Power Management

LT8610AC

Analog Devices

High Efficiency 42V/120mA Synchronous Buck

Manufacturer: Analog Devices

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Rev. 0For more information www.analog.com Document Feedback High Efficiency 42V/120mA Synchronous Buck All registered trademarks and trademarks are the property of their respective owners. TYPICAL APPLICATION FEATURES DESCRIPTION The LT®8604 is a compact, high speed synchronous monolithic step-down switching regulator that delivers up to 120mA with high efficiency at constant switching fre- quency, even up to 2.2MHz. It accepts a wide input voltage range up to 42V, and consumes only 2.5µA of quiescent current. Top and bottom power switches are included with all necessary circuitry to minimize the need for external components. Low ripple Burst Mode operation enables high efficiency down to very low output currents while keeping the output ripple below 10mVP-P. Additional features provide for flexible and robust opera- tion. Internal compensation with peak current mode topol- ogy allows the use of small inductors and results in fast transient response and good loop stability. The EN/UV pin has an accurate 1V threshold and can be used to program VIN under voltage lockout or to shut down the LT8604 reducing the input supply current to 1µA. A PG flag sig - nals when VOUT is within ±7.5% of the programmed out- put voltage as well as fault conditions. Thermal shutdown provides additional protection. The LT8604 is available in a small 10-Lead 3mm × 2mm DFN package with exposed pad for low thermal resistance. 5V, Step-Down Converter

APPLICATIONS

n High Efficiency 2MHz Synchronous Operation n > 90% Efficiency at 50mA, 12VIN to 5VOUT n Ultralow Quiescent Current Burst Mode® Operation n < 2.5µA IQ Regulating 24VIN to 3.3VOUT n Output Ripple < 10mVP-P n Wide Input Voltage Range: 3.2V to 42V n Fast Minimum Switch-On Time: 35ns n Adjustable Switching Frequency: 200kHz to 2.2MHz n Allows Tiny Inductors n Accurate 1V Enable Pin Threshold n Internal Compensation n Output Soft-Start and T racking n Small 10-Lead 3mm × 2mm Side-Wettable n AEC-Q100 Qualified for Automotive Applications n Industrial Sensors n Industrial Internet of Things n 4mA to 20mA Current Loops n Flow Meters n Automotive Housekeeping Supplies Efficiency at VOUT = 5V 100µH 10µF 1µF 60.4k 1µF 47nF 187k V IN EN/UV FB TR/SS RT INTV CC SW GND V OUT 120mA PG V IN 5.9V TO 42V f SW = 700kHz L T8604 BIAS BST ON OFF

8604 TA01a

V IN = 12V V IN = 24V V IN = 36V f SW = 700kHz LOAD CURRENT (mA) 100 120 100 EFFICIENCY (%)

8604 TA01b

Rev. 0 For more information www.analog.com PIN CONFIGURATIONABSOLUTE MAXIMUM RATINGS Operating Junction Temperature Range (Note 2) (Note 1) TOP VIEW DDBM PACKAGE 10-LEAD (3mm × 2mm) PLASTIC DFN θJA = 76°C/W, θJC = 13.5°C/W EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB BST SW BIAS INTVCC RT VIN EN/UV PG TR/SS FB6 ORDER INFORMATION Lead Free Finish TAPE AND REEL (MINI) TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT8604EDDBM#TRMPBF LT8604EDDBM#TRPBF LHNB 10-Lead (3mm × 2mm) Plastic Side-Wettable –40°C to 125°C LT8604IDDBM#TRMPBF LT8604IDDBM#TRPBF LHNB 10-Lead (3mm × 2mm) Plastic Side-Wettable –40°C to 125°C LT8604JDDBM#TRMPBF LT8604JDDBM#TRPBF LHNB 10-Lead (3mm × 2mm) Plastic Side-Wettable –40°C to 150°C AUTOMOTIVE PRODUCTS** LT8604EDDBM#WTRMPBF LT8604EDDBM#WTRPBF LHNB 10-Lead (3mm × 2mm) Plastic Side-Wettable –40°C to 125°C LT8604IDDBM#WTRMPBF LT8604IDDBM#WTRPBF LHNB 10-Lead (3mm × 2mm) Plastic Side-Wettable –40°C to 125°C LT8604JDDBM#WTRMPBF LT8604JDDBM#WTRPBF LHNB 10-Lead (3mm × 2mm) Plastic Side-Wettable –40°C to 150°C TRM = 500 pieces. *Temperature grades are identified by a label on the shipping container . Contact the factory for parts specified with wider operating temperature ranges. Contact the factory for information on lead based finish parts. Tape and reel specifications. Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix. **Versions of this part are available with controlled manufacturing to support the quality and reliability requirements of automotive applications. These models are designated with a #W suffix. Only the automotive grade products shown are available for use in automotive applications. Contact your local Analog Devices account representative for specific product ordering information and to obtain the specific Automotive Reliability reports for these models.

Rev. 0For more information www.analog.com PARAMETER CONDITIONS MIN TYP MAX UNITS Minimum Input Voltage l 2.9 3.2 V VIN Quiescent Current VEN/UV = 0V VEN/UV = 2V, Not Switching l 1.7 µA µA VIN Current in Regulation VIN = 12V, VOUT = 3.3V, ILOAD = 100µA VIN = 12V, VOUT = 3.3V, ILOAD = 1mA 400 µA µA Feedback Reference Voltage l 0.762 0.778 0.798 V FB Voltage Line Regulation VIN = 4V to 42V l ±0.002 ±0.04 %/V FB Pin Input Current VFB = 0.8V l ±20 nA BIAS Pin Current Consumption VBIAS = 3.3V, ILOAD = 30mA, 700kHz 0.9 mA Minimum On-Time l 35 65 ns Minimum Off-Time 90 120 ns Oscillator Frequency RT = 221k RT = 18.2k l l 140 1.85 200 2.00 260 2.15 kHz MHz Top Power NMOS On-Resistance 3.2 Ω Top Power NMOS Current Limit l 185 230 275 mA Bottom Power NMOS On-Resistance 1.2 Ω SW Leakage Current VIN = 24V l 15 µA EN/UV Pin Threshold Pin Voltage Rising l 0.98 1.04 1.11 V V EN/UV Pin Hysteresis 40 mV EN/UV Pin Current VEN/UV = 2V ±20 nA PG Upper Threshold Offset from VFB/OUT VFB/OUT Rising l 5.0 7.5 10.0 % PG Lower Threshold Offset from VFB/OUT VFB/OUT Falling l –5.0 –7.5 –10.0 % PG Hysteresis 0.5 % PG Leakage VPG = 42V l ±200 nA PG Pull-Down Resistance VPG = 0.1V 550 1200 Ω TR/SS Source Current VTR/SS = 0.1V l 1 2 3.5 µA TR/SS Pull-Down Resistance Fault Condition, VTR/SS = 0.1V 300 900 Ω 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 LT8604E 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 LT8604I is guaranteed over the full –40°C to 125°C operating junction temperature range. The LT8604J 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: This IC includes overtemperature protection that is intended to protect the device during overload conditions. Junction temperature will exceed 150°C when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature will reduce lifetime ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C.

Rev. 0 For more information www.analog.com V IN = 12V V IN = 24V V IN = 36V L = 33µH f SW = 2MHz OUTPUT CURRENT (A) 10µ 100µ 10m 100m 100 EFFICIENCY (%)

8604 G04

TEMPERATURE (°C) –50 –25 100 125 150 775 776 777 778 779 780 FB REGULATION VOL TAGE (mV)

8604 G05

OUTPUT CURRENT (mA) 100 120 –0.15 –0.10 –0.05 0.00 0.05 0.10 0.15 CHANGE IN V OUT (%)

8604 G06

TYPICAL PERFORMANCE CHARACTERISTICS No-Load Supply Current Efficiency (5V Output) Efficiency (3.3V Output) Efficiency (3.3V Output) FB Voltage Load Regulation Line Regulation Efficiency (5V Output) V IN = 12V V IN = 24V V IN = 36V L = 47µH f SW = 2MHz OUTPUT CURRENT (mA) 100 120 100 EFFICIENCY (%)

8604 G01

V IN = 12V V IN = 24V V IN = 36V L = 47µH f SW = 2MHz OUTPUT CURRENT (A) 10µ 100µ 10m 100m 100 EFFICIENCY (%)

8604 G02

V IN = 12V V IN = 24V V IN = 36V L = 33µH f SW = 2MHz OUTPUT CURRENT (mA) 100 120 100 EFFICIENCY (%)

8604 G03

V OUT = 3.3V I OUT = 40mA INPUT VOL TAGE (V) –0.20 –0.15 –0.01 –0.05 0.05 0.10 0.15 0.20 CHANGE IN V OUT (%)

8604 G07

V OUT = 3.3V R1 = 309kΩ R2 = 1MΩ INPUT VOL TAGE (V) 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 INPUT CURRENT (µA)

8604 G08

Rev. 0For more information www.analog.com DUTY CYCLE (%) 100 170 180 190 200 210 220 230 CURRENT LIMIT (mA)

8604 G09

TEMPERATURE (°C) –50 –25 100 125 150 200 205 210 215 220 225 230 235 240 SWITCH CURRENT (mA)

8604 G10

SWITCH CURRENT = 120mA TEMPERATURE (°C) –50 –25 100 125 150 100 200 300 400 500 600 700 800 SWITCH DROP (mV)

8604 G11

SWITCH CURRENT (mA) 100 120 100 200 300 400 500 SWITCH DROP (mV)

8604 G12

I LOAD = 100mA TEMPERATURE (°C) –50 –25 100 125 150 MINIMUM ON-TIME (ns)

8604 G13

TEMPERATURE (°C) –50 –25 100 125 150 100 MINIMUM OFF-TIME (ns)

8604 G14

L = LPS5030-333MR V OUT = 3.3V LOAD CURRENT (mA) 100 120 100 200 300 400 500 600 DROPOUT VOL TAGE (mV)

8604 G15

R T = 18.2kΩ TEMPERATURE (°C) –50 –25 100 125 150 1950 1960 1970 1980 1990 2000 2010 2020 2030 SWITCHING FREQUENCY (kHz)

8604 G16

L = 33µH V IN = 12V V OUT = 3.3V LOAD CURRENT (mA) 250 500 750 1000 1250 1500 1750 2000 2250 2500 SWITCHING FREQUENCY (kHz)

8604 G17

TYPICAL PERFORMANCE CHARACTERISTICS Top FET Current Limit vs Duty Cycle Top FET Current Limit vs Temperature Switch Drop vs Temperature Switch Drop vs Switch Current Minimum On-Time vs Temperature Minimum Off-Time vs Temperature Dropout Voltage vs Load Current Burst Frequency vs Load Current Switching Frequency vs Temperature

Rev. 0 For more information www.analog.com TYPICAL PERFORMANCE CHARACTERISTICS Frequency Foldback Soft-Start T racking Soft-Start Current vs Temperature VIN UVLO Start-Up Dropout Start-Up Dropout 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 FB VOL TAGE (V) 250 500 750 1000 1250 1500 1750 2000 2250 2500 SWITCHING FREQUENCY (kHz) R k /uni03A9 T

8604 G18

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 TR/SS VOL TAGE (V) 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 FB VOL TAGE (V)

8604 G19

TEMPERATURE (°C) –50 –25 100 125 150 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 SOFT-START CURRENT (µA)

8604 G20

TEMPERATURE (°C) –50 –25 100 125 150 2.00 2.20 2.40 2.60 2.80 3.00 V IN UVLO (V)

8604 G21

R LOAD = 400Ω V IN V OUT INPUT VOL TAGE (V) INPUT VOL TAGE (V) OUTPUT VOL TAGE (V)

8604 G24

R LOAD = 40Ω V IN V OUT INPUT VOL TAGE (V) INPUT VOL TAGE (V) OUTPUT VOL TAGE (V)

8604 G25

Bias Pin CurrentPG Thresholds V IN = 12V V OUT = 5V I LOAD = 100mA V BIAS = 3.3V V BIAS = 5V SWITCHING FREQUENCY (MHz) 0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8 2.0 2.2 0.6 0.8 1.0 1.2 1.4 1.6 1.8 BIAS PIN CURRENT (mA)

8604 G23

TEMPERATURE (°C) –50 –25 100 125 150 –10.0 –7.5 –5.0 –2.5 2.5 5.0 7.5 10.0 PG THRESHOLD OFFSET FROM V REF (%)

8604 G22

Rev. 0For more information www.analog.com TYPICAL PERFORMANCE CHARACTERISTICS Switching Waveforms Switching Waveforms Switching Waveforms Switching Waveforms Switching Waveforms Switching Waveforms FRONT PAGE APPLICATION 12V IN TO 5V OUT AT 60mA 500ns/DIV I L 20mA/DIV V SW 5V/DIV

8604 G26

I L 50mA/DIV V SW 10V/DIV

8604 G27

5µs/DIV I L 20mA/DIV V SW 5V/DIV

8604 G28

5µs/DIV I L 20mA/DIV V SW 10V/DIV

8604 G29

V IN = 12V 100µs/DIV I LOAD 30mA/DIV V OUT 50mV/DIV

8604 G30

V IN = 24V 100µs/DIV I LOAD 30mA/DIV V OUT 50mV/DIV

8604 G31

Rev. 0 For more information www.analog.com BST (Pin 1): This pin is used to provide a drive volt - age higher than the input voltage, to the topside power switch. Place a 47nF boost capacitor as close as possible to the IC. Do not put resistance in series with this pin. SW (Pin 2): The SW pin is the output of the internal power switches. Connect this pin to the inductor . This node should be kept small on the PCB for good performance. BIAS (Pin 3): The internal regulator will draw current from BIAS instead of VIN when BIAS is tied to a voltage higher than 3.2V. For output voltages of 3.3V to 25V this pin should be tied to VOUT. If this pin is tied to a supply other than VOUT, use a 1µF local bypass capacitor on this pin. If no supply is available, tie this pin to GND. INTVCC (Pin 4): Internal 3.4V Regulator Bypass Pin. The internal power drivers and control circuits are powered from this voltage. INTV CC maximum output current is 2mA. Do not load the INTVCC pin with external circuitry. INTVCC current will be supplied from BIAS if BIAS > 3.2V, otherwise current will be drawn from V IN. Voltage on INTVCC will vary between 2.8V and 3.4V when V BIAS is between 3.0V and 3.6V. Decouple this pin to power ground with a low ESR ceramic capacitor of at least 1μF placed close to the IC. RT (Pin 5): Tie a resistor between RT and ground to set the switching frequency. FB (Pin 6): The LT8604 regulates the FB pin to 0.778V. Connect the feedback resistor divider tap to this pin. TR/SS (Pin 7): Output T racking and Soft-Start Pin. This pin allows user control of output voltage ramp rate dur - ing start-up. A TR/SS voltage below 0.778V forces the LT8604 to regulate the FB pin to equal the TR/SS pin volt- age. When TR/SS is above 0.778V, the tracking function is disabled and the internal reference resumes control of the error amplifier . An internal 2µA pull-up current on this pin allows a capacitor to program output voltage slew rate. This pin is pulled to ground with a 300Ω MOSFET during shutdown and fault conditions; use a series resistor if driving from a low impedance output. PG (Pin 8): The PG pin is the open-drain output of an internal comparator . PG remains low until the FB pin is within ±7.5% of the final regulation voltage, and there are no fault conditions. PG is valid when V IN is above 3.2V, regardless of EN/UV pin state. EN/UV (Pin 9): The LT8604 is shut down when this pin is low and active when high. The hysteretic threshold volt - age is 1.05V rising and 1.00V falling. Tie to V IN if the shutdown feature is not used. An external resistor divider from VIN can be used to program a V IN threshold below which the LT8604 will shut down. VIN (Pin 10): The VIN pin supplies current to the LT8604 internal circuitry and to the internal top side power switch. This pin must be locally bypassed. Be sure to place the positive terminal of the input capacitor as close as pos - sible to the V IN pin, and the negative capacitor terminal as close as possible to the GND pin. GND (Exposed Pad Pin 11): Ground. The exposed pad must be connected to the negative terminal of the input capacitor and soldered to the PCB in order to lower the thermal resistance. PIN FUNCTIONS

Rev. 0For more information www.analog.com BLOCK DIAGRAM R T L C SS 2µA SWITCH LOGIC AND ANTI- SHOOT THROUGH 3.4V REG SW BURST DETECT OSCILLATOR 200kHz TO 2.2MHz SLOPE COMP INTERNAL 0.778V REF V IN EN/UV ±7.5% PG SHDN TSD INTV CC V IN FB RT V C TR/SS ERROR AMP CIN V IN C OUT V OUT SHDN TSD V IN UVLO SHDN BIAS GND INTV CC BST C INTVCC C BST UVLO UVLO 8604 BD

Rev. 0 For more information www.analog.com The LT8604 is a monolithic constant-frequency current mode step-down DC/DC converter . Operation is best understood by referring to the Block Diagram. An internal oscillator turns on the integrated 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 cur- rent 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 reference. When the load current increases, it causes a reduction in the feedback voltage relative to the reference leading the error amplifier to raise the VC volt- age 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 excess current flowing through the bottom switch, the next clock cycle will be delayed until switch current returns to a safe level. To optimize efficiency, the LT8604 enters Burst Mode operation at light loads. Between bursts, all circuitry associated with controlling the output switch is shut down reducing the input supply current to 1.7µA. In a typical application with a 24V input, 2.5µA will be consumed from the input supply when regulating with no load. To improve efficiency across all loads, supply current to internal circuitry can be sourced from the BIAS pin when biased at 3.2V or above. Else, the internal circuitry will draw current from VIN. The BIAS pin should be connected to VOUT if the LT8604 output is programmed to a voltage between 3.3V and 25V. Comparators monitoring the FB pin voltage will pull the PG pin low if the output voltage varies more than ±7.5% (typi- cal) from the set point, or if a fault condition is present. In the LT8604, the oscillator reduces its operating fre - quency when the voltage at the FB pin is low. This fre - quency foldback helps to control the inductor current when the output voltage is lower than the programmed value which occurs during start-up. If the EN/UV pin is low, the LT8604 is shut down and draws 1µA from the input. When the EN/UV pin is above 1.05V, the switching regulator becomes active. OPERATION

Figure 1. SW Burst Mode Frequency vs Load optimize the quiescent current at low loads. 2.2MHz by using a resistor tied from the RT pin to ground. mode the LT8604 consumes 1.7μA. be minimized as it appears to the output as load current. Figure 2. Burst Mode Operation

8604 F01

8604 F02

Table 1. SW Frequency vs RT Value control of inductor current to assure safe operation. dropped to achieve higher duty cycle. speed peak-current mode architecture.

20 where fSW is the switching frequency in MHz, V OUT is the output voltage, V SW(BOT) is the bottom switch drop (~0.14V) 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 applica - tion. In addition, the saturation current (typically labeled

Rev. 0For more information www.analog.com ISAT) rating of the inductor must be higher than the load current plus 1/2 of the inductor ripple current: IL(PEAK) =ILOAD(MAX) + 1 2 ΔL where ∆IL is the inductor ripple current as calculated sev- eral paragraphs below and I LOAD(MAX) is the maximum output load for a given application. As a quick example, an application requiring 120mA out- put should use an inductor with an RMS rating of greater than 120mA and an ISAT of greater than 180mA. To keep the efficiency high, the series resistance (DCR) should be less than 1Ω, and the core material should be intended for high frequency applications. The LT8604 limits the peak switch current in order to protect the switches and the system from overload faults. The top switch current limit (I LIM) is at least 185mA at low duty cycles and decreases linearly to 137mA at D = 0.8. The inductor value must then be sufficient to supply the desired maximum output current (I OUT(MAX)), which is a function of the switch current limit (I LIM) and the ripple current: IOUT(MAX) =ILIM – ΔIL The peak-to-peak ripple current in the inductor can be calculated as follows: ΔIL = VOUT L •fSW 1– VOUT VIN(MAX) where fSW is the switching frequency of the LT8604, and L is the value of the inductor . Therefore, the maximum output current that the LT8604 will deliver depends on the switch current limit, the inductor value, and the input and output voltages. The inductor value may have to be increased if the inductor ripple current does not allow sufficient maximum output current (IOUT(MAX)) given the switching frequency, and maximum input voltage used in the desired application. For more information about maximum output current and discontinuous operation, see Analog Devices Application Note 44. Finally, for duty cycles greater than 50%, a minimum inductance is required to avoid sub-harmonic oscillation: LMIN = VOUT + VSW(BOT) fSW

12.5 where fSW is the switching frequency, VOUT is the output voltage, VSW(BOT) is the bottom switch drop (~0.14V) and LMIN is the inductor value. Input Capacitor Bypass the input of the LT8604 circuit with a ceramic capacitor of X7R or X5R type. Y5V types have poor perfor- mance over temperature and applied voltage, and should not be used. A 1μF to 2.2μF ceramic capacitor is adequate to bypass the LT8604 and will easily handle the ripple current. 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 sup - ply in pulses with very fast rise and fall times. The input capacitor is required to reduce the resulting voltage rip - ple at the LT8604 and to force this very high frequency switching current into a tight local loop, minimizing EMI. A 1μF capacitor is capable of this task, but only if it is placed close to the LT8604 (see the PCB Layout section). A second precaution regarding the ceramic input capaci- tor concerns the maximum input voltage rating of the LT8604. A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT8604 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT8604’s voltage rating. This situation is easily avoided (see Analog Devices 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 LT8604 to produce the DC output. In this role it determines the output ripple, thus low impedance at the switching frequency is important. The second function is APPLICATIONS INFORMATION

Rev. 0 For more information www.analog.com to store energy in order to satisfy transient loads and sta- bilize the LT8604’s control loop. Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance. A good starting value is: COUT = 50 VOUTfSW where fSW is the switching frequency in MHz, V OUT is the output voltage, and COUT is the recommended output capacitance in μF . Use X5R or X7R types. This choice will provide low output ripple and good transient response. T ransient performance can be improved with a higher value output capacitor and the addition of a feedforward capaci- tor placed between V OUT 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 capaci - tance under the relevant operating conditions of voltage bias and temperature. A physically larger capacitor or one with a higher voltage rating may be required. Ceramic Capacitors Ceramic capacitors are small, robust and have very low ESR. However , ceramic capacitors can cause problems when used with the LT8604 due to their piezoelectric nature. When in Burst Mode operation, the LT8604’ s switching frequency depends on the load current, and at very light loads the LT8604 can excite the ceramic capacitor at audio frequencies, generating audible noise. Since the LT8604 operates at a lower current limit during Burst Mode operation, the noise is typically very quiet. If this is unac- ceptable, use a high performance tantalum or electrolytic capacitor at the output. A final precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LT8604. As previously mentioned, a ceramic input capacitor com - bined with trace or cable inductance forms a high qual - ity (under damped) tank circuit. If the LT8604 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT8604’s rating. This situation is easily avoided (see Analog Devices Application Note 88). EN/UV Pin The LT8604 is in shutdown when the EN/UV pin is low and active when the pin is high. The rising threshold of the EN/UV comparator is 1.05V, with 50mV of hysteresis. The EN/UV pin 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 V IN to EN/UV programs the LT8604 to regulate the output only when VIN is above a desired voltage (see Block Diagram). Typically, this threshold, VIN(EN/UV), is used in situations where the input supply is current limited, or has a relatively high source resistance. A switching regulator draws constant power from the source, so source current increases as source voltage drops. This looks like a negative resistance load to the source and can cause the source to current limit or latch low under low source voltage conditions. The VIN(EN/UV) threshold prevents the regulator from operating at source voltages where the problems might occur . This threshold can be adjusted by setting the values R3 and R4 such that they satisfy the following equation: R3 = VIN(EN/UV) 1V –1⎛ ⎠⎟•R4 where the LT8604 will remain off until VIN is above VIN(EN/UV). Due to the comparator’s hysteresis, switching will not stop until the input falls slightly below VIN(EN/UV). For light-load currents, the current through the VIN(EN/UV) resistor network can easily be greater than the supply current consumed by the LT8604. Therefore, the VIN(EN/UV) resis- tors 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 V IN that powers the drivers and the internal bias circuitry. The INTVCC can supply enough cur- rent for the LT8604’s circuitry and must be bypassed to APPLICATIONS INFORMATION

connect an external load to the INTVCC pin. TR/SS can be externally driven by another voltage source. back voltage will regulate to the internal reference voltage. falling too low, or thermal shutdown. upper and lower thresholds include 0.5% of hysteresis. low, VIN is too low, or thermal shutdown. during short circuit conditions. is grounded the SW pin current will drop to near 0.7µA. against a shorted or reversed input.

8604 F03

Figure 3. Reverse VIN Protection

8604 F04

within the circuit board and on the bottom side. Figure 4 shows the basic guidelines for a layout example. Figure 4. Recommended PCB Layout (Not to Scale)

Rev. 0For more information www.analog.com TYPICAL APPLICATIONS 5V Step-Down Converter Typical Performance Minimum Load to Full Frequency Typical Performance Minimum Load to Full Frequency Typical Performance Minimum Load to Full Frequency 3.3V 2MHz Step-Down Converter 5V 2MHz Step-Down Converter L 100µH 10µF 16V 1µF 60.4k 10nF 1µF 47nF 100k 187k L: LPS5030-104MR V IN EN/UV FB TR/SS RT INTV CC SW GND V OUT 120mA PG V IN 5.9V TO 42V f SW = 700kHz L T8604 BIAS BST POWER GOOD X7R 1206

8604 TA03

L 33µH 10µF 16V 1µF 18.2k 10nF 1µF 47nF 100k 309k L: LPS5030-333MR V IN EN/UV FB TR/SS RT INTV CC SW GND V OUT 3.3V 120mA PG V IN 4.2V TO 42V f SW = 2MHz L T8604 BIAS BST POWER GOOD X7R 1206

8604 TA04

L 47µH 10µF 16V 1µF 18.2k 10nF 1µF 47nF 100k 187k L: LPS5030-473MR V IN EN/UV FB TR/SS RT INTV CC SW GND V OUT 120mA PG V IN 5.9V TO 42V f SW = 2MHz L T8604 BIAS BST POWER GOOD X7R 1206