Linear Technology Analog Devices LT8607B Manuale utente

LT8607/LT8607B

Rev. C

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TYPICAL APPLICATION

FEATURES DESCRIPTION

42V, 750mA Synchronous

Step-Down Regulator with

2.5µA Quiescent Current

The LT

8607 is a compact, high efficiency, high speed syn-

chronous monolithic step-down switching regulator that

consumes only 1.7µA of non-switching quiescent current.

The LT8607 can deliver 750mA of continuous current.

Low ripple Burst Mode operation enables high efficiency

down to very low output currents while keeping the output

ripple below 10mVP-P. Internal compensation with peak

current mode topology allows the use of small inductors

and results in fast transient response and good loop sta-

bility. The EN/UV pin has an accurate 1V threshold and

can be used to program V

undervoltage lockout or to

shut down the LT8607 reducing the input supply current

to 1µA. The PG pin signals when VOUT 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 pack-

age omits these pins and can be purchased in pulse-skip-

ping or Burst Mode operation.

APPLICATIONS

Wide Input Voltage Range: 3.0V to 42V

Ultralow Quiescent Current Burst Mode

Operation:

n <3µA IQRegulating 12VIN to 3.3VOUT

n Output Ripple <10mVP-P

High Efficiency 2MHz Synchronous Operation:

n >93% Efficiency at 0.5A, 12VIN to 5VOUT

750mA Maximum Continuous Output

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 Thermally Enhanced 10-Lead MSOP Package or

8-Lead 2mm × 2mm DFN Package

General Purpose Step-Down Converter

Low EMI Step Down

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

5V, 2MHz Step-Down

12VIN to 5VOUT Efficiency

0.1µF

4.7µF

1µF

22µF

X7R

1206

10pF

4.7µH

18.2k

187k

100k

10nF

EN/UV

SYNC

LT8607

INTV

TR/SS

GND

BST

5.7V to 42V

OUT

750mA

POWER

GOOD

= 2MHz

8607 TA01a

= 2MHz

L = 4.7µH

OUT

(mA)

125

250

375

500

625

750

100

EFFICIENCY (%)

8607 TA01b

PACKAGE SYNC FUNCTIONALITY

LT8607MSE MSE Programmable

LT8607DFN DFN Burst Mode Operation

LT8607BDFN DFN Pulse-Skipping Mode

LT8607/LT8607B

Rev. C

For more information www.analog.com

ABSOLUTE MAXIMUM RATINGS

VIN, EN/UV, PG..........................................................42V

FB, TR/SS . .................................................................4V

SYNC Voltage .............................................................6V

(Note 1)

LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE

LT8607EMSE#PBF LT8607EMSE#TRPBF LTGXJ 10-Lead Plastic MSOP –40°C to 125°C

LT8607IMSE#PBF LT8607IMSE#TRPBF LTGXJ 10-Lead Plastic MSOP –40°C to 125°C

LT8607HMSE#PBF LT8607HMSE#TRPBF LTGXJ 10-Lead Plastic MSOP –40°C to 150°C

LT8607EDC#PBF LT8607EDC#TRPBF LGXK 8-Lead Plastic (2mm × 2mm) Plastic DFN –40°C to 125°C

LT8607IDC#PBF LT8607IDC#TRPBF LGXK 8-Lead Plastic (2mm × 2mm) Plastic DFN –40°C to 125°C

LT8607HDC#PBF LT8607HDC#TRPBF LGXK 8-Lead Plastic (2mm × 2mm) Plastic DFN –40°C to 150°C

LT8607BEDC#PBF LT8607BEDC#TRPBF LGXM 8-Lead Plastic (2mm × 2mm) DFN –40°C to 125°C

LT8607BIDC#PBF LT8607BIDC#TRPBF LGXM 8-Lead Plastic (2mm × 2mm) DFN –40°C to 125°C

LT8607BHDC#PBF LT8607BHDC#TRPBF LGXM 8-Lead Plastic (2mm × 2mm) DFN –40°C to 150°C

Contact the factory for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.

Tape and reel specifications. Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix.

ELECTRICAL CHARACTERISTICS

The ldenotes the specifications which apply over the full operating

temperature range, otherwise specifications are at TA= 25°C.

ORDER INFORMATION

PARAMETER CONDITIONS MIN TYP MAX UNITS

Minimum Input Voltage

2.5 3.0

3.2

VIN Quiescent Current VEN/UV = 0V

VEN/UV = 2V, Not Switching, VSYNC = 0V or LT8607 DFN, VIN ≤ 36V

1.7

µA

VIN Current in Regulation VIN = 6V, VOUT = 2.7V, Output Load = 100µA

VIN = 6V, VOUT = 2.7V, Output Load = 1mA

500

700

µA

BST

INTVCC

SYNC

EN/UV

VIN

TR/SS

TOP VIEW

GND

MSE PACKAGE

10-LEAD PLASTIC MSOP

θJA = 40°C/W

EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB

TOP VIEW

BST

INTVCC

EN/UV

VIN

DC PACKAGE

8-LEAD (2mm ×2mm) PLASTIC DFN

θJA = 102°C/W

EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB

GND

PIN CONFIGURATION

Operating Junction Temperature Range (Note 2)

LT8607E............................................ –40°C to 125°C

LT8607I............................................. –40°C to 125°C

LT8607H............................................ –40°C to 150°C

Storage Temperature Range .................. –65°C to 150°C

LT8607/LT8607B

Rev. C

For more information www.analog.com

ELECTRICAL CHARACTERISTICS

The ldenotes the specifications which apply over the full operating

temperature range, otherwise specifications are at TA= 25°C.

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. Absolute Maximum Ratings are those values beyond

which the life of a device may be impaired.

Note 2: The LT8607E 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

LT8607I is guaranteed over the full –40°C to 125°C operating junction

temperature range. The LT8607H 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.

PARAMETER CONDITIONS MIN TYP MAX UNITS

Feedback Reference Voltage MSOP Package

VIN = 6V, ILOAD = 100mA

0.774

0.762

0.778

0.782

0.798

DFN Package

VIN = 6V, ILOAD = 100mA

0.771

0.753

0.778

0.785

0.803

Feedback Voltage Line Regulation VIN = 4.0V to 40V l±0.02 ±0.06 %/V

Feedback Pin Input Current VFB = 1V ±20 nA

Minimum On-Time ILOAD = 750mA, SYNC = 0V or LT8607 DFN

ILOAD = 750mA, SYNC = 1.9V or LT8607B DFN

Minimum Off Time ILOAD = 300mA l93 130 ns

Oscillator Frequency MSOP Package

RT= 221k, ILOAD = 350mA

RT = 60.4k, ILOAD = 350mA

RT = 18.2k, ILOAD = 350mA

155

640

1.90

200

700

2.00

245

760

2.10

kHz

MHz

DFN Package

RT= 221k, ILOAD = 350mA

RT = 60.4k, ILOAD = 350mA

RT = 18.2k, ILOAD = 350mA

140

610

1.85

200

700

2.00

260

790

2.15

kHz

MHz

Top Power NMOS On-Resistance ILOAD = 500mA 375 mΩ

Top Power NMOS Current Limit MSOP Package l1.2 1.6 2.0 A

DFN Package l1.2 1.7 2.2 A

Bottom Power NMOS On-Resistance 240 mΩ

SW Leakage Current VIN = 36V 5 µA

EN/UV Pin Threshold EN/UV Rising l0.99 1.05 1.11 V

EN/UV Pin Hysteresis 50 mV

EN/UV Pin Current VEN/UV = 2V ±20 nA

PG Upper Threshold Offset from VFB VFB Rising l5.0 8.5 13.0 %

PG Lower Threshold Offset from VFB VFB Falling l5.0 8.5 13.0 %

PG Hysteresis 0.5 %

PG Leakage VPG = 42V ±200 nA

PG Pull-Down Resistance VPG = 0.1V 550 1200 Ω

Sync Low Input Voltage MSOP Only l0.4 0.9 V

Sync High Input Voltage INTVCC = 3.5V, MSOP Only l2.7 3.2 V

TR/SS Source Current MSOP Only l1 2 3 µA

TR/SS Pull-Down Resistance Fault Condition, TR/SS = 0.1V, MSOP Only 300 900 Ω

Spread Spectrum Modulation Frequency VSYNC = 3.3V, MSOP Only 0.5 3 6 kHz

LT8607/LT8607B

Rev. C

For more information www.analog.com

TYPICAL PERFORMANCE CHARACTERISTICS

Efficiency (5V Output, Burst Mode

Operation)

Efficiency (5V Output, Burst Mode

Operation)

Efficiency (3.3V Output,

Burst Mode Operation)

Efficiency (3.3V Output,

Burst Mode Operation) FB Voltage Load Regulation

Line Regulation

No-Load Supply Current

(3.3V Output Switching)

No Load Supply Current vs

Temperature (Not Switching)

INPUT VOLTAGE (V)

–0.20

–0.15

–0.10

–0.05

0.00

0.05

0.10

0.15

0.20

CHANGE IN V

OUT

(%)

8607 G07

TEMPERATURE (°C)

–50

–10

110

150

775

776

777

778

779

780

FB REGULATION VOLTAGE (mV)

8607 G05

OUTPUT CURRENT (mA)

125

250

375

500

625

750

–0.25

–0.20

–0.15

–0.10

–0.05

0.00

0.05

0.10

0.15

0.20

0.25

CHANGE IN V

OUT

(%)

8607 G06

TA= 25°C, unless otherwise noted.

= 2MHz

= 12V

= 24V

L = 4.7µH

SYNC = 0V OR LT8607 DFN

OUT

(mA)

125

250

375

500

625

750

100

EFFICIENCY (%)

8607 G01

= 2MHz

= 12V

= 24V

L = 4.7µH

OUT

(mA)

0.001

0.01

0.1

100

900

100

EFFICIENCY (%)

8607 G02

SYNC = 0V OR LT8607 DFN

= 2MHz

= 12V

= 24V

L = 2.2µH

OUT

(mA)

125

250

375

500

625

750

100

EFFICIENCY (%)

8607 G03

SYNC = 0V OR LT8607 DFN

= 2MHz

= 12V

= 24V

L = 2.2µH

OUT

(mA)

0.001

0.01

0.1

100

900

100

EFFICIENCY (%)

8607 G04

SYNC = 0V OR LT8607 DFN

L = 2.2µH

INPUT VOLTAGE (V)

2.00

2.25

2.50

2.75

3.00

3.25

3.50

3.75

4.00

4.25

4.50

(µA)

8607 G08

SYNC = 0V OR LT8607 DFN

TEMPERATURE (°C)

–50

–10

110

150

1.3

1.5

1.7

1.9

2.1

2.3

2.5

2.7

2.9

3.1

3.3

INPUT CURRENT (µA)

8607 G09

SYNC = 0V OR LT8607 DFN

LT8607/LT8607B

Rev. C

For more information www.analog.com

DUTY CYCLE (%)

100

1.20

1.30

1.40

1.50

1.60

1.70

TOP FET CURRENT LIMIT (A)

8607 G10

SWITCH CURRENT (mA)

125

250

375

500

625

750

100

150

200

250

300

SWITCH DROP (mV)

8607 G13

TOP SW

BOT SW

DUTY CYCLE = 0

TEMPERATURE (°C)

–50

–10

110

150

1.50

1.55

1.60

1.65

1.70

TOP FET CURRENT LIMIT (A)

8607 G11

OUT

= 750mA

TEMPERATURE (°C)

–50

–30

–10

110

130

150

MINIMUM ON-TIME (ns)

8607 G14

SWITCH CURRENT = 750mA

TOP SW

BOT SW

TEMPERATURE (°C)

–50

–30

–10

110

130

150

100

150

200

250

300

350

400

450

SWITCH DROP (mV)

8607 G12

OUT

= 350mA

TEMPERATURE (°C)

–50

–30

–10

110

130

150

100

105

110

MINIMUM OFF-TIME (ns)

8607 G15

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 Output Current

Switching Frequency

vs Temperature

Burst Frequency vs

Output Current

L: XFL4020-472ME

OUTPUT CURRENT (A)

125

250

375

500

625

750

100

150

200

250

300

350

DROPOUT VOLTAGE (mV)

8607 G16

= 18.2kΩ

TEMPERATURE (°C)

–50

–10

110

150

1975

1980

1985

1990

1995

2000

2005

2010

2015

2020

2025

SWITCHING FREQUENCY (kHz)

8607 G17

TA= 25°C, unless otherwise noted.

SYNC = 0V OR LT8607 DFN

OUTPUT CURRENT (mA)

100

125

250

500

750

1000

1250

1500

1750

2000

2250

2500

SWITCHING FREQUENCY (kHz)

8607 G18

= 12V

L = 2.2µH

OUT

= 3.3V

LT8607/LT8607B

Rev. C

For more information www.analog.com

TYPICAL PERFORMANCE CHARACTERISTICS

Minimum Load to Full

Frequency (SYNC Float to 1.9V)

(MSOP Package) or LT8607B DFN Frequency Foldback

Soft-Start Tracking

(MSOP Package)

Soft-Start Current vs Temperature

(MSOP Package)

Start-Up Dropout

VIN UVLO vs Temperature

Start-Up Dropout

INPUT VOLTAGE (V)

100

125

150

OUTPUT CURRENT (mA)

8607 G19

= 12V

L = 2.2µH

OUT

= 3.3V

= 18.2kΩ

SS VOLTAGE (V)

0.1

0.2

0.4

0.5

0.6

0.7

0.8

1.0

1.1

1.2

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

FB VOLTAGE (V)

8607 G21

TEMPERATURE (°C)

–50

–30

–10

110

130

150

1.5

1.6

1.7

1.8

1.9

2.0

2.1

2.2

2.3

2.4

2.5

SOFT-START CURRENT (µA)

8607 G22

OUT

LOAD

= 50Ω

INPUT VOLTAGE (V)

OUTPUT VOLTAGE (V)

8607 G24

TEMPERATURE (°C)

–50

–30

–10

110

130

150

2.00

2.25

2.50

2.75

3.00

3.25

UVLO (V)

8607 G23

OUT

LOAD

= 6.66Ω

INPUT VOLTAGE (V)

OUTPUT VOLTAGE (V)

8607 G25

TA= 25°C, unless otherwise noted.

FB VOLTAGE (V)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

250

500

750

1000

1250

1500

1750

2000

2250

2500

FREQUENCY (kHz)

8607 G20

SYNC = 0V OR LT8607 DFN

LT8607/LT8607B

Rev. C

For more information www.analog.com

TYPICAL PERFORMANCE CHARACTERISTICS

Switching Waveforms Switching Waveforms

Transient Response

Switching Waveforms

(Burst Mode)

Transient Response

200ns/DIV

ILOAD

200mA/DIV

VSW

5V/DIV

8607 G26

12VIN TO 5VOUT AT 500mA

2MHz

200µs/DIV

ILOAD

200mA/DIV

VOUT

100mV/DIV

8607 G29

VIN =12V

VOUT = 5V

50mA TO 550mA

COUT = 22µF

= 2MHz

100µs/DIV

ILOAD

200mA/DIV

VOUT

100mV/DIV

8607 G30

VIN =12V

VOUT = 5V

250mA TO 750mA

COUT = 22µF

= 2MHz

200ns/DIV

ILOAD

200mA/DIV

VSW

10V/DIV

8607 G27

36VIN TO 5VOUT AT 500mA

2MHz

2µs/DIV

VOUT

20mV/DIV

ILOAD

200mA/DIV

VSW

10V/DIV

8607 G28

12VIN TO 5VOUT AT 7.5mA

22µF C

OUT

TA= 25°C, unless otherwise noted.

Radiated EMI Performance

(CISPR 25 Radiated Emission Test with Class 5 Peak Limits)

FREQUENCY (MHz)

AMPLITUDE (dBµV/m)

–5

–10

8607 G31

0500 900300 700

1000

400 800200 600100

VERTICAL POLARIZATION

PEAK DETECTOR

CLASS 5 PEAK LIMIT

SPREAD SPECTRUM MODE

FIXED FREQUENCY

DC2565A DEMO BOARD

WITH EMI FILTER INSTALLED

14V INPUT TO 5V OUTPUT AT 500mA, f

= 2MHz

LT8607/LT8607B

Rev. C

For more information www.analog.com

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. The internal

power drivers and control circuits are powered from this

voltage. INTVCC max output current is 20mA. Voltage on

INTVCC will vary between 2.8V and 3.5V. Decouple this

pin to power ground with at least a 1µF low ESR ceramic

capacitor. Do not load the INTVCC pin with external circuitry.

RT:A resistor is tied between RT and ground to set the

switching frequency. When synchronizing, the R

resistor

should be chosen to set the LT8607 switching frequency

to equal or below the lowest synchronization input.

SYNC (MSOP Only):External Clock Synchronization

Input. Ground this pin for low ripple Burst Mode operation

at low output loads. Tie to a clock source for synchroni-

zation to an external frequency. Leave floating for pulse-

skipping mode with no spread spectrum modulation. Tie

to INTVCC or tie to a voltage between 3.2V and 5.0V for

pulse-skipping mode with spread spectrum modulation.

When in pulse-skipping mode, the IQregulating no load

will increase to several mA. There is no SYNC pin on

the LT8607 DFN package. The LT8607 DFN internally

ties SYNC to ground. The LT8607B package internally

floatsSYNC.

FB: The LT8607 regulates the FB pin to 0.778V. Connect

the feedback resistor divider tap to this pin.

TR/SS (MSOP Only):Output Tracking and Soft-Start Pin.

This pin allows user control of output voltage ramp rate

during start-up. A TR/SS voltage below 0.778V forces the

LT8607 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 from

INTVCC on this pin allows a capacitor to program out-

put 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.

There is no TR/SS pin on the LT8607 or LT8607B DFN

and the node is internally floated.

PG:The PG pin is the open-drain output of an internal

comparator. PG remains low until the FB pin is within

±8.5% of the final regulation voltage, and there are no

fault conditions. PG is valid when VIN is above 3.2V and

when EN/UV is high. PG is pulled low when VIN is above

3.2V and EN/UV is low. If VIN is near zero, PG will be high

impedance.

VIN:The VIN pin supplies current to the LT8607 internal

circuitry and to the internal 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

VIN pins, and the negative capacitor terminal as close as

possible to the GND pins.

EN/UV:The LT8607 is shut down when this pin is low and

active when this pin is high. The hysteretic threshold volt-

age is 1.05V going up and 1.00V going down. Tie to VIN

if the shutdown feature is not used. An external resistor

divider from VIN can be used to program a VIN threshold

below which the LT8607 will shut down.

GND: Exposed Pad Pin. The exposed pad must be con-

nected to the negative terminal of the input capaci-

tor and soldered to the PCB in order to lower the

thermalresistance.

LT8607/LT8607B

Rev. C

For more information www.analog.com

BLOCK DIAGRAM

OPERATION

The LT8607 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. The error amplifier servos the VC node

by comparing the voltage on the VFB pin with an inter-

nal 0.778V 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.

If the EN/UV pin is low, the LT8607 is shut down and

draws 1µA from the input. When the EN/UV pin is above

1.05V, the switching regulator becomes active.

To optimize efficiency at light loads, the LT8607 enters

Burst Mode operation during light load situations. 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, 3.0µA will be consumed

from the input supply when regulating with no load. The

SYNC pin is tied low to use Burst Mode operation and can

be floated to use pulse-skipping mode. If a clock is applied

to the SYNC pin the part will synchronize to an external

clock frequency and operate in pulse-skipping mode. While

in pulse-skipping mode the oscillator operates continu-

ously and positive SW transitions are aligned to the clock.

During light loads, switch pulses are skipped to regulate

the output and the quiescent current will be several mA.

The SYNC pin may be tied high for spread spectrum modu-

lation mode, and the LT8607 will operate similar to pulse-

skipping mode but vary the clock frequency to reduce EMI.

The LT8607 DFN has no SYNC pin and will always operate

in Burst Mode operation. The LT8607B has no SYNC pin

and will operate in pulse-skipping mode.

Comparators monitoring the FB pin voltage will pull the PG

pin low if the output voltage varies more than ±8.5% (typi

cal) from the set point, or if a fault condition is present.

The oscillator reduces the LT8607’s operating frequency

when the voltage at the FB pin is low and the part is in Burst

Mode operation. This frequency foldback helps to control

the inductor current when the output voltage is lower than

the programmed value which occurs duringstart-up.

–

SLOPE COMP

INTERNAL 0.778V REF

OSCILLATOR

200kHz TO 2.2MHz

BURST

DETECT

3.5V

REG

CBST

COUT

VOUT

8607 BD

SW L

BST

SWITCH

LOGIC

AND

ANTI-

SHOOT

THROUGH

ERROR

AMP

SHDN

±8.5%

SHDN

TSD

INTVCC UVLO

VIN UVLO

SHDN

TSD

VIN UVLO

EN/UV

1V +

–INTVCC

GND

RPG

CSS

TR/SS (MSOP ONLY)

2µA

SYNC (MSOP ONLY)

VIN

VOUT

CIN

CFF

CVCC

OPT

LT8607/LT8607B

Rev. C

For more information www.analog.com

APPLICATIONS INFORMATION

Achieving Ultralow Quiescent Current

To enhance efficiency at light loads, the LT8607 enters

into low ripple Burst Mode operation, which keeps the

output capacitor charged to the desired output voltage

while minimizing the input quiescent current and mini-

mizing output voltage ripple. In Burst Mode operation the

LT8607 delivers single small pulses of current to the out-

put capacitor followed by sleep periods where the output

power is supplied by the output capacitor. While in sleep

mode the LT8607 consumes 1.7µA.

As the output load decreases, the frequency of single cur-

rent pulses decreases (see Figure1) and the percentage

of time the LT8607 is in sleep mode increases, result-

ing in much higher light load efficiency than for typical

converters. By maximizing the time between pulses, the

converter quiescent current approaches 3.0µA for a typi-

cal 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 250mA resulting in output

voltage ripple shown in Figure3. Increasing the output

capacitance will decrease the 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

Table1. The output load at which the LT8607 reaches the

programmed frequency varies based on input voltage,

output voltage, and inductor choice.

For some applications it is desirable for the LT8607 to

operate in pulse-skipping mode, offering two major differ-

ences 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

as shown in Figure2. To enable pulse-skipping mode the

SYNC pin is floated. To achieve spread spectrum modula-

tion with pulse-skipping mode, the SYNC pin is tied high.

While a clock is applied to the SYNC pin the LT8607 will

also operate in pulse-skipping mode. The LT8607 DFN is

Figure1. Burst Frequency vs Output Current

= 18.2kΩ

= 12V

L = 2.2µH

OUT

= 3.3V

INPUT VOLTAGE (V)

100

125

150

OUTPUT CURRENT (mA)

8607 F02

2µs/DIV

VOUT

20mV/DIV

ILOAD

200mA/DIV

VSW

10V/DIV

8607 F03

programmed for Burst Mode operation and cannot enter

pulse-skipping mode. The LT8607B DFN is programmed

for pulse-skipping mode and cannot enter Burst Mode

operation.

OUTPUT CURRENT (mA)

100

125

250

500

750

1000

1250

1500

1750

2000

2250

2500

SWITHCING FREQUENCY (kHz)

8607 F01

= 12V

L = 2.2µH

OUT

= 3.3V

SYNC = 0V OR LT8607 DFN

Figure2. Minimum Load to Full Frequency

(SYNC Float to 1.9V) (MSOP or LT8607B DFN)

Figure3. Burst Mode Operation

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