Datasheet

ManualsBrandsLinear Technology ManualsDC / DC Adjustable Switching RegulatorsSwitching Regulator IC Step-Down QFN- 41.6V

LT8640/LT8640-1

Rev.C

For more information www.analog.com

TYPICAL APPLICATION

FEATURES DESCRIPTION

42V, 5A Synchronous

Step-Down Silent Switcher

with 2.5µA Quiescent Current

The LT

8640/LT8640-1 step-down regulator features Si-

lent Switcher architecture designed to minimize EMI/EMC

emissions while delivering high efficiency at frequencies up

to 3MHz. An ultralow 2.5µA quiescent current—with the

output in full regulation—enables applications requiring

highest efficiency at very small load currents.

The LT8640/LT8640-1 allows high V

to low V

OUT

conversion

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

30ns. The SYNC/MODE pin selects between Burst Mode

operation, spread spectrum mode, synchronization to

an external clock, and either pulse-skipping (LT8640) or

forced continuous mode (LT8640-1).

PACKAGE

SYNC/

MODE ≠ 0 V

COMP H-GRADE CLKOUT

INTERNAL

CAPS

LT8640 QFN

Pulse-

Skipping

Internal Yes No No

LT8640-1 QFN FCM Internal Yes No No

LT8640S LQFN FCM Internal No Yes Yes

LT8643S LQFN FCM External No Yes Yes

LT8640S-2 LQFN FCM Internal Yes Yes No

LT8643S-2 LQFN FCM External Yes Yes No

5V 5A Step-Down Converter

APPLICATIONS

Silent Switcher

Architecture

Ultralow EMI/EMC Emissions

Spread Spectrum Frequency Modulation

High Efficiency at High Frequency

Up to 96% Efficiency at 1MHz, 12V

to 5V

OUT

Up to 95% Efficiency at 2MHz, 12V

to 5V

OUT

Wide Input Voltage Range: 3.4V to 42V

5A Maximum Continuous Output, 7A 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: 30ns

Low Dropout Under All Conditions: 100mV at 1A

Forced Continuous Mode (LT8640-1 Only)

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 and Side Wettable 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

LT8640/

LT8640-1

8640 TA01a

BST

SYNC/MODE SW

TR/SS BIAS

INTV

RT GND

1µF

0.1µF

4.7pF

47µF

OUT

1µF

4.7µF

5.5V TO 42V

10nF

41.2k

1µF

3.3µH

243k

GND2GND1

= 1MHz

EFFICIENCY

POWER LOSS

1MHz, L = 3.3µH

2MHz, L = 2.2µH

3MHz, L = 1µH

LOAD CURRENT (A)

0.5

1.5

2.5

3.5

4.5

100

0.38

0.75

1.13

1.50

1.88

2.25

2.63

3.00

EFFICIENCY (%)

POWER LOSS (W)

12V

to 5V

OUT

Efﬁciency

8640 TA01b

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Summary of content (30 pages)

PAGE 1
LT8640/LT8640-1 42V, 5A Synchronous Step-Down Silent Switcher with 2.5µA Quiescent Current DESCRIPTION FEATURES Silent Switcher® Architecture nn Ultralow EMI/EMC Emissions nn Spread Spectrum Frequency Modulation nn High Efficiency at High Frequency nn Up to 96% Efficiency at 1MHz, 12V to 5V IN OUT nn Up to 95% Efficiency at 2MHz, 12V to 5V IN OUT nn Wide Input Voltage Range: 3.4V to 42V nn 5A Maximum Continuous Output, 7A Peak Transient Output nn Ultralow Quiescent Current Burst Mode® Operation nn 2.
PAGE 2
LT8640/LT8640-1 PIN CONFIGURATION VIN, EN/UV, PG...........................................................42V BIAS...........................................................................25V FB, TR/SS ...................................................................4V SYNC Voltage ..............................................................6V Operating Junction Temperature Range (Note 2) LT8640E/LT8640-1E........................... –40°C to 125°C LT8640I/LT8640-1I.............................
PAGE 3
LT8640/LT8640-1 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. PARAMETER CONDITIONS Minimum Input Voltage VIN Quiescent Current TYP MAX l MIN 2.9 3.4 V l 0.75 0.75 3 10 µA µA l 1.7 1.7 4 10 µA µA 0.3 0.5 mA 21 220 50 350 µA µA 0.970 0.970 0.976 0.982 V V 0.004 0.
PAGE 4
LT8640/LT8640-1 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 LT8640-1 Output Sink Current in Forced Continuous Mode VFB = 1.01V, L = 6.8µH, RT = 60.4k 0.25 0.6 1 A LT8640-1 VIN to Disable Forced Continuous Mode VIN Rising 35 37 39 V LT8640-1 VFB Offset from Feedback Reference Voltage to Disable Forced Continuous Mode VFB Rising 7 9.
PAGE 5
LT8640/LT8640-1 TYPICAL PERFORMANCE CHARACTERISTICS 12VIN to 3.3VOUT Efficiency vs Frequency EFFICIENCY 95 2.25 90 1.13 75 POWER LOSS 70 65 60 0.5 1 1.75 85 1.40 80 1.05 75 POWER LOSS 65 60 0.5 1 90 L = WE–LHMI1040 0.70 1MHz, L = 2.2μH 2MHz, L = 1μH 0.35 3MHz, L = 1μH 0 1.5 2 2.5 3 3.5 4 4.5 5 LOAD CURRENT (A) 8640 G01 1.8 75 70 60 55 0 0.5 Efficiency at 3.3VOUT Efficiency at 5VOUT 90 90 80 80 2.1 70 70 1.8 75 1.5 65 60 55 50 0 0.5 1 1.2 VIN = 12V 0.9 VIN = 24V 0.
PAGE 6
LT8640/LT8640-1 TYPICAL PERFORMANCE CHARACTERISTICS EN Pin Thresholds 0.12 0.10 1.02 0.10 1.00 0.99 0.98 0.08 CHANGE IN VOUT (%) EN RISING 1.01 CHANGE IN VOUT (%) EN THRESHOLD (V) Line Regulation Load Regulation 0.15 1.03 0.05 0 –0.05 0.97 EN FALLING 0.95 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) –0.15 0.02 0 –0.04 VOUT = 5V VIN = 12V R1/R2 = 100k/24.3k 0 0.5 1.5 2 2.5 3 3.5 LOAD CURRENT (A) 1 4 4.5 5 –0.
PAGE 7
LT8640/LT8640-1 TYPICAL PERFORMANCE CHARACTERISTICS Switching Frequency Dropout Voltage 740 600 VIN = 5V VOUT SET TO REGULATE AT 5V L = IHLP3232DZ-01, 1µH FRONT PAGE APPLICATION VIN = 12V 1000 VOUT = 5V 300 200 100 SWITCHING FREQUENCY (kHz) 400 0 RT = 60.4k 730 SWITCHING FREQUENCY (kHz) DROPOUT VOLTAGE (mV) 500 Burst Frequency 1200 720 710 700 690 680 670 0 0.5 1.5 2 2.5 3 3.5 LOAD CURRENT (A) 1 4 4.5 660 –50 –25 5 0 20 600 1.
PAGE 8
LT8640/LT8640-1 TYPICAL PERFORMANCE CHARACTERISTICS RT Programmed Switching Frequency 3.6 225 8.5 3.4 200 125 100 75 BIAS PIN CURRENT (mA) 150 3.0 2.8 2.6 2.4 50 0 0.2 0.6 1.4 1.8 2.2 2.6 1 SWITCHING FREQUENCY (MHz) 2.0 –55 –25 3 95 65 35 TEMPERATURE (°C) 5 125 Bias Pin Current 80 10 5 2.6 1.4 1.8 2.2 0.
PAGE 9
LT8640/LT8640-1 TYPICAL PERFORMANCE CHARACTERISTICS Transient Response; Load Current Stepped from 100mA (Burst Mode Operation) to 1.1A Transient Response; Load Current Stepped from 1A to 2A IL 1A/DIV IL 1A/DIV VOUT 100mV/DIV VOUT 200mV/DIV 50µs/DIV FRONT PAGE APPLICATION 1A TO 2A TRANSIENT 12VIN, 5VOUT COUT = 47µF 50µs/DIV FRONT PAGE APPLICATION 100mA (Burst Mode OPERATION) TO 1.
PAGE 10
LT8640/LT8640-1 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
PAGE 11
LT8640/LT8640-1 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. INTVCC (Pin 2): Internal 3.4V Regulator Bypass Pin. The internal power drivers and control circuits are powered from this voltage.
PAGE 12
LT8640/LT8640-1 PIN FUNCTIONS Diagram for internal pull-up and pull-down resistance. 3) Spread spectrum mode. Tie this pin high to INTVCC (~3.4V) or an external supply of 3V to 4V for pulse-skipping mode with spread spectrum modulation. 4) Synchronization mode. Drive this pin with a clock source to synchronize to an external frequency. During synchronization the part will operate in pulse-skipping mode. During synchronization the part will operate in forced continuous mode.
PAGE 13
LT8640/LT8640-1 OPERATION The LT8640/LT8640-1 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 14
LT8640/LT8640-1 APPLICATIONS INFORMATION Low EMI PCB Layout Note that large, switched currents flow in the LT8640/ LT8640-1 VIN1, VIN2, GND1, and GND2 pins and the input capacitors (CIN1, CIN2). The loops formed by the input capacitors 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.
PAGE 15
LT8640/LT8640-1 APPLICATIONS INFORMATION The exposed pad on the bottom of the package should be soldered to SW to reduce thermal resistance to ambient. To keep thermal resistance low, extend the ground plane from GND1 and GND2 as much as possible, and add thermal vias to additional ground planes within the circuit board and on the bottom side.
PAGE 16
LT8640/LT8640-1 APPLICATIONS INFORMATION While in Burst Mode operation the current limit of the top switch is approximately 900mA (as shown in Figure 3), resulting in low output voltage ripple. Increasing the output capacitance will decrease output ripple proportionally. As load ramps upward 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 2a.
PAGE 17
LT8640/LT8640-1 APPLICATIONS INFORMATION Spread Spectrum Mode FB Resistor Network The LT8640/LT8640-1 features spread spectrum operation to further reduce EMI/EMC emissions. To enable spread spectrum operation, the SYNC/MODE pin should be tied high to INTVCC (~3.4V) or an external supply of 3V to 4V. In this mode, triangular frequency modulation is used to vary the switching frequency between the value programmed by RT to approximately 20% higher than that value.
PAGE 18
LT8640/LT8640-1 APPLICATIONS INFORMATION Setting the Switching Frequency The LT8640/LT8640-1 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. A table showing the necessary RT value for a desired switching frequency is in Table 1. The RT resistor required for a desired switching frequency can be calculated using: RT = 46.5 – 5.2 fSW (3) where RT is in kΩ and fSW is the desired switching frequency in MHz.
PAGE 19
LT8640/LT8640-1 APPLICATIONS INFORMATION Inductor Selection and Maximum Output Current The LT8640/LT8640-1 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 LT8640/LT8640-1 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: ⎛ VOUT + VSW(BOT) ⎞ L =⎜ ⎟•0.
PAGE 20
LT8640/LT8640-1 APPLICATIONS INFORMATION 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 LT8640/LT8640-1 may operate with higher ripple current. This allows use of a physically smaller inductor, or one with a lower DCR resulting in higher efficiency.
PAGE 21
LT8640/LT8640-1 APPLICATIONS INFORMATION Ceramic Capacitors Ceramic capacitors are small, robust and have very low ESR. However, ceramic capacitors can cause problems when used with the LT8640/LT8640-1 due to their piezoelectric nature. When in Burst Mode operation, the LT8640/LT8640-1’s switching frequency depends on the load current, and at very light loads the LT8640/LT86401 can excite the ceramic capacitor at audio frequencies, generating audible noise.
PAGE 22
LT8640/LT8640-1 APPLICATIONS INFORMATION Output Voltage Tracking and Soft-Start The LT8640/LT8640-1 allows the user to program its output voltage ramp rate by means of the TR/SS pin. An internal 1.9µA 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.
PAGE 23
LT8640/LT8640-1 APPLICATIONS INFORMATION Thermal Considerations and Peak Output Current Temperature rise of the LT8640/LT8640-1 is worst when operating at high load, high VIN, and high switching frequency. If the case temperature is too high for a given application, then either VIN, switching frequency, or load current can be decreased to reduce the temperature to an acceptable level. Figure 6 shows examples of how case temperature rise can be managed by reducing VIN, switching frequency, or load.
PAGE 24
LT8640/LT8640-1 TYPICAL APPLICATIONS 5V 5A Step-Down Converter VIN 5.5V TO 42V 4.7µF EN/UV VIN1 1µF 0603 GND1 PG 10nF VIN2 GND2 LT8640/ LT8640-1 BST SYNC/MODE TR/SS 0.1µF 3.3µH VOUT 5V 5A SW BIAS 1µF 4.7pF 1M FB INTVCC 41.2k 1µF 0603 RT 243k GND fSW = 1MHz L: VISHAY IHLP2525EZ-01 47µF 1210 X5R/X7R 8640 TA08 3.3V, 5A Step-Down Converter VIN 3.8V TO 42V 4.7µF EN/UV VIN1 1µF 0603 GND1 PG 10nF VIN2 GND2 LT8640/ LT8640-1 BST SYNC/MODE TR/SS 0.1µF 2.2µH 4.
PAGE 25
LT8640/LT8640-1 TYPICAL APPLICATIONS Ultralow EMI 5V, 5A Step-Down Converter VIN 5.6V TO 42V FB1 BEAD 10µF 1210 10µF 1210 10µF 1206 1µF 0603 EN/UV VIN1 GND1 PG 10nF VIN2 GND2 LT8640/ LT8640-1 BST SYNC/MODE TR/SS 0.1µF 1.5µH VOUT 5V 5A SW BIAS 1µF 4.7pF 1M FB INTVCC 18.2k 1µF 0603 RT 243k GND fSW = 2MHz FB1 BEAD: WE-MPSB 100Ω 8A 1812 L: WE-LHMI7030 47µF 1210 X5R/X7R 8640 TA02 2MHz 5V, 5A Step-Down Converter VIN 5.5V TO 42V 4.
PAGE 26
LT8640/LT8640-1 TYPICAL APPLICATIONS 2MHz 3.3V, 5A Step-Down Converter VIN 3.8V TO 42V 4.7µF EN/UV VIN1 1µF 0603 GND1 PG 10nF VIN2 GND2 LT8640/ LT8640-1 BST SYNC/MODE TR/SS 0.1µF 1µH VOUT 3.3V 5A SW BIAS 1µF 4.7pF 1M FB INTVCC 18.2k 1µF 0603 RT 412k GND fSW = 2MHz L: VISHAY IHLP2525CZ-01 47µF 1210 X5R/X7R 8640 TA06 12V, 5A Step-Down Converter VIN 12.5V TO 42V 4.7µF EN/UV VIN1 1µF 0603 GND1 PG 10nF VIN2 GND2 LT8640/ LT8640-1 BST SYNC/MODE TR/SS 0.1µF 4.7µH 4.
PAGE 27
LT8640/LT8640-1 PACKAGE DESCRIPTION UDC Package 18-Lead Plastic QFN (3mm × 4mm) (Reference LTC DWG # 05-08-1956 Rev B) 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 ±0.10 1 PIN 1 TOP MARK (NOTE 5) 4.00 ±0.
PAGE 28
LT8640/LT8640-1 PACKAGE DESCRIPTION UDCF Package 20(18)-Lead Plastic Side Wettable QFN (3mm × 4mm) (Reference LTC DWG # 05-08-1617) 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 PIN 1 ID 0.12 × 45° 1.50 REF 0.40 ±0.10 1 PIN 1 TOP MARK (NOTE 5) 0.220 ±0.05 2.127 ±0.
PAGE 29
LT8640/LT8640-1 REVISION HISTORY REV DATE DESCRIPTION A 10/15 Added LT8640-1 Version to Title Clarified Minimum On-Time to 35ns Added LT8640-1 Version 01/18 Added LT8640-1 Version Specifications 4 Clarified Minimum Load Graph to Pulse Skipping 7 Clarified Efficiency Conditions 11 to 25 1 1,3 Added LT8640HUDC-1 2 Added New Note 3, Note 3 became Note 4 4 Clarified Minimum On-Time Graph 6 Clarified Case Temperature Graph 8 Clarified SYNC/MODE Pin Description 12 Clarified Block Drawing 1
PAGE 30
LT8640/LT8640-1 TYPICAL APPLICATION VIN 3.4V TO 22V (42V TRANSIENT) 2MHz 1.8V, 5A Step-Down Converter 4.7µF EN/UV VIN1 1µF 0603 GND1 PG 10nF VIN2 GND2 LT8640/ LT8640-1 BST SYNC/MODE TR/SS SW BIAS 1µF 18.2k INTVCC RT FB 1µF 0603 1µH 0.1µF EXTERNAL 1µF SOURCE >3.1V OR GND VOUT 1.8V 5A 10pF 866k 1M GND fSW = 2MHz L: VISHAY IHLP2525CZ-01 100µF 1210 X5R/X7R 8640 TA07 RELATED PARTS PART DESCRIPTION COMMENTS LT8609 42V, 2A, 94% Efficiency, 2.