6 – 6.1 fSW 180 160 140 120 100 80 60 20 The LT8616 uses a constant frequency PWM architecture that can be programmed to switch from 200kHz to 3MHz by using a resistor tied from the RT pin to ground. Table 1 and Figure 3 show the necessary RT value for a desired switching frequency. 0.6 200 40 Setting the Switching Frequency RT = 220 RT RESISTOR (kΩ) Assuming channel 2 feedback divider contributes 2.5µA to the quiescent current, then the total quiescent current is 6.5µA. 240 (3) where RT is in kΩ and fSW is the desired switching frequency in MHz. 0 0 1 2 3 4 SWITCHING PERIOD (µs) 5 8616 F03 Figure 3. Switching Frequency vs RT Operating Frequency Selection and Trade-Offs Selection of the operating frequency is a trade-off between efficiency, component size, and input voltage range. The advantage of high frequency operation is that smaller inductor and capacitor values may be used. The disadvantages are lower efficiency and a smaller input voltage range for full frequency operation. 8616f For more information www.linear.com/LT8616 13 LT8616 Applications Information The highest switching frequency (fSW(MAX)) for a given application can be calculated as follows: fSW(MAX) = ( VOUT + VSW(BOT) tON(MIN) VIN – VSW(TOP) + VSW(BOT) ) (4) where VIN is the typical input voltage, VOUT is the output voltage, VSW(TOP) and VSW(BOT) are the internal switch drops (~0.53V, ~0.38V, respectively at maximum load for channel 1 and ~0.78V, ~0.48V for channel 2) and tON(MIN) is the minimum top switch on-time of 55ns (see the Electrical Characteristics). This equation shows that a lower switching frequency is necessary to accommodate a high VIN/VOUT ratio. Choose the lower frequency between channel 1 and 2. For transient operation, VIN may go as high as the absolute maximum rating of 42V regardless of the RT value, however the LT8616 will reduce switching frequency on each channel independently as necessary to maintain control of inductor current to assure safe operation. The LT8616 is capable of a maximum duty cycle of greater than 99%, and the VIN to VOUT dropout is limited by the RDS(ON) of the top switch. In this mode the channel that enters dropout skips switch cycles, resulting in a lower than programmed switching frequency. For applications that cannot allow deviation from the programmed switching frequency at low VIN/VOUT ratios, use the following formula to set switching frequency: VIN(MIN) = VOUT + VSW(BOT) 1– fSW • tOFF(MIN) – VSW(BOT) + VSW(TOP) (5) where VIN(MIN) is the minimum input voltage without skipped cycles, VOUT is the output voltage, VSW(TOP) and VSW(BOT) are the internal switch drops (~0.53V, ~0.38V, respectively at maximum load for channel 1 and ~0.78V, ~0.48V for channel 2), fSW is the switching frequency (set by RT), and tOFF(MIN) is the minimum switch off-time. Note that higher switching frequency will increase the minimum input voltage below which cycles will be dropped to achieve higher duty cycle. 14 Note there is no minimum VIN2 voltage requirement as it does not supply the internal common bias circuits, making channel 2 uniquely capable of operating from very low input voltages. Inductor Selection and Maximum Output Current The LT8616 is designed to minimize solution size by allowing the inductor to be chosen based on the output load requirements of the application. During overload or short-circuit conditions the LT8616 safely tolerates operation with a saturated inductor through the use of a high speed peak-current mode architecture. A good first choice for the inductor value is: VOUT1 + VSW1(BOT) L1= •1.6 fSW L2 = VOUT2 + VSW2(BOT) fSW (6a) (6b) where fSW is the switching frequency in MHz, VOUT is the output voltage, VSW(BOT) is the bottom switch drop (~0.38V, ~0.48V) and L is the inductor value in μH. To avoid overheating and poor efficiency, an inductor must be chosen with an RMS current rating that is greater than the maximum expected output load of the application. In addition, the saturation current (typically labeled ISAT) rating of the inductor must be higher than the load current plus 1/2 of in inductor ripple current: 1 IL(PEAK) =ILOAD(MAX) + ΔIL 2 (7) where ∆IL is the inductor ripple current as calculated in equation 9 and ILOAD(MAX) is the maximum output load for a given application. As a quick example, an application requiring 1A output should use an inductor with an RMS rating of greater than 1A and an ISAT of greater than 1.3A. During long duration overload or short-circuit conditions, the inductor RMS rating requirement is greater to avoid overheating of the inductor. To keep the efficiency high, the series resistance (DCR) should be less than 0.04Ω, and the core material should be intended for high frequency applications. 8616f For more information www.linear.com/LT8616 LT8616 Applications Information The LT8616 limits the peak switch current in order to protect the switches and the system from overload faults. The top switch current limit (ILIM) is 4.2A at 0% duty cycle and decreases linearly to 2.9A at DC = 80% (channel 2 current limit are 5.5A at 0% duty cycle and 3.7A at DC = 80%). The inductor value must then be sufficient to supply the desired maximum output current (IOUT(MAX)), which is a function of the switch current limit (ILIM) and the ripple current. IOUT(MAX) =ILIM – ΔIL 2 (8) The peak-to-peak ripple current in the inductor can be calculated as follows: ΔIL = VOUT L • fSW ⎛ ⎞ V • ⎜ 1– OUT ⎟ ⎝ VIN(MAX) ⎠ (9) where fSW is the switching frequency of the LT8616, and L is the value of the inductor. Therefore, the maximum output current that the LT8616 will deliver depends on the switch current limit, the inductor value, and the input and output voltages. Each channel has a secondary valley current limit. After the top switch has turned off, the bottom switch carries the inductor current. If for any reason the inductor current is too high, the bottom switch will remain on, delaying the top switch turning on until the inductor current returns to a safe level. This level is specified as the valley Current Limit, and is independent of duty cycle. Maximum output current in the application circuit is limited to this valley current plus one half of the inductor ripple current. In most cases current limit is enforced by the top switch. The bottom switch limit controls the inductor current when the minimum on-time condition is violated (high input voltage, high frequency or saturated inductor). The bottom switch current limit is designed to avoid any contribution to the maximum rated current of the LT8616. The optimum inductor for a given application may differ from the one indicated by this design guide. A larger value inductor provides a higher maximum load current and reduces the output voltage ripple. For applications requiring smaller load currents, the value of the inductor may be lower and the LT8616 may operate with higher ripple current. This allows use of a physically smaller inductor, or one with a lower DCR resulting in higher efficiency. Be aware that low inductance may result in discontinuous mode operation, which further reduces maximum load current. For more information about maximum output current and discontinuous operation, see Linear Technology’s Application Note 44. Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5), a minimum inductance is required to avoid sub-harmonic oscillation. See Application Note 19. Table 2. Inductor Manufacturers VENDOR URL Coilcraft www.coilcraft.com Sumida www.sumida.com Toko www.toko.com Würth Elektronik www.we-online.com Vishay www.vishay.com Input Capacitor Bypass the input of the LT8616 circuit with a ceramic capacitor of X7R or X5R type placed as close as possible to the VIN and GND pins. Y5V types have poor performance over temperature and applied voltage, and should not be used. A 2.2μF to 10μF ceramic capacitor is adequate to bypass the LT8616 and will easily handle the ripple current. Note that larger input capacitance is required when a lower switching frequency is used. If the input power source has high impedance, or there is significant inductance due to long wires or cables, additional bulk capacitance may be necessary. This can be provided with a low performance electrolytic capacitor. Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. The input capacitor is required to reduce the resulting voltage ripple at the LT8616 and to force this very high frequency 8616f For more information www.linear.com/LT8616 15 LT8616 Applications Information switching current into a tight local loop, minimizing EMI. A 2.2μF capacitor is capable of this task, but only if it is placed close to the LT8616 (see the PCB Layout section). A second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the LT8616. A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT8616 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT8616’s voltage rating. This situation is easily avoided (see Linear Technology Application Note 88). Output Capacitor and Output Ripple The output capacitor has two essential functions. Along with the inductor, it filters the square wave generated by the LT8616 to produce the DC output. In this role it determines the output voltage ripple, thus, low impedance at the switching frequency is important. The second function is to store energy in order to satisfy transient loads and stabilize the LT8616’s control loop. Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance. For good starting values, see the Typical Applications section. Use X5R or X7R types. This choice will provide low output ripple and good transient response. Transient performance can be improved with a higher value output capacitor and the addition of a feed forward capacitor placed between VOUT and FB. Increasing the output capacitance will also decrease the output voltage ripple. A lower value of output capacitor can be used to save space and cost but transient performance will suffer and may cause loop instability. See the Typical Applications in this data sheet for suggested capacitor values. When choosing a capacitor, special attention should be given to the data sheet to calculate the effective capacitance under the relevant operating conditions of voltage bias and temperature. A physically larger capacitor or one with a higher voltage rating may be required. 16 Ceramic Capacitors Ceramic capacitors are small, robust and have very low ESR. However, ceramic capacitors can cause problems when used with the LT8616 due to their piezoelectric nature. When in Burst Mode operation, the LT8616’s switching frequency depends on the load current, and at very light loads the LT8616 can excite the ceramic capacitor at audio frequencies, generating audible noise. Since the LT8616 operates at a lower current limit during Burst Mode operation, the noise is typically very quiet to a casual ear. If this is unacceptable, use a high performance tantalum or electrolytic capacitor at the output. Low noise ceramic capacitors are also available. Table 3. Ceramic Capacitor Manufacturers MANUFACTURER WEB Taiyo Yuden www.t-yuden.com AVX www.avxcorp.com Murata www.murata.com TDK www.tdk.com Enable Pin The LT8616 is in shutdown when both EN/UV pins are low and active when either pin is high. The rising threshold of the EN/UV comparator is 1.03V, with 50mV of hysteresis. The EN/UV pins can be tied to VIN if the shutdown feature is not used, or tied to a logic level if shutdown control is required. Adding a resistor divider from VIN to EN/UV programs the LT8616 to operate only when VIN is above a desired voltage (see the Block Diagram). Typically, this threshold, VIN(EN), is used in situations where the input supply is current limited, or has a relatively high source resistance. A switching regulator draws constant power from the source, so source current increases as source voltage drops. This looks like a negative resistance load to the source and can cause the source to current limit or latch low under low 8616f For more information www.linear.com/LT8616 LT8616 Applications Information ⎛ VIN1(EN) ⎞ R5 = R6 ⎜ –1 ⎝ 1.03V ⎟⎠ (10) where the corresponding channel will remain off until VIN is above VIN(EN). Due to the comparator’s hysteresis, switching will not stop until the input falls slightly below VIN(EN). When operating in Burst Mode operation for light load currents, the current through the VIN(EN) resistor network can easily be greater than the supply current consumed by the LT8616. Therefore, the VIN(EN) resistors should be large to minimize their effect on efficiency at low loads. INTVCC Regulator An internal low dropout (LDO) regulator produces the 3.4V supply from VIN1 that powers the drivers and the internal bias circuitry. For this reason, VIN1 must be present and valid to use either channel. The INTVCC pin supplies current for the LT8616’s circuitry and must be bypassed to ground with a 1μF ceramic capacitor. Good bypassing is necessary to supply the high transient currents required by the power MOSFET gate drivers. To improve efficiency, the internal LDO will draw current from the BIAS pin when the BIAS pin is at 3.1V or higher. Typically, the BIAS pin is tied to the lowest output or external supply above 3.1V. If BIAS is connected to a supply other than VOUT, bypass it with a local ceramic capacitor. If the BIAS pin is below 3.0V, the internal LDO will consume current from VIN1. Applications with high input voltage and high switching frequency where the internal LDO pulls current from VIN1 will increase die temperature because of the higher power dissipation across the LDO. Do not connect an external load to the INTVCC pin. Output Voltage Tracking and Soft-Start The LT8616 allows the user to program its output voltage ramp rate with the TR/SS pin. An internal 2μA current pulls up the TR/SS pin to INTVCC. Putting an external capacitor on TR/SS enables soft starting the output to prevent current surge on the input supply. During the soft-start ramp the output voltage will proportionally track the TR/SS pin voltage. For output tracking applications, TR/SS can be externally driven by another voltage source. From 0V to 0.790V, the TR/SS voltage will override the internal 0.790V reference input to the error amplifier, thus regulating the FB pin voltage to that of TR/SS pin (figure 4). When TR/SS is above 0.790V, tracking is disabled and the feedback voltage will regulate to the internal reference voltage. The TR/SS pin may be left floating if the function is not needed. Note the LT8616 will not discharge the output to regulate to a lower TR/SS voltage (figure 5). An active pull-down circuit is connected to the TR/SS pin which will discharge the external soft-start capacitor in the case of fault conditions and restart the ramp when the faults are cleared. Fault conditions that clear the soft-start capacitor are the corresponding EN/UV pin below 0.92V, VIN1 voltage falling too low, or thermal shutdown. 900 800 700 FB VOLTAGE (mV) source voltage conditions. The VIN(EN) threshold prevents the regulator from operating at source voltages where the problems might occur. This threshold can be adjusted by setting the values R5 and R6 (R7, R8 for channel 2) such that they satisfy the following equation: 600 500 400 300 200 100 0 0 100 200 300 400 500 600 700 800 900 1000 TR/SS VOLTAGE (mV) 8616 F04 Figure 4. FB Tracking TR/SS Voltage Until 0.790V 8616f For more information www.linear.com/LT8616 17 LT8616 Applications Information Synchronization TR/SS 500mV/DIV VOUT 2V/DIV 2ms/DIV 8616 F05 Figure 5. TR/SS Does Not Discharge VOUT Channel 1 will synchronize its positive switch edge transitions to the positive edge of the SYNC signal, and channel 2 will synchronize to the negative edge of the SYNC signal. PG THRESHOLD RELATIVE TO FB (%) 15 10 5 PG LOW FALLING PG HIGH RISING PG LOW RISING PG HIGH FALLING 0 –5 –10 –15 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 8616 F06 Figure 6. Power-Good Thresholds Output Power Good When the LT8616’s output voltage is within the ±10% window of the regulation point, which is a FB voltage in the range of 0.72V to 0.88V (typical), the output voltage is considered good and the open-drain PG pin goes high impedance and is typically pulled high with an external resistor. Otherwise, the internal pull-down device will pull the PG pin low. To prevent glitching, both the upper and lower thresholds include 1% of hysteresis. See figure 6. The PG pin is also actively pulled low during several fault conditions: corresponding EN/UV pin below 0.92V, INTVCC voltage falling too low, VIN1 UVLO, or thermal shutdown. 18 To select low ripple Burst Mode operation, tie the SYNC/ MODE pin below 0.4V (this can be ground or a logic low output). To select pulse skip mode, tie the SYNC/MODE pin above 2.4V (SYNC/MODE can be tied to INTVCC). To synchronize the LT8616 oscillator to an external frequency connect a square wave (with 20% to 80% duty cycle) to the SYNC/MODE pin. The square wave amplitude should have valleys that are below 0.4V and peaks above 2.4V (up to 6V). The LT8616 will not enter Burst Mode operation at low output loads while synchronized to an external clock, but instead will pulse skip to maintain regulation. The LT8616 may be synchronized over a 200kHz to 3MHz range. The RT resistor should be chosen to set the LT8616 switching frequency to 20% below the lowest synchronization input. For example, if the synchronization signal will be 500kHz and higher, the RT should be selected for 400kHz. The slope compensation is set by the RT value, while the minimum slope compensation required to avoid subharmonic oscillations is established by the inductor size, input voltage, and output voltage. Since the synchronization frequency will not change the slopes of the inductor current waveform, if the inductor is large enough to avoid subharmonic oscillations at the frequency set by RT, then the slope compensation will be sufficient for all synchronization frequencies. The duty cycle of the SYNC signal can be used to set the relative phasing of the two channels for minimizing input ripple. The LT8616 does not operate in forced continuous mode regardless of the SYNC signal. Never leave the SYNC/ MODE pin floating. 8616f For more information www.linear.com/LT8616 LT8616 Applications Information Pulse-Skipping Mode Pulse-skipping mode is activated by applying logic high (above 2.4V) or an external clock to the SYNC/MODE pin. While in pulse-skipping mode, the oscillator operates continuously and SW transitions are aligned to the clock. During light loads, switch pulses are skipped to regulate the output and the quiescent current per channel will be several hundred µA. Full switching frequency is reached at lower output load than in Burst Mode operation. Figure 7 shows a connection of the VIN and EN/UV pins that will allow the LT8616 to run only when the input voltage is present and that protects against a shorted or reversed input. D1 VIN1 D2 VIN1 VIN2 VIN2 LT8616 EN/UV1 EN/UV2 GND 8616 F07 Shorted and Reversed Input Protection The LT8616 will tolerate a shorted output. The bottom switch current is monitored such that if inductor current is beyond safe levels, turn on of the top switch will be delayed until the inductor current falls to safe levels. A fault condition of one channel will not affect the operation of the other. There is another situation to consider in systems where the output will be held high when the input to the LT8616 is absent. This may occur in battery charging applications or in battery-backup systems where a battery or some other supply is OR-ed with channel 1's output. If the VIN1 pin is allowed to float and either EN/UV pin is held high (either by a logic signal or because it is tied to VIN1), then the LT8616’s internal circuitry will pull its quiescent current through its SW1 pin. This is acceptable if the system can tolerate current draw in this state. If both EN/UV pins are grounded the SW1 pin current will drop to near 1µA. However, if the VIN1 pin is grounded while channel 1 output is held high, regardless of EN/UV1, parasitic body diodes inside the LT8616 can pull current from the output through the SW1 pin and the VIN1 pin, damaging the IC VIN2 is not connected to the shared internal supply and will not draw any current if left floating. If both VIN1 and VIN2 are floating, regardless of EN/UV pins states, no-load will be present at the output of channel 2. However, if the VIN2 pin is grounded while channel 2 output is held high, parasitic body diodes inside the LT8616 can pull current from the output through the SW2 pin and the VIN2 pin, damaging the IC Figure 7. Reverse VIN Protection for Two Independent Input Voltages PCB Layout For proper operation and minimum EMI, care must be taken during printed circuit board layout. Figure 8 shows the recommended component placement with trace, ground plane and via locations. Note that large, switched currents flow in the LT8616’s VIN pins, GND pins, and the input capacitors (CIN1 and CIN2). The loop formed by the input capacitor should be as small as possible. When using a physically large input capacitor the resulting loop may become too large in which case using a small case/value capacitor placed close to the VIN and GND pins plus a larger capacitor further away is preferred. 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 under the application circuit on the layer closest to the surface layer. The SW and BOOST nodes should be as small as possible. Finally, keep the FB and RT nodes small so that the ground traces will shield them from the SW and BOOST nodes. The exposed pad acts as a heat sink and is connected electrically to ground. The exposed pad of the TSSOP package is the only electrical connection to ground and must be soldered to ground. To keep thermal resistance low, extend the ground plane as much as possible, and add thermal vias under and near the LT8616 to additional ground planes within the circuit board and on the bottom side. 8616f For more information www.linear.com/LT8616 19 LT8616 Applications Information High Temperature Considerations For higher ambient temperatures, care should be taken in the layout of the PCB to ensure good heat sinking of the LT8616. The exposed pad on the bottom of the package must be soldered to a ground plane. This ground should be tied to large copper layers below with thermal vias; these layers will spread heat dissipated by the LT8616. Placing additional vias can reduce thermal resistance further. The maximum load current should be derated as the ambient temperature approaches the maximum junction rating. Power dissipation within the LT8616 can be estimated by calculating the total power loss from an efficiency measurement and subtracting the inductor loss. The die temperature is calculated by multiplying the LT8616 power dissipation by the thermal resistance from junction to ambient. The LT8616 will stop switching and indicate a fault condition if safe junction temperature is exceeded. R2 RPG1 R1 RT COUT1 CFF1 L1 CIN1 CBST1 CBST2 CIN2 L2 COUT2 CFF2 RPG2 R3 R4 8616 F08 NOTE: CVCC IS BELOW THE PACKAGE ON THE BACK SIDE Figure 8. Recommended Layout Open Pins and Shorting Neighboring Pins The LT8616 in TSSOP package is designed to tolerate faults to each pin. Output voltages will stay at or below regulation if adjacent pins are shorted or a pin is left floating. See Table 4 for pin fault behavior when the LT8616 in the TSSOP package is connected in the application shown on Figure 9. 5V, 3.3V, 700KHZ STEP-DOWN CONVERTER VIN 12V VIN1 4.7µF 1M BOOST1 0.1µF EN/UV1 10µH SW1 VIN2 4.7µF 200k 1M EN/UV2 FB1 10nF PG1 PG2 SYNC/MODE BIAS BOOST2 0.1µF TR/SS1 1µF 56.2k TR/SS2 INTVCC 4.7µH SW2 1M RT 5.6pF VOUT1 5V, 1.5A 47µF 187k LT8616 1M TOP VIEW GND 10pF VOUT2 3.3V, 2.5A FB2 2 × 47µF 316k 8616 F09 EN/UV2 1 28 TR/SS2 PG2 2 27 FB2 SW2 3 26 FB2 SW2 4 25 NC SW2 5 24 VIN2 BOOST2 6 23 NC NC 7 BOOST1 8 SW1 9 29 GND 22 BIAS 21 INTVCC 20 NC SW1 10 19 VIN1 PG1 11 18 NC TR/SS1 12 17 SYNC/MODE FB1 13 16 EN/UV1 FB1 14 15 RT FE PACKAGE 28-LEAD PLASTIC TSSOP Figure 9. See Table 4 for Open and Short Pin Behavior of this Application in the TSSOP Package 20 8616f For more information www.linear.com/LT8616 LT8616 Applications Information Table 4. LT8616xFE Pin Fault Behavior For Circuit In Figure 9 LT8616 Pin Float Short to Next Pin EN/UV2 1 Part May Be On or Off PG2 2 No Change No Change SW2 3 No Change No Change Part May Be On or Off SW2 4 No Change No Change SW2 5 No Change OUT2 Below Regulation BOOST2 6 OUT2 Below Regulation No Change No Change NC 7 No Change BOOST1 8 OUT1 Below Regulation OUT1 Below Regulation SW1 9 No Change No Change SW1 10 No Change No Change PG1 11 No Change No Change TR/SS1 12 No Change No Change FB1 13 No Change OUT1 Below Regulation FB1 14 No Change RT 15 Switching Frequency Reduces CH1, CH2 Off EN/UV1 16 Part May Be On or Off CH1, CH2 Off SYNC/MODE 17 No Change No Change NC 18 No Change No Change VIN1 19 CH1, CH2 Off No Change NC 20 No Change No Change INTVCC 21 OUT1, OUT2 Below Regulation No Change BIAS 22 No Change No Change NC 23 No Change No Change VIN2 24 CH2 Off No Change NC 25 No Change No Change FB2 26 No Change No Change FB2 27 No Change OUT2 Below Regulation TR/SS2 28 No Change 8616f For more information www.linear.com/LT8616 21 LT8616 Typical Applications 5V, 2.5V, 2.05MHz Step-Down Converter VIN 5.8V TO 42V 4.7µF 4.7µF VIN1 BOOST1 0.1µF EN/UV1 3.3µH SW1 VIN2 EN/UV2 IHLP-2525CZ-01 10nF 1µF 14.7k 47µF 10V, 1210 BIAS BOOST2 0.1µF TR/SS1 TR/SS2 INTVCC RT 5.6pF 187k LT8616 PG1 PG2 SYNC/MODE 1M 1M FB1 VOUT1 5V, 1.5A 1.5µH SW2 IHLP-2020BZ-01 1M VOUT2 2.5V, 2.5A 10pF FB2 GND 100µF 6.3V, 1210 464k 8616 TA02 3.3V, 0.79V, 1MHz 2-Stage Step-Down Converter, Sequenced Start-Up 0.7A VIN 4.2V TO 42V VIN1 4.7µF 0.1µF EN/UV1 VOUT1 SW1 EN/UV2 1M 10nF TR/SS1 RT 5.6pF VOUT1 3.3V, 0.8A 47µF 10V, 1210 BIAS BOOST2 0.1µF 1µH SW2 INTVCC 37.4k 1M 316k LT8616 TR/SS2 1µF XFL4020-472ME FB1 PG1 PG2 SYNC/MODE 10nF 3.3V 1.5A 4.7µH VIN2 4.7µF 1M BOOST1 GND VOUT2 0.79V, 2.5A XFL4020-102ME FB2 2 × 100µF 6.3V, 1210 8616 TA03 22 8616f For more information www.linear.com/LT8616 LT8616 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. FE Package 28-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663 Rev K) Exposed Pad Variation EB 9.60 – 9.80* (.378 – .386) 4.75 (.187) 4.75 (.187) 28 27 26 2524 23 22 21 20 1918 17 16 15 6.60 ±0.10 4.50 ±0.10 2.74 (.108) SEE NOTE 4 0.45 ±0.05 EXPOSED PAD HEAT SINK ON BOTTOM OF PACKAGE 6.40 2.74 (.252) (.108) BSC 1.05 ±0.10 0.65 BSC RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50* (.169 – .177) 0.09 – 0.20 (.0035 – .0079) 0.50 – 0.75 (.020 – .030) NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS 2. DIMENSIONS ARE IN MILLIMETERS (INCHES) 3. DRAWING NOT TO SCALE 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0.25 REF 1.20 (.047) MAX 0° – 8° 0.65 (.0256) BSC 0.195 – 0.30 (.0077 – .0118) TYP 0.05 – 0.15 (.002 – .006) FE28 (EB) TSSOP REV K 0913 4. RECOMMENDED MINIMUM PCB METAL SIZE FOR EXPOSED PAD ATTACHMENT *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.150mm (.006") PER SIDE 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. 8616f For more information www.linear.com/LT8616 23 LT8616 Typical Application 5V, 3.3V, 2.05MHz Step-Down Converter VIN 5.8V TO 42V 0.1µF 2.2µF 0.1µF VIN1 BOOST1 0.1µF EN/UV1 3.3µH SW1 0.1µF 2.2µF 0.1µF VIN2 XAL4030-332ME 1M FB1 EN/UV2 10nF BIAS BOOST2 0.1µF TR/SS1 2.2µH SW2 TR/SS2 1µF 14.7k 22µF 10V, 1210 PG1 PG2 SYNC/MODE 10nF 4.7pF 187k LT8616 1M VOUT1 5V, 1.5A XFL4020-222ME INTVCC RT GND 1M 4.7pF VOUT2 3.3V, 2.5A 47µF 10V, 1210 FB2 316k 8616 TA04 Related Parts PART NUMBER DESCRIPTION COMMENTS LT8609 42V, 2A, 95% Efficiency, 2.2MHz Synchronous MicroPower Step-Down DC/DC Converter with IQ = 2.5µA VIN(MIN) = 3V, VIN(MAX) = 42V, VOUT(MIN) = 0.8V, IQ = 2.5µA, ISD = <1µA, MSOP-10E Package LT8610A/AB 42V, 3.5A, 96% Efficiency, 2.2MHz Synchronous MicroPower VIN(MIN) = 3.4V, VIN(MAX) = 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA, ISD = <1µA, MSOP-10E Package Step-Down DC/DC Converter with IQ = 2.5µA LT8610AC 42V, 3.5A, 96% Efficiency, 2.2MHz Synchronous MicroPower VIN(MIN) = 3V, VIN(MAX) = 42V, VOUT(MIN) = 0.8V, IQ = 2.5µA, ISD = <1µA, MSOP-10E Package Step-Down DC/DC Converter with IQ = 2.5µA LT8610 42V, 2.5A, 96% Efficiency, 2.2MHz Synchronous MicroPower VIN(MIN) = 3.4V, VIN(MAX) = 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA, ISD = <1µA, MSOP-10E Package Step-Down DC/DC Converter with IQ = 2.5µA LT8611 42V, 2.5A, 96% Efficiency, 2.2MHz Synchronous MicroPower VIN(MIN) = 3.4V, VIN(MAX) = 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA, Step-Down DC/DC Converter with IQ = 2.5µA and Input/Output ISD = <1µA, 3 × 5 QFN-24 Package Current Limit/Monitor LT8620 65V, 2.5A, 96% Efficiency, 2.2MHz Synchronous MicroPower VIN(MIN) = 3.4V, VIN(MAX) = 65V, VOUT(MIN) = 0.97V, IQ = 2.5µA, ISD = <1µA, 3 × 5 QFN-24 Package Step-Down DC/DC Converter with IQ = 2.5µA LT8614 42V, 4A, 96% Efficiency, 2.2MHz Synchronous MicroPower Step-Down DC/DC Converter with IQ = 2.5µA VIN(MIN) = 3.4V, VIN(MAX) = 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA, ISD = <1µA, 3 × 5 QFN-18 Package LT8612 42V, 6A, 96% Efficiency, 2.2MHz Synchronous MicroPower Step-Down DC/DC Converter with IQ = 2.5µA VIN(MIN) = 3.4V, VIN(MAX) = 42V, VOUT(MIN) = 0.97V, IQ = 3.0µA, ISD = <1µA, 3 × 6 QFN-28 Package LT8640 42V, 6A, 96% Efficiency, 3MHz Synchronous MicroPower Step-Down DC/DC Converter with IQ = 2.5µA VIN(MIN) = 3.4V, VIN(MAX) = 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA, ISD = <1µA, 3 × 4 QFN-18 Package LT8602 42V, Quad Output (2.5A+1.5A+1.5A+1.5A) 95% Efficiency, 2.2MHz Synchronous MicroPower Step-Down DC/DC Converter with IQ = 25µA VIN(MIN) = 3V, VIN(MAX) = 42V, VOUT(MIN) = 0.8V, IQ = 25µA, ISD = <1µA, 6 × 6 QFN-40 Package 24 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 For more information www.linear.com/LT8616 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LT8616 8616f LT 0415 • PRINTED IN USA LINEAR TECHNOLOGY CORPORATION 2015