Datasheet LTC3862 Datasheet (LINEAR TECHNOLOGY)

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LINEAR TECHNOLOGY

JANUARY 2009 VOLUME XVIII NUMBER 4

IN THIS ISSUE…

COVER ARTICLE 2-Phase, Non-Synchronous Boost

Controller Simpliﬁes Design of High Voltage, High Current Supplies

...........................................................1

Muthu Subramanian and Tick Houk

Linear in the News… ...........................2

DESIGN FEATURES Reliable Precision Voltage Reference

with 5ppm/°C Drift is Factory Trimmed and Tested at –40°C, 25°C and 125°C

...........................................................7

Michael B. Anderson and Brendan Whelan

Ultralow Noise 15mm ×15mm × 2.8mm µModule™ Step-Down Regulators Meet the Class B of CISPR 22 and Yield High Efﬁciency at up to 36V

Judy Sun, Jian Yin, Sam Young and Henry Zhang

Dual Hot Swap™ Controller Brings Digital Monitoring to AdvancedTCA,

µTCA and AMC Applications ..............15

Josh Simonson

±32V Triple-Output Supply for LCDs, CCDs and LEDs Includes Fault

Protection in a 3mm × 3mm QFN ......20

Eko T Lisuwandi

PD Interface for PoE+ Includes 25.5W Classiﬁcation and Protection Features in a Low Proﬁle 4mm × 3mm DFN

.........................................................26

Kirk Su

DESIGN IDEAS

....................................................

(complete list on page 30)

Design Tools ......................................43

Sales Ofﬁces .....................................

..........10

30–42

2-Phase, Non-Synchronous Boost Controller Simpliﬁes Design of High Voltage, High Current Supplies

by Muthu Subramanian and Tick Houk

Introduction

Due to an increasing need for high power step-up power supplies in automotive and industrial applications, Linear Technology has recently introduced the LTC3862 family of 2-phase, single output non-synchronous boost DC/DC controllers. The LTC3862 provides a ﬂexible, high performance step-up controller in three convenient package options: GN24, 5mm × 5mm 24-pin exposed pad QFN and 24-pin exposed pad TSSOP. The LTC3862 is optimized for power MOSFETs that require 5V gate drive, whereas the LTC3862-1 is designed for 10V gate drive MOSFETs.

The LTC3862 utilizes a ﬁxed frequency, peak current mode control topology to drive ground-referenced power MOSFETs, each with a current sense resistor in its source. The use of a precision transconductance (gm) error ampliﬁer allows for easy loop compensation and facilitates the parallel connection of several ICs in multiphase applications. The operating frequency can be programmed from 75kHz to 500kHz using a single resistor, and a phase lock loop allows the switching frequency to be synchronized to an external clock over a 50kHz to 650kHz range.

The LTC3862 is a versatile

control IC optimized for a

wide variety of step-up

DC/DC converter

applications. This makes

it easy to optimize

efﬁciency, size and weight

of the power supply, while

keeping component and

manufacturing costs low.

A 24V, 5A Car Audio Power Supply

Today’s high end car audio systems require signiﬁcant power to drive upwards of seven speakers inside the passenger compartment. High frequency speakers such as tweeters are generally very efﬁcient, but low frequency drivers such as subwoofers require substantial power to achieve high volume. In addition to the need for high power, the car audio system should be itmmune to changes in the battery voltage. These requirements can be met through the use of a step-up converter for the power

continued on page 3

, LT, LTC, LTM, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered trademarks of Linear Technology Corporation. Adaptive Power, Bat-Track, BodeCAD, C-Load, DirectSense, Easy Drive, FilterCAD, Hot Swap, LinearView, µModule, Micropower SwitcherCAD, Multimode Dimming, No Latency ΔΣ, No Latency Delta-Sigma, No R Filter, PanelProtect, PowerPath, PowerSOT, SmartStart, SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT, TimerBlox, True Color PWM, UltraFast and VLDO are trademarks of Linear Technology Corporation. Other product names may be trademarks of the companies that manufacture the products.

, Operational

SENSE

Page 2

LTC3862, continued from page 1

SENSE1

3V8

SLOPE

BLANK

CLKOUT SYNC PLLFLTR

PHASEMODE

MAX

10nF

SENSE1

–

SENSE2

–

SENSE2

RUN

FREQ

45.3k 24.9k

84.5k

10Ω

5V TO 24V

MBRD835L

10Ω

4.2µH

CDEP145-4R2

ITH

SGND

OUT

6.98k

26.7k

130k

10nF

1µF

1nF

22µF 25V

0.007Ω 1W

Q1 Si7386DP

Q2 Si7386DP

0.007Ω 1W

OUT

24V 5A (MAX)

100µF

35V

100µF

35V

22µF 25V

10µF 50V

LTC3862

1nF

100pF

INTV

GATE1

GATE2

PGND

4.7µF

10nF

10µF 50V

4.2µH

CDEP145-4R2

LOAD CURRENT (mA)

EFFICIENCY (%)

POWER LOSS (mW)

100

10000

1000

100

10000

100 1000

EFFICIENCY

POWER LOSS

VIN = 12V V

OUT

= 24V

LOAD

5A/DIV

OUT

500mV/DIV

500µs/DIVVIN = 12V

OUT

= 24V

LOAD

= 2A TO 5A

DESIGN FEATURES L

Figure 1. A 120W 2-phase, 24V/5A output car audio power supply

ampliﬁers. Figure 1 shows a 2-phase, 24V/5A output audio power supply that operates from a car battery, and Figure 2 shows the efﬁciency curve for this converter.

A 2-phase design with an operating frequency of 300kHz allows the use of signiﬁcantly smaller output capacitors and inductors than a single-phase design. To keep the output ripple voltage below 60mV peak-to-peak and satisfy the RMS ripple current demand, a combination of two 100µF, 35V aluminum electrolytic capacitors are connected in parallel with four 10µF, 50V ceramic capacitors. The 4.2µH,

10.6A inductor (CDEP145-4R2) from Sumida Inductors is chosen for its high

Figure 2. Efﬁciency and power loss vs load current for the 120W car audio power supply

Linear Technology Magazine • January 2009

A 2-phase design with an

operating frequency of

300kHz allows the use of

signiﬁcantly smaller output

capacitors and inductors

than a single-phase design.

Also, the output current

of this converter can

easily be scaled by adding

additional power stages and controllers, without

modifying the basic design.

Figure 3. Inductor current waveforms in load step show accurate current matching between load sharing channels

saturation current rating and surface mount package design.

The MOSFET is a Vishay Si7386DP,

which has a maximum R

DS(ON)

of 7mΩ

at VGS = 10V and 9.5mΩ at VGS =

4.5V. The 35V, 8A Schottky from On Semiconductor (MBRD835L) offers surface mount capability and small size. It should be noted that the output current of a converter such as this can easily be scaled by adding additional power stages and controllers, without modifying the basic design.

Excellent Channel-to-Channel Current Matching Ensures a Balanced Thermal Design

In order to provide the best channelto-channel inductor current matching, the LTC3862 is designed to make the transfer function from the output of the error ampliﬁer (the ITH pin) to the current comparator inputs (SENSE+ and SENSE– pins) as accurate as possible. The speciﬁcation for the maximum current sense threshold is 75mV, and the channel-to-channel (V tion is ±10mV, over the –40°C to 150°C temperature range. This excellent matching ensures balanced inductor currents and a thermally stable design, even with multiple regulators daisychained together. Figure 3 shows how

SENSE1

– V

) mismatch speciﬁca-

SENSE2

Page 3

L DESIGN FEATURES

SENSE1

SLOPE

BLANK

CLKOUT

SYNC

PLLFLTR

PHASEMODE

MAX

10nF

SENSE1

–

SENSE2

–

SENSE2

RUN

FREQ

45.3k 24.9k

150k

10Ω

8.5V TO 28V

B3100

10Ω

58µH

PA2050-583

ITH

SGND

OUT

5.62k

45.3k

324k

0.1µF

1µF

6.8µF 50V

0.020Ω 1W

Q1 HAT2267H

Q2 HAT2267H

0.020Ω 1W

OUT

72V

47µF 100V

6.8µF 50V

2.2µF 100V

LTC3862-1

1.5nF

100pF

INTV

GATE1

GATE2

PGND

4.7µF

10nF

58µH

PA2050-583

3V8

1nF

SYNC

10V/DIV

GATE1

10V/DIV

GATE2

10V/DIV

CLKOUT 10V/DIV

2µs/DIVVIN = 12V

OUT

= 48V 1A

PHASEMODE = SGND

SW1

10V/DIV

SW2

10V/DIV

1A/DIV

1µs/DIVVIN = 17V

OUT

= 24V

LIGHT LOAD (10mA)

2.5µs/DIVVIN = 24V

OUT

= 48V, 1.5A

SW1

50V/DIV

SW2

50V/DIV

OUT

100mV/DIV

AC COUPLED

5A/DIV

SW1

50V/DIV

SW2

50V/DIV

2A/DIV

1µs/DIVVIN = 12V

OUT

= 48V

LOAD

= 100mA

5A/DIV

OUT

20V/DIV

500µs/DIV

VIN = 24V V

OUT

= 72V

Figure 4. Inductor current and switch node voltage waveforms at heavy load, continuous conduction mode (CCM)

well matched the inductor currents are for the car audio power supply during a load step.

Constant Frequency Operation over a Wide Load Current Range

Constant frequency operation eases the task of input and output ﬁlter design, and prevents a power supply from becoming audible at light load. At heavy load, the inductor currents are generally continuous (CCM), as shown in Figure 4. At light load, the inductor current will go discontinuous (DCM), as shown in Figure 5. When the load current drops below what can be supported by the minimum on-time of the converter (approximately 180ns), the controller will begin to skip cycles in order to maintain output regulation,

Figure 8. An 8.5V–28V input, 72V/1.5A output low emissions diesel fuel injector actuator supply

Figure 5. Inductor and switch node voltage waveforms at light load, discontinuous

Figure 6. Inductor and switch node voltage waveforms at light load (pulse-skipping)

conduction mode (DCM)

the better, since constant frequency operation is maintained down to this threshold. In Figure 6 the onset of pulse-skipping occurs at a relatively low 0.2% of the maximum load current.

For systems where synchronization to an external clock is required, the LTC3862 contains a phase lock loop (PLL). Figure 7 illustrates the switching waveforms with an external sync signal applied to the SYNC pin.

Figure 7. Synchronizing the LTC3862 to an external clock using the phase lock loop

Strong Gate Drivers and a High Current Internal LDO

as shown in Figure 6. This is a normal operating condition that doesn’t cause any problems in the system, as long as the peak inductor current is low.

In general, the lower the load cur-

rent at the onset of pulse-skipping,

Complete the Package

In high output voltage systems, switching losses in the power MOSFETs can sometimes exceed the conduction losses. In order to reduce switching losses as much as possible, the LTC3862 incorporates strong gate drivers. The PMOS pull-up transistor has a typical R the NMOS pull-down transistor has a typical R

of 0.7Ω. In addition

DS(ON)

to reducing switching losses, these strong gate drivers allow two power MOSFETs to be connected in paral-

Figure 9. Load step waveforms for diesel fuel injector actuator supply

Linear Technology Magazine • January 2009

of 2.1Ω, and

DS(ON)

Page 4

DESIGN FEATURES L

SENSE1

SLOPE

BLANK

CLKOUT SYNC PLLFLTR

PHASEMODE

MAX

10nF

ON/OFF

CONTROL

SENSE1

–

SENSE2

–

SENSE2

RUN

FREQ

45.3k

10Ω

4.5V TO 5.5V

MBRB2515LT41

10Ω

2.7µH

CDEP145-2R7

ITH

SGND

OUT

18.7k

3.83k

165k

10nF

1µF

33µF 10V

0.005Ω 1W

Q1 HAT2165H

Q3 HAT2165H

0.005Ω 1W

OUT

12V 15A

220µF

16V

220µF

16V

LTC3862

10nF

330pF

INTV

GATE1

GATE2

PGND

4.7µF

10nF

15µF 25V

2.7µH

CDEP145-2R7

SENSE1

SLOPE

BLANK

CLKOUT SYNC PLLFLTR

PHASEMODE

MAX

10nF

SENSE1

–

SENSE2

–

SENSE2

RUN

FREQ

45.3k

10Ω

MBRB2515LT41

10Ω

2.7µH

CDEP145-2R7

ITH

SGND

10k

1µF

33µF 10V

0.005Ω 1W

Q1 HAT2165H

Q3 HAT2165H

0.005Ω 1W

220µF

16V

220µF

16V

LTC3862

10nF

INTV

GATE1

GATE2

PGND

4.7µF

330pF

10nF

15µF 25V

2.7µH

CDEP145-2R7

3V8

1nF

3V8

1nF

lel for each channel in high current applications.

In order to simplify operation in single-supply systems, the LTC3862 includes a 5V low dropout regulator (LDO) that can support output currents up to 50mA. The use of a PMOS output transistor ensures that the full supply voltage is available for driving the power MOSFETs under low supply conditions, such as during automotive cold cranking. An undervoltage lockout circuit detects when the LDO output voltage falls below 3.3V and shuts off the gate drivers, thereby

protecting the power MOSFETs from switching at low VGS.

The LTC3862 is capable of operation over a 4V to 36V input voltage range, making it suitable for a wide variety of boost applications.

Lower Emissions Diesel Fuel Injection: A 8.5V–28V Input, 72V, 1.5A Output Boost

Tomorrow’s low emissions diesel fuel injection systems require more precise and faster actuation of the fuel injectors than do their gasoline counterparts. Stepping up the voltage

of the system is an easy way to achieve fast actuation by increasing di/dt in the actuator, since the energy stored on a capacitor is CV2/2. Boosting the car battery voltage from 13V to 72V signiﬁcantly increases the di/dt, enabling faster actuation. The actuation of the injector typically discharges the supply capacitor by 10V–20V, after which the boost converter recharges the output cap to 72V. Figure 8 illustrates this

8.5V to 28V input, 72V/1.5A output 2-phase boost converter. Figure 9 illustrates the load step for a simulated injector.

Figure 10. A 4-phase, 12V/15A industrial power supply that operates from a 5V input

Linear Technology Magazine • January 2009

Page 5

L DESIGN FEATURES

5V/DIV

IL1 MASTER

5A/DIV

IL2 MASTER

5A/DIV

IL1 SLAVE

5A/DIV

IL2 SLAVE

5A/DIV

OUT

10V/DIV

1ms/DIVVIN = 5V

OUT

= 12V

LOAD

= 10Ω

LOAD

2.5A-5A 5A DIV

IL1 MASTER

5A/DIV

IL2 MASTER

5A/DIV

IL1 SLAVE

5A/DIV

IL2 SLAVE

5A/DIV

OUT

200mV/DIV

250µs/DIVVIN = 5V

OUT

= 12V

Figure 11. Power supply start-up waveforms for 4-phase, 12V/15A industrial power supply

This power supply operates at a switching frequency of 300kHz in order to reduce switching losses and uses a

57.8µH, 5A inductor (PA2050-583). An 80V Renesas HAT2267H MOSFET was chosen for this application, in order to provide sufﬁcient guardband above the 72V output. The MOSFET has a maximum R

of 13mΩ at VGS =

DS(ON)

10V. The Diodes Inc surface mount diode (B3100) was chosen for the 3A output current level. A combination of a two 47µF, 100V electrolytic and six 2.2µF, 100V low ESR ceramic capacitors are used to reduce the output ripple to below 100mV peak-to-peak and satisfy the RMS ripple current requirement.

A 4-Phase, 5V Input, 12V/15A Output, Industrial Power Supply

Figure 10 illustrates an industrial power supply that converts a 5V input to a 12V output at up to 15A of load current. The use of four phases greatly eases the task of choosing the power components, and reduces output ripple signiﬁcantly. Figure 11

Figure 12. Load step waveforms for 4-phase, 12V/15A industrial power supply

In high output voltage

systems, switching losses

in the power MOSFETs

can sometimes exceed the

conduction losses. In order

to reduce switching losses

as much as possible, the

LTC3862 incorporates

strong gate drivers. The

PMOS pull-up transistor has

a typical R

DS(ON)

of 2.1Ω,

and the NMOS pull-down

transistor has a typical

of 0.7Ω. In addition to

DS(ON)

reducing switching losses,

these strong gate drivers

allow two power MOSFETs to

be connected in parallel for

high current applications.

shows the start-up waveforms for this converter. Figure 12 shows the load step waveforms.

Multiphase operation is made possible using the PHASEMODE, SYNC and CLKOUT pins. The PHASEMODE pin controls the phase relationship between GATE 1 and GATE 2, as well as between GATE 1 and CLKOUT. The CLKOUT pin of a master controller is connected to the SYNC pin of a slave, where the phase lock loop ensures proper synchronization. The PHASEMODE pin can be used to program 2-, 3-, 4-, 6- and 12-phase operation.

48V/5A Demo Circuit

The DC1286A demonstration circuit board is designed for high power applications, providing a 48V/5A output using the GN24 package option of the LTC3862 or LTC3862-1. The 6-layer PCB design ensures proper routing of the SENSE lines, and exhibits minimal jitter even at 50% duty cycle. Jumpers are provided to easily change the BLANK time, PHASE, maximum duty, and SLOPE compensation. There is an optional onboard 12V VIN supply to power the IC, and the component footprint provides ﬂexibility to use various inductors, MOSFET’s and diodes.

Conclusion

The LTC3862 is a versatile control IC optimized for a wide variety of stepup DC/DC converter applications. Its

Figure 13. 48V/5A output, high power demonstration circuit

ﬂexible, high performance operation and three convenient package options make it possible to optimize efﬁciency, size and weight of the power supply, while keeping the total component and manufacturing costs low.

Linear Technology Magazine • January 2009