Datasheet

ManualsBrandsLinear Technology ManualsDC / DC Adjustable Switching RegulatorsSwitching Regulator IC Step-Down MSOP- 41.5V

LT8608/LT8608B

Rev. D

For more information www.analog.com

TYPICAL APPLICATION

FEATURES DESCRIPTION

42V, 1.5A Synchronous Step-Down Regulator

with 2.5µA Quiescent Current

The LT

8608 is a compact, high efficiency, high speed

synchronous monolithic step-down switching regulator

that consumes only 1.7µA of quiescent current. The LT8608

can deliver 1.5A of continuous current. Low ripple Burst

Mode operation enables high efficiency down to very low

output currents while keeping the output ripple below

10mV. Internal compensation with peak current mode

topology 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 V

undervoltage lockout or to shut down the

LT8608. The PG flag signals when V

OUT

is within ±8.5% of

the programmed output voltage as well as fault conditions.

The MSOP package includes a SYNC pin to synchronize

to an external clock, or to select Burst Mode operation

orpulse-skipping with or without spread-spectrum; the

TR/SS pin programs soft-start or tracking. The DFN

package omits these pins and can be purchased in pulse-

skipping or Burst Mode operation varieties.

PACKAGE SYNC FUNCTIONALITY

LT8608MSE MSE Programmable

LT8608DFN DFN Burst Mode Operation

LT8608BDFN DFN Pulse-Skipping Mode

APPLICATIONS

Wide Input Voltage Range: 3.0V to 42V

Ultralow Quiescent Current Burst Mode

Operation

<2.5µA I

Regulating 12V

to 3.3V

OUT

Output Ripple <10mV

P-P

High Efﬁciency 2MHz Synchronous Operation

>92% Efﬁciency at 0.5A, 5V

OUT

from 12V

1.5A Continuous Output Current

Fast Minimum Switch-On Time: 35ns

Adjustable and Synchronizable: 200kHz to 2.2MHz

Spread Spectrum Frequency Modulation for Low EMI

Allows Use of Small Inductors

Low Dropout

Peak Current Mode Operation

Accurate 1V Enable Pin Threshold

Internal Compensation

Output Soft-Start and Tracking

Small 10-Lead MSOP Package or 8-Lead

2mm×2mm DFN Package

General Purpose Step Down

Low EMI Step Down

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

5V, 2MHz Step-Down

12V

to 5V

OUT

Efficiency

= 2MHz

OUT

(A)

0.25

0.50

0.75

1.00

1.25

1.50

100

EFFICIENCY (%)

OUT

8608 TA01b

BST

EN/UV

ON OFF

0.1µF

22µF

10pF

4.7µF

5.5V TO 42V

1µF

OUT

1.5A

187k

8608 TA01a

2.2µH

SYNC

INTV

TR/SS

LT8608

GND

18.2k

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Summary of content (24 pages)

PAGE 1
LT8608/LT8608B 42V, 1.5A Synchronous Step-Down Regulator with 2.5µA Quiescent Current FEATURES DESCRIPTION Wide Input Voltage Range: 3.0V to 42V nn Ultralow Quiescent Current Burst Mode® Operation nn <2.5µA I Regulating 12V to 3.3V Q IN OUT nn Output Ripple <10mV P-P nn High Efficiency 2MHz Synchronous Operation nn >92% Efficiency at 0.5A, 5V OUT from 12VIN nn 1.5A Continuous Output Current nn Fast Minimum Switch-On Time: 35ns nn Adjustable and Synchronizable: 200kHz to 2.
PAGE 2
LT8608/LT8608B ABSOLUTE MAXIMUM RATINGS (Note 1) VIN, EN/UV, PG...........................................................42V FB, TR/SS ...................................................................4V SYNC Voltage ..............................................................6V Operating Junction Temperature Range (Note 2) LT8608E ............................................ –40°C to 125°C LT8608I.............................................. –40°C to 125°C LT8608H....................................
PAGE 3
LT8608/LT8608B 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 Feedback Reference Voltage MSOP Package VIN = 6V, ILOAD = 100mA VIN = 6V, ILOAD = 100mA l 0.774 0.762 0.778 0.778 0.782 0.798 V V DFN Package VIN = 6V, ILOAD = 100mA VIN = 6V, ILOAD = 100mA l 0.773 0.753 0.778 0.778 0.783 0.803 V V Feedback Voltage Line Regulation VIN = 4.
PAGE 4
LT8608/LT8608B TYPICAL PERFORMANCE CHARACTERISTICS Efficiency (5V Output, Burst Mode Operation) Mode Operation) Efficiency (5V Output, Burst Mode 100 100 VIN = 12V 95 95 80 75 70 65 50 0.00 0.25 0.50 0.75 1.00 IOUT (A) 1.25 60 50 40 30 0 0.001 0.01 1.50 85 0.1 8608 G01 1 10 IOUT (mA) 100 1k 75 70 65 60 50 0.00 10k 50 40 30 0 0.001 0.01 SYNC = 0V OR LT8608 DFN L = 2.2µH fSW = 2MHz 0.1 1 10 IOUT (mA) 100 1k 778 777 0.2 0.1 0.0 –0.1 –0.2 –0.
PAGE 5
LT8608/LT8608B TYPICAL PERFORMANCE CHARACTERISTICS Top FET Current Limit vs TopDuty Fet Cycle Current Limit vs Duty Cycle Top FET Current Limit 3.1 DUTY CYCLE = 0 SWITCH DROP (mV) 2.75 2.50 2.8 2.7 2.25 2.
PAGE 6
LT8608/LT8608B TYPICAL PERFORMANCE CHARACTERISTICS Minimum Load to Full Frequency (SYNC Float to 1.9V) (MSOP Package) L = 2.2µH VOUT = 5V RT = 18.2k FREQUENCY (kHz) LOAD CURRENT (mA) 100 75 50 25 2500 1.0 2250 0.9 2000 0.8 1750 0.7 FB VOLTAGE (V) 125 Frequency Foldback 1500 1250 1000 0 0 5 10 15 20 25 30 INPUT VOLTAGE (V) 35 0.6 0.5 0.4 750 0.3 500 0.2 0.1 SYNC = 0V OR LT8608 DFN RT = 18.2k 250 0 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.
PAGE 7
LT8608/LT8608B TYPICAL PERFORMANCE CHARACTERISTICS Switching Waveforms Switching Waveforms Switching Waveforms IL IL 1A/DIV IL 1A/DIV 200mA/DIV SW SW 5V/DIV 5V/DIV 200ns/DIV SW 2V/DIV 10μs/DIV 8608 G28 8608 G29 12VIN TO 5VOUT AT 3mA 12VIN TO 3.3 VOUT AT 1A 2MHz 200ns/DIV 8608 G30 36VIN TO 3.3VOUT AT 1A 2MHz Transient Response Transient Response ILOAD 500mA/DIV ILOAD 500mA/DIV VOUT 50mV/DIV VOUT 50mV/DIV 50µs/DIV 8608 G31 VIN = 12V 0.
PAGE 8
LT8608/LT8608B PIN FUNCTIONS BST: This pin is used to provide a drive voltage, higher than the input voltage, to the topside power switch. Place a 0.1µF boost capacitor as close as possible to the IC. Do not place a resistor in series with this pin. SW: The SW pin is the output of the internal power switches. Connect this pin to the inductor and boost capacitor. This node should be kept small on the PCB for good performance. INTVCC: Internal 3.5V Regulator Bypass Pin.
PAGE 9
LT8608/LT8608B BLOCK DIAGRAM VIN VIN CIN – + INTERNAL 0.778V REF R3 (OPT) EN/UV R4 (OPT) PG 1V + – SHDN ±8.5% SLOPE COMP ERROR AMP + + – RPG R2 R1 CSS RT FB TR/SS VC SHDN TSD INTVCC UVLO VIN UVLO 2µA INTVCC CVCC OSCILLATOR 200kHz TO 2.2MHz VOUT CFF 3.5V REG BST BURST DETECT SWITCH LOGIC AND ANTISHOOT THROUGH CBST M1 L SW VOUT COUT M2 GND SHDN TSD VIN UVLO RT SYNC 8608 BD Rev. D For more information www.analog.com Downloaded from Arrow.com.
PAGE 10
LT8608/LT8608B OPERATION The LT8608 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 11
LT8608/LT8608B APPLICATIONS INFORMATION Burst Frequency vs Load Current 2500 SWITHCING FREQUENCY (kHz) VOUT 20mV/DIV L = 2.2µH VIN = 12V VOUT = 3.3V SYNC = 0V 2250 2000 1750 SW 5V/DIV INDUCTOR CURRENT 1500 1250 500mA/DIV 1000 20μs/DIV 8608 F03 Figure 3. Burst Mode Operation 750 500 250 0 0 100 200 300 400 LOAD CURRENT (mA) 500 VOUT 20mV/DIV SW 5V/DIV 8608 F01 Figure 1. SW Burst Mode Frequency vs Load 500mA/DIV 125 L = 2.2µH VOUT = 5V RT = 18.
PAGE 12
LT8608/LT8608B APPLICATIONS INFORMATION FB Resistor Network Operating Frequency Selection and Trade-Offs The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the resistor values according to: 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.
PAGE 13
LT8608/LT8608B APPLICATIONS INFORMATION Inductor Selection and Maximum Output Current The LT8608 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 LT8608 safely tolerates operation with a saturated inductor through the use of a high speed peak-current mode architecture.
PAGE 14
LT8608/LT8608B APPLICATIONS INFORMATION A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT8608 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT8608’s voltage rating. This situation is easily avoided (see Analog Devices Application Note 88). loads the LT8608 can excite the ceramic capacitor at audio frequencies, generating audible noise.
PAGE 15
LT8608/LT8608B APPLICATIONS INFORMATION When 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 LT8608. Therefore, the VIN(EN) resistors should be large to minimize their effect on efficiency at low loads. is considered good and the open-drain PG pin goes high impedance and is typically pulled high with an external resistor. Otherwise, the internal drain pull-down device will pull the PG pin low.
PAGE 16
LT8608/LT8608B APPLICATIONS INFORMATION For some applications, reduced EMI operation may be desirable, which can be achieved through spread spectrum modulation. This mode operates similar to pulse skipping mode operation, with the key difference that the switching frequency is modulated up and down by a 3kHz triangle wave. The modulation has the frequency set by RT as the low frequency, and modulates up to approximately 20% higher than the frequency set by RT.
PAGE 17
LT8608/LT8608B APPLICATIONS INFORMATION Figure 7 shows the basic guidelines for a layout example that can pass CISPR25 radiated emission test with class 5 limits. Thermal Considerations For higher ambient temperatures, care should be taken in the layout of the PCB to ensure good heat sinking of the LT8608. Figure 7 shows the recommended component placement with trace, ground plane, and via locations. The exposed pad on the bottom of the package must be soldered to a ground plane.
PAGE 18
LT8608/LT8608B APPLICATIONS INFORMATION GROUND PLANE ON LAYER 2 COUT L CIN CBST 1 CVCC CIN(OPT) RT RPG R4 R3 R1 CFF GND VIA VIN VIA VOUT VIA EN/UV VIA R2 CSS OTHER SIGNAL VIA 8609S F07 Figure 7. PCB Layout (Not to Scale) 18 Downloaded from Arrow.com. Rev. D For more information www.analog.
PAGE 19
LT8608/LT8608B TYPICAL APPLICATIONS 3.3V Step Down VIN 3.9V TO 42V C1 0.1µF VIN C2 4.7µF EN/UV BST SYNC SW L1 2.2µH R4 100k LT8608 INTVCC C3 1µF PG C5 10pF TR/SS C6 10nF R1 18.2k RT GND FB 8608 TA03 fSW = 2MHz R3 309k R2 1M L1 = XFL4020-222ME VOUT 3.3V 1.5A POWER GOOD C4 22µF X7R 1206 5V Step Down VIN 5.6V TO 42V C1 0.1µF VIN C2 4.7µF EN/UV BST SYNC SW L1 2.2µH R4 100k LT8608 INTVCC C3 1µF PG C5 10pF TR/SS C6 10nF R1 18.
PAGE 20
LT8608/LT8608B TYPICAL APPLICATIONS 1.8V 2MHz Step-Down Converter VIN 3.1V TO 20V (42V TRANSIENT) C2 4.7µF PSKIP R1 18.2k L1 2.2µH VOUT 1.8V 1.5A SW INTVCC C6 10nF M1 NFET BST EN/UV SYNC C3 1µF C1 0.1µF VIN LT8608 (MSOP) R4 100k PG C5 10pF TR/SS RT GND FB 8608 TA06 fSW = 2MHz R3 768k R2 1M L1 = XFL4020-222ME POWER GOOD C4 22µF X7R 1206 Ultralow EMI 5V 1.5A Step-Down Converter VIN 5.8V TO 42V L2 BEAD L3 4.7µH C8 4.7µF C7 4.7µF VIN C2 4.
PAGE 21
LT8608/LT8608B PACKAGE DESCRIPTION MSE Package 10-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1664 Rev I) BOTTOM VIEW OF EXPOSED PAD OPTION 1.88 ±0.102 (.074 ±.004) 5.10 (.201) MIN 1 0.889 ±0.127 (.035 ±.005) 1.68 ±0.102 (.066 ±.004) 0.05 REF 10 0.305 ± 0.038 (.0120 ±.0015) TYP RECOMMENDED SOLDER PAD LAYOUT 3.00 ±0.102 (.118 ±.004) (NOTE 3) DETAIL “B” CORNER TAIL IS PART OF DETAIL “B” THE LEADFRAME FEATURE.
PAGE 22
LT8608/LT8608B PACKAGE DESCRIPTION DC8 Package 8-Lead Plastic DFN (2mm × 2mm) (Reference LTC DWG # 05-08-1939 Rev Ø) Exposed Pad Variation AA 1.8 REF 0.90 REF 0.23 REF 0.85 ±0.05 2.60 ±0.05 PACKAGE OUTLINE 0.335 REF 0.25 ±0.05 0.45 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 2.00 ±0.05 (4 SIDES) PIN 1 BAR TOP MARK (SEE NOTE 6) 2.00 SQ ±0.05 1.8 REF 5 8 0.23 0.335 REF REF 0.55 ±0.05 PIN 1 NOTCH R = 0.15 (DC8MA) DFN 0113 REV Ø 4 0.200 REF 0.
PAGE 23
LT8608/LT8608B REVISION HISTORY REV DATE DESCRIPTION A 12/16 Clarified minimum on time to 35ns Clarified condition on feedback voltage line regulation Clarified transient response graphs PAGE NUMBER B 01/18 Added DFN Package Option Added H-Grade Temperature Options Clarified Graphs for MSOP Package Option Only Clarified Pin Functions for MSOP and DFN Package Options Clarified Operation and Applications Information Section for MSOP and DFN Package Options Added Figure 6 Clarified Typical Application
PAGE 24
LT8608/LT8608B TYPICAL APPLICATION 3.3V and 1.8V with Ratio Tracking VIN 3.9V TO 42V C2 4.7µF C1 0.1µF VIN R1 18.2k R4 100k LT8608 INTVCC C6 10nF VOUT 3.3V, 1.5A SW SYNC C3 1µF L1 2.2µH BST EN/UV PG C5 10pF TR/SS RT GND FB R3 309k R2 1M POWER GOOD C4 47µF fSW = 2MHz C8 4.7µF R9 31.6k L2 2.2µH BST EN/UV SW SYNC INTVCC C12 1µF C7 0.1µF VIN R8 100k LT8608 PG C11 10pF TR/SS R10 10k R5 18.2k RT GND FB 8608 TA02 R7 768k R6 1M VOUT 1.8V 1.