Datasheet

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LT8641

Rev B

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TYPICAL APPLICATION

FEATURES DESCRIPTION

65V, 3.5A Synchronous Step-Down

Silent Switcher with 2.5µA Quiescent Current

The LT

8641 step-down regulator features Silent Switcher

architecture designed to minimize EMI emissions while

delivering high efficiency at frequencies up to 3MHz. As-

sembled in a 3mm × 4mm QFN, the monolithic construc-

tion with integrated power switches and inclusion of all

necessary circuitry yields a solution with a minimal PCB

footprint. An ultralow 2.5µA quiescent current—with the

output in full regulation—enables applications requiring

highest efficiency at very small load currents. Transient

response remains excellent and output voltage ripple is

below 10mV

P-P

at any load, from zero to full current.

The LT8641 allows high V

to low V

OUT

conversion at

high frequency with a fast minimum top switch on-time of

35ns. Operation is safe in overload even with a saturated

inductor.

Essential features are included and easy to use: An

open-drain PG pin signals when the output is in regula-

tion. The SYNC/MODE pin selects between Burst Mode,

pulse-skipping, or spread spectrum mode, and also al-

lows synchronization to an external clock. Soft-start and

tracking functionality is accessed via the TR/SS pin. An

accurate enable threshold can be set using the EN/UV pin

and a resistor at the RT pin programs switch frequency.

5V 3.5A Step-Down Converter

12V

to 5V

OUT

Efficiency

APPLICATIONS

Silent Switcher

Architecture

Ultralow EMI Emissions

Spread Spectrum Frequency Modulation

High Efﬁciency at High Frequency

Up to 95% Efﬁciency at 1MHz, 12V

to 5V

OUT

Up to 94% Efﬁciency at 2MHz, 12V

to 5V

OUT

Wide Input Voltage Range: 3V to 65V

3.5A Maximum Continuous Output,

5A Peak Transient Output

Ultralow Quiescent Current Burst Mode

Operation

2.5μA I

Regulating 12V

to 3.3V

OUT

Output Ripple < 10mV

P-P

Fast Minimum Switch On-Time: 35ns

Low Dropout Under All Conditions: 130mV at 1A

Safely Tolerates Inductor Saturation in Overload

Adjustable and Synchronizable: 200kHz to 3MHz

Peak Current Mode Operation

Output Soft-Start and Tracking

Small 18-Lead 3mm × 4mm QFN

Automotive and Industrial Supplies

General Purpose Step-Down

GSM Power Supplies

All registered trademarks and trademarks are the property of their respective owners. Protected

by U.S. patents, including 8823345.

IN2

IN1

EN/UV

LT8641

8641 TA01a

BST

SYNC/MODE SW

TR/SS BIAS

INTV

RT GND

1µF

0.1µF

4.7pF

47µF

OUT

3.5A

1µF

4.7µF

5.5V TO 65V

10nF

41.2k

1µF

4.7µH

191k

GND2GND1

= 1MHz

EFFICIENCY

POWER LOSS

1MHz, L = 3.3μH

2MHz, L = 2.2μH

3MHz, L = 1.5μH

LOAD CURRENT (A)

0.5

1.5

2.5

3.5

100

0.325

0.650

0.975

1.300

1.625

1.950

2.275

2.600

EFFICIENCY (%)

POWER LOSS (W)

8641 TA01b

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Summary of content (26 pages)

PAGE 1
LT8641 65V, 3.5A Synchronous Step-Down Silent Switcher with 2.5µA Quiescent Current FEATURES DESCRIPTION Silent Switcher® Architecture nn Ultralow EMI Emissions nn Spread Spectrum Frequency Modulation nn High Efficiency at High Frequency nn Up to 95% Efficiency at 1MHz, 12V to 5V IN OUT nn Up to 94% Efficiency at 2MHz, 12V to 5V IN OUT nn Wide Input Voltage Range: 3V to 65V nn 3.5A Maximum Continuous Output, 5A Peak Transient Output nn Ultralow Quiescent Current Burst Mode® Operation nn 2.
PAGE 2
LT8641 PIN CONFIGURATION VIN, EN/UV.................................................................65V PG..............................................................................42V BIAS...........................................................................25V FB, TR/SS ...................................................................4V SYNC/MODE Voltage ..................................................6V Operating Junction Temperature Range (Note 2) LT8641E....................................
PAGE 3
LT8641 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 Feedback Pin Input Current VFB = 1V –20 BIAS Pin Current Consumption VBIAS = 3.3V, 2MHz Minimum On-Time ILOAD = 1.5A, SYNC = 0V ILOAD = 1.5A, SYNC = 2V RT = 221k RT = 60.4k RT = 18.2k Top Power NMOS On-Resistance ISW = 1A MAX 20 9 l l Minimum Off-Time Oscillator Frequency TYP l l l 180 665 1.
PAGE 4
LT8641 TYPICAL PERFORMANCE CHARACTERISTICS 12VIN to 5VOUT Efficiency vs Frequency 12VIN to 3.3VOUT Efficiency vs Frequency 2.3 95 1.9 80 1.3 1.0 75 POWER LOSS 65 60 0.5 1 L = WE–LHMI7050 0.7 1MHz, L = 3.3μH 2MHz, L = 2.2μH 0.3 3MHz, L = 1.5μH 0 1.5 2 2.5 3 3.5 LOAD CURRENT (A) 1.2 80 0.9 75 POWER LOSS 70 L = WE–LHMI7050 0.6 1MHz, L = 2.2µH 2MHz, L = 1.5µH 0.3 3MHz, L = 1µH 0 1.5 2 2.5 3 3.5 LOAD CURRENT (A) 65 60 0.5 1 8641 G01 Efficiency at 3.
PAGE 5
LT8641 TYPICAL PERFORMANCE CHARACTERISTICS EN EN Pin Pin Thresholds Thresholds 1.02 0.99 0.98 EN FALLING 0 0.05 0 –0.05 0 0.5 1 1.5 2 2.5 LOAD CURRENT (A) 3 –0.15 2.0 15 25 35 45 INPUT VOLTAGE (V) 55 65 8641 G12 Top Top FET FET Current Current Limit Limit 10 9 CURRENT LIMIT (A) 2.5 5 8.0 CURRENT LIMIT (A) 7.5 7.0 6.5 8 5% DC 7 6.0 0 10 20 30 40 50 INPUT VOLTAGE (V) 60 5.5 0.1 0.3 0.5 DUTY CYCLE 0.7 6 –50 –25 0.
PAGE 6
LT8641 TYPICAL PERFORMANCE CHARACTERISTICS Dropout Voltage VIN = 5V VOUT SET TO REGULATE AT 5V L = WE–LHMI7050, 1µH 400 300 200 100 0 720 710 700 690 680 0 0.5 1 1.5 2 2.5 LOAD CURRENT (A) 3 660 –50 –25 3.5 0 400 200 0 25 50 75 100 125 150 TEMPERATURE (°C) 60 40 20 200 400 600 LOAD CURRENT (mA) 600 800 Soft-Start Tracking 1.0 0.8 FB VOLTAGE (V) SWITCHING FREQUENCY (kHz) 80 0 8641 G21 VOUT = 3.3V VIN = 12V VSYNC = 0V RT = 60.
PAGE 7
LT8641 TYPICAL PERFORMANCE CHARACTERISTICS RT Programmed Switching Frequency VMinimum IN UVLO Input Voltage 250 Bias Pin Current 3.2 7.5 3.0 7.0 VBIAS = 5V VOUT = 5V ILOAD = 1A fSW = 1MHz 175 INPUT VOLTAGE (V) RT PIN RESISTOR (kΩ) 200 150 125 100 75 50 BIAS PIN CURRENT (mA) 225 2.8 2.6 2.4 0 0.2 0.6 1.4 1.8 2.2 2.6 1 SWITCHING FREQUENCY (MHz) 2.0 –50 –25 3 6.0 5.5 5.0 2.2 25 6.5 0 4.
PAGE 8
LT8641 TYPICAL PERFORMANCE CHARACTERISTICS Transient Response; Load Current Stepped from 300mA (Burst Mode Operation) to 1.3A Transient Response; Load Current Stepped from 1A to 2A ILOAD 1A/DIV ILOAD 1A/DIV VOUT 100mA/DIV VOUT 200mA/DIV 8641 G37 50µs/DIV 50µs/DIV FRONT PAGE APPLICATION 300mA (Burst Mode OPERATION) TO 1.
PAGE 9
LT8641 TYPICAL PERFORMANCE CHARACTERISTICS Radiated EMI Performance (CISPR25 Radiated Emission Test with Class 5 Peak Limits) 50 VERTICAL POLARIZATION PEAK DETECTOR 45 AMPLITUDE (dBµV/m) 40 35 30 25 20 15 10 5 CLASS 5 PEAK LIMIT FIXED FREQUENCY MODE SPREAD SPECTRUM MODE 0 –5 0 100 200 300 400 500 600 700 800 900 1000 FREQUENCY (MHz) 50 HORIZONTAL POLARIZATION PEAK DETECTOR 45 AMPLITUDE (dBµV/m) 40 35 30 25 20 15 10 5 CLASS 5 PEAK LIMIT FIXED FREQUENCY MODE SPREAD SPECTRUM MODE 0 -5 0
PAGE 10
LT8641 PIN FUNCTIONS BIAS (Pin 1): The internal regulator will draw current from BIAS instead of VIN when BIAS is tied to a voltage higher than 3.1V. 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 to GND. GND2 (10, 11): Power Switch Ground. These pins are the return path of the internal bottom side power switch and must be tied together.
PAGE 11
LT8641 PIN FUNCTIONS SYNC/MODE (Pin 17): This pin programs four different operating modes: 1) Burst Mode. Tie this pin to ground for Burst Mode operation at low output loads—this will result in ultralow quiescent current. 2) Pulse-skipping mode. Float this pin for pulse-skipping mode. This mode offers full frequency operation down to low output loads before pulse skipping occurs. When floating, pin leakage currents should be <1µA. See Block Diagram for internal pull-up and pull-down resistance.
PAGE 12
LT8641 OPERATION The LT8641 is a monolithic, constant frequency, 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.
PAGE 13
LT8641 APPLICATIONS INFORMATION Low EMI PCB Layout should be as small as possible by placing the capacitors adjacent to the VIN1/2 and GND1/2 pins. Capacitors with small case size such as 0603 are optimal due to lowest parasitic inductance. The LT8641 is specifically designed to minimize EMI emissions and also to maximize efficiency when switching at high frequencies. For optimal performance the LT8641 requires the use of multiple VIN bypass capacitors.
PAGE 14
LT8641 APPLICATIONS INFORMATION Achieving Ultralow Quiescent Current Burst Frequency As the output load decreases, the frequency of single current pulses decreases (see Figure 2a) and the percentage of time the LT8641 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 2.5µA for a typical application when there is no output load.
PAGE 15
LT8641 APPLICATIONS INFORMATION For some applications it is desirable for the LT8641 to operate in pulse-skipping mode, offering two major differences from Burst Mode operation. First is the clock stays awake at all times and all switching cycles are aligned to the clock. In this mode much of the internal circuitry is awake at all times, increasing quiescent current to several hundred µA. Second is that full switching frequency is reached at lower output load than in Burst Mode operation (see Figure 2b).
PAGE 16
LT8641 APPLICATIONS INFORMATION The highest switching frequency (fSW(MAX)) for a given application can be calculated as follows: fSW (M AX ) = VOUT + VSW (BOT ) t ON(M IN ) ( 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 (~0.3V, ~0.15V, respectively at maximum load) and tON(MIN) is the minimum top switch on-time (see the Electrical Characteristics).
PAGE 17
LT8641 APPLICATIONS INFORMATION (8) 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 LT8641 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.
PAGE 18
LT8641 APPLICATIONS INFORMATION 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. A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT8641 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT8641’s voltage rating.
PAGE 19
LT8641 APPLICATIONS INFORMATION to the source and can cause the source to current limit or latch low under low 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 R3 and R4 such that they satisfy the following equation: ⎛ R3 ⎞ VIN(EN ) = ⎜ + 1⎟ • 1.01V ⎝ R4 ⎠ (11) where the LT8641 will remain off until VIN is above VIN(EN).
PAGE 20
LT8641 APPLICATIONS INFORMATION The LT8641 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 LT8641 may be synchronized over a 200kHz to 3MHz range. The RT resistor should be chosen to set the LT8641 switching frequency equal to or below the lowest synchronization input. For example, if the synchronization signal will be 500kHz and higher, the RT should be selected for 500kHz.
PAGE 21
LT8641 APPLICATIONS INFORMATION Case Temperature Rise D1 DC2373A DEMO BOARD VIN = 12V, fSW = 1MHz VIN = 24V, fSW = 1MHz VIN = 12V, fSW = 2MHz VIN = 24V, fSW = 2MHz LT8641 EN/UV GND 8641 F04 Figure 4. Reverse VIN Protection Thermal Considerations and Peak Output Current For higher ambient temperatures, care should be taken in the layout of the PCB to ensure good heat sinking of the LT8641. The ground pins on the bottom of the package should be soldered to a ground plane.
PAGE 22
LT8641 TYPICAL APPLICATIONS 5V 3.5A Step-Down Converter VIN 5.5V TO 65V 4.7µF EN/UV VIN1 1µF 0603 GND1 PG 10nF VIN2 GND2 LT8641 SYNC/MODE TR/SS BST 0.1µF 4.7µH VOUT 5V 3.5A SW BIAS 1µF 4.7pF 1M FB INTVCC 41.2k 1µF 0603 RT 191k GND fSW = 1MHz L: VISHAY IHLP2525EZ-01 47µF 1210 X5R/X7R 8641 TA08 3.3V, 3.5A Step-Down Converter VIN 3.8V TO 65V 4.7µF EN/UV VIN1 1µF 0603 GND1 PG 10nF VIN2 GND2 LT8641 SYNC/MODE TR/SS BST 0.1µF 2.2µH VOUT 3.3V 3.5A SW BIAS 1µF 4.
PAGE 23
LT8641 TYPICAL APPLICATIONS 2MHz 5V, 3.5A Step-Down Converter VIN 5.5V TO 65V 4.7µF VIN1 1µF 0603 EN/UV GND1 PG 10nF VIN2 GND2 LT8641 SYNC/MODE TR/SS BST 0.1µF 2.2µH VOUT 5V 3.5A SW BIAS 1µF 4.7pF 1M FB INTVCC 18.2k 1µF 0603 RT 191k GND fSW = 2MHz L: VISHAY IHLP2525CZ-01 47µF 1210 X5R/X7R 8641 TA03 2MHz 3.3V, 3.5A Step-Down Converter VIN 3.8V TO 65V 4.7µF VIN1 1µF 0603 EN/UV GND1 PG 10nF VIN2 GND2 LT8641 SYNC/MODE TR/SS BST 0.1µF 1.5µH VOUT 3.3V 3.5A SW BIAS 1µF 4.
PAGE 24
LT8641 PACKAGE DESCRIPTION UDC Package Variation: UDC20(18) 20(18)-Lead Plastic QFN (3mm × 4mm) (Reference LTC DWG # 05-08-1956 Rev C) Exposed Pad Variation AA 0.055 BSC 0.70 ±0.05 3.50 ±0.05 2.10 ±0.05 1.50 REF 0.770 BSC 0.220 ±0.05 0.356 ±0.05 0.400 ±0.05 PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 2.50 REF 3.10 ±0.05 4.50 ±0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 3.00 ±0.10 0.75 ±0.05 1.50 REF PIN 1 ID 0.12 × 45° 0.40 ±0.
PAGE 25
LT8641 REVISION HISTORY REV DATE DESCRIPTION A 05/18 Clarified benefits Clarified Power Loss scale Added LT8641HUDC Added new Note 3, Note 3 became Note 4 Clarified SYNC/MODE (Pin 17) description Clarified Block Diagram Clarified Applications Synchronization and Spread Spectrum section Clarified Figure 4 Clarified Applications section Clarified output capacitor in Typical Applications Clarified Package page moves to page 24 B 08/18 Clarified conditions of BIAS Pin Current Consumption and Oscillator
PAGE 26
LT8641 TYPICAL APPLICATIONS 2MHz 1.8V, 3.5A Step-Down Converter VIN 3V TO 22V (65V TRANSIENT) 4.7µF VIN1 1µF 0603 EN/UV GND1 PG 10nF VIN2 GND2 LT8641 SYNC/MODE TR/SS BST SW BIAS 1µF 18.2k INTVCC RT FB 1µF 0603 1µH 0.1µF EXTERNAL 1µF SOURCE >3.1V OR GND VOUT 1.8V 3.