Linear LTC3875 Datasheet

Dual, 2-Phase, Synchronous

3875 TA01b

Controller with Low Value DCR Sensing

and Temperature Compensation

FEATURES DESCRIPTION

LTC3875

n

Low Value DCR Current Sensing

n

Programmable DCR Temperature Compensation

n

±0.5% 0.6V Output Voltage Accuracy

n

Dual True Remote Sensing Differential Amplifiers

n

Optional Fast Transient Operation

n

Phase-Lockable Fixed Frequency 250kHz to 720kHz

n

Dual, 180° Phased Controllers Reduce Required

Input Capacitance and Power Supply Induced Noise

n

Dual N-Channel MOSFET Synchronous Drive

n

Wide VIN Range: 4.5V to 38V Operation

n

Output Voltage Range with Low DCR: 0.6V to 3.5V,

without Low DCR: 0.6V to 5V

n

Adjustable Soft-Start Current Ramping or Tracking

n

Foldback Output Current Limiting

n

Clock Input and Output for Up to 12-Phase Operation

n

Short-Circuit Soft Recovery

n

Output Overvoltage Protection

n

Power Good Output Voltage Monitor

n

40-Lead QFN Packages

APPLICATIONS

n

Servers and Instruments

n

Telecom Systems

n

DC Power Distribution Systems

The LT C®3875 is a dual output current mode synchronous
step-down DC/DC controller that drives all N-channel
synchronous power MOSFET stages. It employs a unique
architecture which enhances the signal-to-noise ratio of
the current sense signal, allowing the use of very low DC
resistance power inductors to maximize the efficiency in
high current applications. This feature also reduces the
switching jitter commonly found in low DCR applications.

The LTC3875 features two high speed remote sense differ­ential amplifiers, programmable current sense limits from
10mV to 30mV and DCR temperature compensation to limit
the maximum output current precisely over temperature.

A unique thermal balancing function adjusts per phase cur­rent in order to minimize the

thermal stress for multichip
single output applications. The LTC3875 also features a
precise 0.6V reference with guaranteed accuracy of ±0.5%
that provides an accurate output voltage from 0.6V to 3.5V.
A 4.5V to 38V input voltage range allows it to support a
wide variety of bus voltages. The LTC3875 is available
in a low profile 40-lead 6mm × 6mm (0.5mm pitch) and
40-lead 5mm × 5mm (0.4mm pitch) QFN packages.

L, LT, LT C , LT M, Linear Technology, the Linear logo OPTI-LOOP, Burst Mode and PolyPhase
are registered trademarks and No R
All other trademarks are the property of their respective owners. Protected by U.S. Patents
including 5481178, 5705919, 5929620, 6100678, 6144194, 6177787, 6304066, 6580258.

is a trademark of Linear Technology Corporation.

SENSE

TYPICAL APPLICATION

High Efficiency Dual Phase 1.2V/60A Step-Down Converter

INTV

CC

4.7µF

(OPTIONAL) (OPTIONAL)

THERMAL

SENSOR

0.3µH

(0.32mΩ DCR)

V

OUT

+

470µF

2.5V ×2
SP

INTV

V

RUN1,2
ILIM
ENTMPB
TG1

BOOST1 BOOST2

SW1
EXTV
BG1
TAVG
TRSET1
SNSA1

SNS1
SNSD1

TCOMP1
V

V
I

TH1

IN

OSNS1
OSNS1

LTC3875

CC

+

–

+

+
–

TK/SS2TK/SS1

PHASMD

CC

CLKOUT

PGOOD

IFAST

MODE/PLLIN

TG2

SW2

BG2

PGND
TRSET2
SNSA2

SNS2

SNSD2

TCOMP2

FREQ

V

OSNS2

V

OSNS2

I

0.1µF

+
–
+

+
–

TH2

122k

1500pF

15k

For more information www.linear.com/LTC3875

V
6V TO 14V

22µF
16V ×4

THERMAL

SENSOR

0.3µH

(0.32mΩ DCR)

20k

20k

IN

Efficiency and Power Loss

vs Load Current

50 60

14

12

POWER LOSS (W)

10

8

6

4

2

0

3875fb

100

12V

IN

1.8V

O

95

~400kHz
CCM

90

85

EFFICIENCY (%)

80

0.32mΩ

75

V

OUT

1.2V
60A

+

470µF

2.5V ×2
SP

3875 TA01a

70

0

20 30 40

10

LOAD CURRENT (A)

1.5mΩ

0.32mΩ PLOSS

1.5mΩ PLOSS

1

LTC3875

ABSOLUTE MAXIMUM RATINGS

(Note 1)

Input Supply Voltage (VIN) ......................... 40V to –0.3V

Topside Driver Voltages

(BOOST1, BOOST2).................................... 46V to –0.3V

Switch Voltage (SW1, SW2)
INTV

, RUN(s), PGOOD, EXTVCC

CC

.......................... 40V to –5V

(BOOST-SW1), (BOOST2-SW2).................... 6V to –0.3V

+

SNSA
SNS

(s), SNSD+(s),

–

(s) Voltages .................................. INTVCC to –0.3V

PIN CONFIGURATION

TOP VIEW

TK/SS1

V

OSNS1

V

OSNS1

V

OSNS2

V

OSNS2

TK/SS2

SNSA2

SNS2

SNSA1+SNS1–SNSD1+TCOMP1/ITEMP1

3940 38 37 36 35 34 33 32 31

1

+

2

–

3

I

4

TH1

I

5

TH2

+

6

–

7

8

+

9

–

10

12 13 14 15

11 20

+

SNSD2

SGND/PGND

TAVG

TRSET2

TRSET1

ILIM

RUN1

41

16 17 18 19

FREQ

RUN2

IFAST

MODE/PLLIN

PHASMD

CLKOUT

SW2

PGOOD

ENTMPB

30

29

28

27

26

25

24

23

22

21

SW1

TG1

BOOST1

BG1

V

IN

INTV

CC

EXTV

CC

BG2

BOOST2

TG2

MODE/PLLIN, ILIM, FREQ, IFAST, ENTMPB

+

V

OSNS(s)

I

TH1

, V

OSNS(s)

, I

, PHASMD, TRSET1, TRSET2,

TH2

TCOMP1, TCOMP2, TAVG Voltages
INTV

Peak Output Current ................................100mA

CC

–

Voltages ...............INTVCC to –0.3V

.......INTVCC to –0.3V

Operating Junction Temperature Range
(Notes 2, 3)
Storage Temperature Range

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

.................. –65°C to 125°C

TOP VIEW

TK/SS1

V

OSNS1

V

OSNS1

V

OSNS2

V

OSNS2

TK/SS2

SNSA2

SNS2

SNSA1+SNS1–SNSD1+TCOMP1/ITEMP1

3940 38 37 36 35 34 33 32 31

1

+

2

–

3

I

4

TH1

I

5

TH2

+

6

–

7

8

+

9

–

10

12 13 14 15

11 20

+

SNSD2

SGND/PGND

TAVG

TRSET2

TRSET1

ILIM

RUN1

41

16 17 18 19

FREQ

RUN2

IFAST

MODE/PLLIN

PHASMD

CLKOUT

SW2

PGOOD

ENTMPB

30

29

28

27

26

25

24

23

22

21

SW1

TG1

BOOST1

BG1

V

IN

INTV

CC

EXTV

CC

BG2

BOOST2

TG2

2

TCOMP2/ITEMP2

40-LEAD (6mm × 6mm) PLASTIC QFN

T

EXPOSED PAD (PIN 41) IS SGND/PGND, MUST BE SOLDERED TO PCB

JMAX

UJ PACKAGE

= 125°C, θJA = 33°C/W, θJC = 2.0°C/W

For more information www.linear.com/LTC3875

TCOMP2/ITEMP2

EXPOSED PAD (PIN 41) IS SGND/PGND, MUST BE SOLDERED TO PCB

40-LEAD (5mm × 5mm) PLASTIC QFN

T

JMAX

UH PACKAGE

= 125°C, θJA = 44°C/W, θJC = 7.3°C/W

3875fb

LTC3875

ORDER INFORMATION

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

LTC3875EUH#PBF LTC3875EUH#TRPBF 3875 40-Lead (5mm × 5mm) Plastic QFN –40°C to 125°C
LTC3875IUH#PBF LTC3875IUH#TRPBF 3875 40-Lead (5mm × 5mm) Plastic QFN –40°C to 125°C

LTC3875EUJ#PBF LTC3875EUJ#TRPBF LTC3875

LTC3875IUJ#PBF LTC3875IUJ#TRPBF LTC3875

40-Lead (6mm × 6mm) Plastic QFN
40-Lead (6mm × 6mm) Plastic QFN

Consult LTC Marketing for parts specified with wider operating temperature ranges.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/

For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/. Some packages are available in 500 unit reels through
designated sales channels with #TRMPBF suffix.

The l denotes the specifications which apply over the specified operating

ELECTRICAL CHARACTERISTICS

junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 15V, V

= 5V unless otherwise noted.

RUN1,2

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Main Control Loops

V

IN

V

OUT

V

OSNS1,2

I

OSNS1,2

V

REFLNREG

V

LOADREG

g

m1,2

+

+

Input Voltage Range 4.5 38 V
Output Voltage Range SNSD+ Pin to V

Regulated V

Feedback Voltage

OUT

Including Diffamp Error

+

SNSD

Pin to GND

(Note 4); I
(Note 4); I

OUT

Voltage = 1.2V, –40°C to 85°C

TH1,2

Voltage = 1.2V,–40°C to 125°C

TH1,2

l

Feedback Current (Note 4) –30 –100 nA
Reference Voltage Line Regulation VIN = 4.5V to 38V (Note 4) 0.002 0.005 %/V
Output Voltage Load Regulation (Note 4)

Measured in Servo Loop; ∆I
Measured in Servo Loop; ∆I

Transconductance Amplifier g

I

m

= 1.2V; Sink/Source 5µA (Note 4) 2.2 mmho

TH1,2

Voltage = 1.2V to 0.7V

TH

Voltage = 1.2V to 1.6V

TH

l
l

Thermal Functions

I

TCOMP1,2

T

SHDN

T

HYS

Thermal Sensor Current 29 30 31 µA
Internal Thermal Shutdown (Note 8) 160 °C
Internal TS Hysteresis (Note 8) 10 °C

Fast Transient Functions

I

FAST

Fast Transient Program Current

l

Current Sensing Functions

I

SENSE(AC)

I

SENSE(DC)

A

VT(SNS)

AC Sense Pins Bias Current Each Channel; V
DC Sense Pins Bias Current Each Channel; V
Total Sense Gain to Current Comp 5 V/V

= 3.3V ±0.5 ±2 µA

SNSA+(S)

SNSD+(S)

= 3.3V

l

–40°C to 125°C

to 125°C

–40°C

0.6

0.6

0.597

0.5965

0.600

0.600

0.01

–0.01

3.5
5

0.603

0.6045

0.1

–0.1

9 10 11 µA

±30 ±50 nA

V
V

V
V

%
%

For more information www.linear.com/LTC3875

3875fb

3

LTC3875

ELECTRICAL CHARACTERISTICS

The l denotes the specifications which apply over the specified operating

junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 15V, V

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

V

SENSE(MAX)(DC)

V

SENSE(MAX)(NODE)

I

MISMATCH

Maximum Current Sense Threshold
with SNSD

Maximum Current Sense Threshold
with SNSD

+

Pin to V

+

Pin to GND

OUT

Channel-to-Channel Current
Mismatch

I

Q

Input DC Supply Current
Normal Mode

Shutdown
UVLO Undervoltage Lockout V
UVLO Hyst UVLO Hysteresis 0.5 V
V

OVL

I

TK/SS1,2

V

RUN1,2

V

RUN1,2HYS

Feedback Overvoltage Lockout Measured at V

Soft-Start Charge Current V

RUN Pin On Threshold V

RUN Pin On Hysteresis 80 mV

Driver Functions

TG1,2 t
TG1,2 t

BG1,2 t
BG1,2 t

TG/BG t

r
f

r
f

1D

TG Transition Time

Rise Time

Fall Time

BG Transition Time

Rise Time

Fall Time

Top Gate Off to Bottom Gate On

Delay Synchronous Switch-On

Delay Time
BG/TG t

2D

Bottom Gate Off to Top Gate On

Delay Synchronous Switch-On

Delay Time
t

ON(MIN)

Minimum On-Time (Note 7) 90 ns

0°C to 85°C
V
V

= 1.2V, ILIM = 0V

SNS–(s)

= 1.2V, ILIM = 1/4 INTV

SNS–(s)

V

= 1.2V, ILIM = 1/2 INTV

SNS–(s)

V

= 1.2V, ILIM = 3/4 INTV

SNS–(s)

V

= 1.2V, ILIM = INTV

SNS–(s)

CC
CC
CC

CC

–40°C to 125°C
V
V

V
V
V
V
V

= 1.2V, ILIM = 0V

SNS–(s)

= 1.2V, ILIM = 1/4 INTV

SNS–(s)

V

= 1.2V, ILIM = 1/2 INTV

SNS–(s)

V

= 1.2V, ILIM = 3/4 INTV

SNS–(s)

V

= 1.2V, ILIM = INTV

SNS–(s)

= 1.2V, ILIM = 0V

SNS–(s)

= 1.2V, ILIM = 1/4 INTV

SNS–(s)

= 1.2V, ILIM = 1/2 INTV

SNS–(s)

= 1.2V, ILIM = 3/4 INTV

SNS–(s)

= 1.2V, ILIM = INTV

SNS–(s)

CC
CC
CC

CC

CC
CC
CC

CC

ILIM = Float, ENTMPB = Float
(Thermal Balance Disabled)

(Note 5)
V

= 15V (without EXTVCC Enabled)

IN

V

= 0V

RUN1,2

Ramping Down 3.5 3.7 4.0 V

INTVCC

+

OSNS1,2

= 0V

TK/SS1,2

, V

RUN2

Rising

RUN1

(Note 6)
C

= 3300pF

LOAD

C

= 3300pF

LOAD

(Note 6)
C

= 3300pF

LOAD

C

= 3300pF

LOAD

C

= 3300pF Each Driver 30 ns

LOAD

C

= 3300pF Each Driver 30 ns

LOAD

= 5V unless otherwise noted.

RUN1,2

9
14
19

23.5

28.5

l

8.5

l

13.5

l

17.5

l

22

l

26.5

l

45

l

70

l

95

l

117.5

l

142.5

l

0.625 0.645 0.665 V

l

l

100
125
150

1.0 1.25 1.5 µA

1.1 1.22 1.35 V

10
15
20
25
30

10
15
20
25
30

50
75

7

40

25
25

25
25

11
16
21

26.5

31.5

11.5

16.5

22.5
28

33.5
55

80

105

132.5

157.5
5 %

10
60

mV
mV
mV
mV
mV

mV
mV
mV
mV
mV

mV
mV
mV
mV
mV

mA

µA

ns
ns

ns
ns

4

3875fb

For more information www.linear.com/LTC3875

LTC3875

ELECTRICAL CHARACTERISTICS

The l denotes the specifications which apply over the specified operating
junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 15V, V

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

Linear Regulator

INTV

CC

V

INTVCC

INT INTVCC Load Regulation ICC = 0mA to 20mA 0.5 2.0 %

V

LDO

V

EXTVCC

EXT EXTVCC Voltage Drop ICC = 20mA, V

V

LDO

V

LDOHYS

Oscillator and Phase-Locked Loop

f

NOM

f

LOW

f

HIGH

R

MODE/PLLIN

I

FREQ

CLKOUT Phase (Relative to Controller 1) PHASMD = GND

CLK High Clock Output High Voltage V
CLK Low Clock Output Low Voltage 0.2 V
V

PGL

I

PGOOD

V

PG

On-Chip Driver

TG R

UP

TG R

DOWN

BG R

UP

BG R

DOWN

Internal VCC Voltage 6V < VIN < 38V 5.3 5.5 5.7 V

EXTVCC Switchover Voltage EXTVCC Ramping Positive

= 5.5V 50 100 mV

EXTVCC

EXTVCC Hysteresis 200 mV

Nominal Frequency V
Lowest Frequency V
Highest Frequency V

= 1.2V 450 500 550 kHz

FREQ

= 0V 220 250 270 kHz

FREQ

≥ 2.4V 650 720 790 kHz

FREQ

MODE/PLLIN Input Resistance 250 kΩ
Frequency Setting Current 9.5 10 10.5 µA

PHASMD = FLOAT

PGOOD Voltage Low I
PGOOD Leakage Current V
PGOOD Trip Level, Either Controller V

TG Pull-Up R
TG Pull-Down R
BG Pull-Up R
BG Pull-Down R

DS(ON)

DS(ON)

DS(ON)

DS(ON)

PHASMD = INTV

= 5.5V 4.5 5.5 V

INTVCC

= 2mA 0.1 0.3 V

PGOOD

= 5.5V ±2 µA

PGOOD

+

with Respect to Set Output Voltage

OSNS

V

OSNS

V

OSNS

TG High 2.6 Ω
TG Low 1.5 Ω
BG High 2.4 Ω
BG Low 1.1 Ω

CC

+

Ramping Negative

+

Ramping Positive

= 5V unless otherwise noted.

RUN1,2

l

4.5 4.7 V

60
90

120

–7.5

7.5

Deg
Deg
Deg

%
%

Stresses beyond those listed under Absolute Maximum Ratings

Note 1:

may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.

Note 2: The LTC3875 is tested under pulsed load conditions such that
T

≈ TA. The LTC3875E is guaranteed to meet specifications from

J

0°C to 85°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
LTC3875I is guaranteed over the full –40° to 125° operating junction
temperature range. Note that the maximum ambient temperature
consistent with these specifications is determined by specific operating
conditions in conjunction with board layout, the rated package thermal
impedance and other environmental factors.

For more information www.linear.com/LTC3875

Note 3: T
dissipation, P

is calculated from the ambient temperature, TA, and power

J

, according to the formula:

D

TJ = TA + (PD • θJA°C/W)
where θ

= 44°C/W for the 5mm × 5mm QFN and θJA = 33°C/W for

JA

the6mm×6mmQFN.
Note 4: The LTC3875 is tested in a feedback loop that servos V

specified voltage and measures the resultant V

OSNS1,2

+

.

ITH1,2

to a

Note 5: Dynamic supply current is higher due to the gate charge being
delivered

at the switching frequency. See Applications Information.

Note 6: Rise and fall times are measured using 10% and 90% levels. Delay
times are measured using 50% levels.

Note 7: The minimum on-time condition is specified for an inductor
peak-to-peak ripple current ≥40% of I

(see Minimum On-Time

MAX

Considerations in the Applications Information section).
Note 8: Guaranteed by design.

3875fb

5

LTC3875

3875 G01

3875 G02

TYPICAL PERFORMANCE CHARACTERISTICS

Efficiency vs Output Current and
Mode (Figure 16 Application Circuit)

100

Burst Mode

90

OPERATION

80

70

60

50

40

EFFICIENCY (%)

30

20

10

0

0.01 1 10 100

0.1
LOAD CURRENT (A)

CCM

PULSE-SKIPPING

VIN = 12V
V

OUT

= 1.5V

Efficiency vs Output Current and
Mode (Figure 16 Application Circuit)

100

90

80

Burst Mode

70

OPERATION

60

50

40

EFFICIENCY (%)

30

20

10

0

0.01 1 10 100

0.1

CCM

PULSE­SKIPPING

LOAD CURRENT (A)

VIN = 12V

= 1V

V

OUT

I

LOAD

40A/DIV

5A TO 30A

I

L1, IL2

10A/DIV

V

OUT

100mV/DIV

AC-COUPLED

Load Step
(Figure 16 Application Circuit)
(Burst Mode Operation)

VIN = 12V

= 1.5V

V

OUT

10µs/DIV

3875 G03

Load Step
(Figure 16 Application Circuit)
(Forced Continuous Mode)

I

LOAD

40A/DIV

5A TO 30A

I

L1, IL2

10A/DIV

V

OUT

100mV/DIV

AC-COUPLED

VIN = 12V
V

OUT

Coincident Tracking

RUN

2V/DIV

V

OUT1

V

OUT2

1V/DIV

VIN = 12V

= 1.5V, R

V

OUT1

= 1V, R

V

OUT2

= 1.5V

LOAD

2.5ms/DIV
= 12Ω, CCM

LOAD

= 6Ω, CCM

10µs/DIV

V

V

OUT1

OUT2

3875 G07

3875 G04

40A/DIV

5A TO 30A

I

10A/DIV

100mV/DIV

AC-COUPLED

TK/SS1
TK/SS2

2V/DIV

V

OUT1

V

OUT2

500mV/DIV

Load Step
(Figure 16 Application Circuit)
(Pulse-Skipping Mode)

I

LOAD

L1, IL2

V

OUT

VIN = 12V

= 1.5V

V

OUT

10µs/DIV

Tracking Up and Down
with External Ramp

V

OUT2

V

OUT1

VIN = 12V

= 1V, 1Ω LOAD

V

OUT1

= 1.5V, 1.5Ω LOAD

V

OUT2

10ms/DIV

3875 G08

500mV/DIV

500mV/DIV

3875 G05

SUPPLY CURRENT (mA)

Prebiased Output at 1V

V

OUT

1V/DIV

+

V

OSNS

TK/SS

VIN = 12V

= 1.5V

V

OUT

CCM: NO LOAD

2.5ms/DIV

Quiescent Current
vs Temperature without EXTV

10.0

9.5

9.0

8.5

8.0

7.5

7.0

6.5

6.0

5.5

5.0
–30 10

–50

–10

TEMPERATURE (°C)

50 130

30

3875 G06

CC

90

110

70

3875 G09

6

3875fb

For more information www.linear.com/LTC3875

TYPICAL PERFORMANCE CHARACTERISTICS

LTC3875

Line Regulation

INTV

CC

6

5

4

3

VOLTAGE (V)

CC

2

INTV

1

0

0

10 20

5 15

INPUT VOLTAGE (V)

30

25

Maximum Current Sense
Threshold vs Feedback Voltage
(Current Foldback)

35

30

25

20

15

10

5

MAXIMUM CURRENT SENSE THRESHOLD (mV)

0

0

0.1 0.2
FEEDBACK VOLTAGE (V)

ILIM = INTV

ILIM = 3/4 INTV

ILIM = 1/2 INTV

ILIM = 1/4 INTV

ILIM = GND

0.4 0.6

0.3 0.5

35

3875 G10

CC

CC

CC

CC

3875 G13

Current Sense Threshold
vs ITH Voltage

40

35

30

25

20

15

10

5

0

CURRENT SENSE THRESHOLD (mV)

–5

40

–10

0

TK/SS Pull-Up Current
vs Temperature

1.40

1.35

1.30

1.25

1.20

1.15

TK/SS CURRENT (µA)

1.10

1.05

1.00
–30 130

–50

ILIM = 0
ILIM = 1/4 INTV
ILIM = 1/2 INTV
ILIM = 3/4 INTV
ILIM = INTV

–10

CC

0.5
ITH VOLTAGE (V)

30

10

TEMPERATURE (°C)

Maximum Current Sense Threshold
vs Common Mode Voltage

35

CC
CC
CC

1

1.5

2

3875 G11

30

25

20

15

10

5

CURRENT SENSE THRESHOLD (mV)

0

0

V

ILIM = INTV

ILIM = 3/4 INTV

ILIM = 1/2 INTV

ILIM = 1/4 INTV

1 2 4

COMMON MODE VOLTAGE (V)

SENSE

CC

CC

CC

CC

ILIM = GND

3

3875 G12

Shutdown (RUN) Threshold
vs Temperature

1.30

1.25

1.20

1.15

1.10

RUN PIN THRESHLD (V)

1.05

50

110

70

90

3875 G14

1.00
–50

ON

OFF

0 50

TEMPERATURE (°C)

100 150

4320 G01

Regulated Feedback Voltage
vs Temperature

0.6045

0.6035

0.6025

0.6015

0.6005

0.5995

0.5985

FEEDBACK VOLTAGE (V)

0.5975

0.5965

0.5955
–50 –30 –10

10 30 50 130

TEMPERATURE (°C)

FREQUENCY (kHz)

70 90 110

3875 G16

For more information www.linear.com/LTC3875

Oscillator Frequency
vs Temperature

900

800

700

600

500

400

300

200

100

0

–50

V

0

TEMPERATURE (°C)

FREQ

V

V

FREQ

FREQ

= INTV

= 1.22V

= GND

50

CC

100

150

3875 G17

3875fb

7

LTC3875

3875 G22

TYPICAL PERFORMANCE CHARACTERISTICS

Undervoltage Lockout Threshold
(INTVCC) vs Temperature

5.0

4.8

4.6

4.4

4.2

4.0

3.8

3.6

UVLO THRESHOLD (V)

3.4

3.2

3.0
–50

RISING

FALLING

0

50

TEMPERATURE (°C)

100

3875 G18

150

Oscillator Frequency
vs Input Voltage

900

V

800

700

600

500

400

300

200

OSCILLATOR FREQUENCY (kHz)

100

0

0

10

= INTV

FREQ

V

FREQ

V

FREQ

INPUT VOLTAGE (V)

= 1.22V

= GND

20

CC

30

40

3875 G19

Shutdown Current
vs Input Voltage

50

45

40

35

30

25

20

15

SHUTDOWN CURRENT (µA)

10

5

0

0

5 15

10

INPUT VOLTAGE (V)

20

25

Quiescent Current vs Input

Shutdown Current vs Temperature Very Low Output Voltage Ripple

50

45

40

35

30

25

20

SHUTDOWN CURRENT (µA)

15

10

–30 130

–50

–10

30

50

10

TEMPERATURE (°C)

70

110

90

3875 G21

Voltage without EXTV

8

7

6

5

4

3

2

QUIESCENT CURRENT (mA)

1

0

5

0

10

15

INPUT VOLTAGE (V)

CC

V

OUT

TYPICAL

FRONT PAGE

10mV/DIV

AC-COUPLED

V

OUT

LOW RIPPLE

FIGURE 20

10mV/DIV

AC-COUPLED

VIN = 12V

= 2.5V

V

20

25

30

40

35

OUT

2µs/DIV

35

30

40

3875 G20

3875 G23

8

3875fb

For more information www.linear.com/LTC3875

PIN FUNCTIONS

LTC3875

TK/SS1, TK/SS2 (Pin 1, Pin 8): Output Voltage Tracking
and Soft-Start Inputs. When one channel is configured to
be the master, a capacitor to ground at this pin sets the
ramp rate for the master channel’s output voltage. When
the channel is configured to be the slave, the feedback
voltage of the master channel is reproduced by a resistor
divider and applied to

this pin. Internal soft-start currents

of 1.25µA charge these pins.

V

OSNS1

+

, V

OSNS2

+

(Pin 2, Pin 6): Positive Inputs of Remote

Sensing Differential Amplifiers. These pins receive the
remotely sensed feedback voltage from external resistive
divider across the output. The differential amplifier out­puts are connected directly to the error amplifiers’ inputs
internally inside the IC.

V

OSNS1

–

, V

OSNS2

–

(Pin 3, Pin 7): Negative Inputs of Re­mote Sensing Differential Amplifiers. Connect these pins
to the negative terminal of the output capacitors when
remote sensing is desired. Connect these pins to local
signal ground if remote sensing is not used.

, I

I

TH1

(Pin 4, Pin 5): Current Control Threshold and

TH2

Error Amplifier Compensation Points. The current com­parators’ tripping thresholds increase with these control
voltages.

TAVG (Pin 13): Average Temperature Summing Point

. Con­nect a resistor to ground to sum all currents together for
multi-channels or multi-IC operations when temperature
balancing function is enabled. The value of the resistor
should be the TRSET resistor value divided by the number
of channels in the system. Float this pin if thermal balanc­ing is not used.

FREQ (Pin 15): There is a precision 10µA current flowing
out of this

pin. A resistor to ground sets a voltage which

in turn programs the frequency. Alternatively, this pin can

be driven with a DC voltage to vary the frequency of the

internal oscillator.

IFAST (Pin 17): Programmable Pin for Fast Transient Op­eration for Channel 2 Only. A resistor to ground programs
the threshold of the output load transient excursion. Float
this pin to disable this function. See the Applications

Information section for more details.

ENTMPB (Pin 18): Enable Pin for Temperature Balanc­ing Function. Ground this pin to enable the temperature
balancing function. Float this pin for normal operation.

PGOOD (Pin 19): Power Good Indicator Output. Open-drain
logic that is pulled to ground when either channel’s output
exceeds ±7.5% regulation window, after the internal 20µs
power bad mask timer expires.

EXTV

Connected to INTV

(Pin 24): External Power Input to an Internal Switch

CC

. This switch closes and supplies the

CC

IC power, bypassing the internal low dropout regulator,
whenever EXTV
on this pin and make sure that EXTV

INTV

(Pin 25): Internal 5.5V Regulator Output. The con-

CC

is higher than 4.7V. Do not exceed 6V

CC

< VIN at all times.

CC

trol circuits are powered from this voltage. Decouple this
pin to PGND with

a minimum of 4.7µF low ESR tantalum

or ceramic capacitor.

(Pin 26): Main Input Supply. Decouple this pin to

V

IN

PGND with a capacitor (0.1µF to 1µF).

BG1, BG2 (Pin 27, Pin 23): Bottom Gate Driver Outputs.
These pins drive the gates of the bottom N-channel
MOSFETs between INTV

and PGND.

CC

BOOST1, BOOST2 (Pin 28, Pin 22): Boosted Floating
Driver Supplies. The (+) terminal of the booststrap capaci-

connect to these pins. These pins swing from a diode

tors
voltage drop below INTV

up to VIN + INTVCC.

CC

TG1, TG2 (Pin 29, Pin 21): Top Gate Driver Outputs. These
are the outputs of floating drivers with a voltage swing
equal to INTV

superimposed on the switch node voltage.

CC

SW1, SW2 (Pin 30, Pin 20): Switch Node Connections to
Inductors. Voltage swing at these pins is from a Schottky
diode (external) voltage drop below ground to V

CLKOUT (Pin 31): Clock Output Pin. Clock output with
phase changeable by PHASMD to enable usage of multiple
LTC3875s in multiphase systems signal swing is from
INTV

to ground.

CC

IN

.

For more information www.linear.com/LTC3875

3875fb

9

LTC3875

PIN FUNCTIONS

PHASMD (Pin 32): Phase Programmable Pin. This pin can
be tied to SGND, INTV

or left floating. It determines the

CC

relative phases between the internal controllers as well
as the phasing of the CLKOUT signal. See Table 1 in the
Operation section for details.

MODE/PLLIN (Pin 33): Forced Continuous Mode, Burst
Mode or Pulse-Skipping Mode Selection Pin and External
Synchronization Input to Phase Detector Pin.

Connect
this pin to SGND to force the IC into continuous mode of
operation. Connect to INTV

to enable pulse-skipping

CC

mode of operation. Leave the pin floating to enable Burst

Mode operation. A clock on the pin will force the IC into

continuous mode of operation and synchronize the internal
oscillator with the clock on this pin. The PLL compensation

network is integrated into the IC

.

RUN1, RUN2 (Pin 34, Pin 16): Run Control Inputs. A volt-

age above 1.22V on either pin turns on the IC. However,

forcing both pins below 1.14V causes the IC to shut down.
There is a 1.0µA pull-up current for both pins. Once the
RUN pin rises above 1.22V, an additional 4.5µA pull-up
current is added to the pin.

ILIM (Pin 35): Current Comparators’ Sense Voltage Range

Input. A resistor divider sets the maximum current sense
threshold to five different levels for the current comparators.

TRSET1, TRSET2 (Pin 36, Pin 14): Input of the Tempera­ture Balancing Circuitries. Connect these pins through
resistors to ground to convert the TCOMP pin voltages
to currents. These currents are then mirrored to pin TAVG
and are added together for all channels. Float this pin if

thermal balancing is

not used.

TCOMP1/ITEMP1, TCOMP2/ITEMP2 (Pin 37, Pin 12):
Input of the Temperature Balancing Circuitries. Connect
these pins to external NTC resistors or temperature sensing
ICs placed near inductors. These pins are used to sense
temperature of each channel and balance the temperature
of the whole system accordingly. When thermal balancing
function is disabled, these pins can be programmed to
compensate the temperature coefficient of the DCR. Con
nect to an NTC (negative tempco) resistor placed near the
output inductor to compensate for its DCR change over
temperature. Floating this pin

disables the DCR temperature

compensation function.

+

SNSD1

, SNSD2+ (Pin 38, Pin 11): DC Current Sense Com-

parator Inputs. The (+) input to the DC current comparator
is normally connected to a DC current sensing network.
Ground these pins to disable the novel DCR sensing
enable normal DCR sensing with five times current limit.

–

, SNS2– (Pin 39, Pin 10): AC and DC Current

SNS1

Sense Comparator Inputs. The (–) inputs to the current
comparators are connected to the output.

+

SNSA1

Comparator Inputs. The (+) input to the AC current com-

, SNSA2+ (Pin 40, Pin 9): AC Current Sense

parator is normally connected to a DCR sensing network.

+

When combined with the SNSD

pin, the DCR sensing
network can be skewed to increase the AC ripple voltage
by a factor of 5.

SGND/PGND (Exposed Pad Pin 41): Signal/Power Ground
Pin. Connect this pin closely to the sources of the bot­tom N-channel MOSFETs, the (–) terminal of C
the (–) terminal of C

. All small-signal components and

IN

compensation components should connect to this ground.

VCC

-

and

and

10

3875fb

For more information www.linear.com/LTC3875

LTC3875

BLOCK DIAGRAM

FREQ

CLKOUT

IFAST

(CHANNEL 2

ONLY)

ILIM

V

IN

0.6V
REF

0.55V

PLL-SYNC

1

50k

I

THB

SLEEP

–

I

MODE/PLLIN

OSC

+

CMP

–

INTV

+

MODE/SYNC

DETECT

5k

CC

PHASMD

–

+

SLOPE

COMPENSATION

ACTIVE CLAMP

EA

++

(Functional diagram shows one channel only)

EXTV

+

1.22V

CC

4.7V

+

–

F

BURST EN

FCNT

ON

SWITCH

LOGIC

AND

ANTISHOOT-

THROUGH

RUN

OV

RUN

–

+

1.25µA

I

REV

UVLO

–

S
R

TCOMP/ITEMP

TEMPSNS

Q

0.5V

0.6V

–

F

SS

–

+

5.5V
REG

V

IN

+

INTV

CC

BOOST

TG

SW

+

SNSA

–

SNS

BG

PGND

PGOOD

+

0.555V

UV

C

C

B

D

B

C

VCC

–

V

IN

IN

M1

V

OUT

M2

+

+

SNSD

R2

C

OUT

R1

SGND

+

AMP

+

OV

0.66V

–

DIFFAMP

–

SNS

–

+

20k

20k

+

+

V

OSNS

–

–

–

V

OSNS

20k

V

FB

1µA/5.5µA

20k

TCOMP/ITEMP

ENTMPB

30µA

C

ITH

C1

R

C

MIRROR

RUN TK/SS

C

SS

+

AMP

–

TRSET

R

TCOMP

For more information www.linear.com/LTC3875

*n EQUALS THE NUMBER
OF CHANNELS IN PARALLEL

TAVG

+

R

TCOMP

n*

–

REPEAT FOR
MULTICHIP OPERATIONS

g

m

3875 BD

3875fb

11

LTC3875

OPERATION

Main Control Loop

The LTC3875 is a constant frequency, current mode step-

down controller with two channels operating 180° or 240°

out of phase. During normal operation, each top MOSFET

is turned on when the clock for that channel sets the R

latch, and turned off when the main current comparator,

, resets the RS latch. The peak inductor current at

I

CMP

which I

on the I

resets the RS latch is controlled by the voltage

CMP

pin, which is the output of each error amplifier

TH

EA. The remote sense amplifier (DIFFAMP) converts the
sensed differential voltage across the output feedback
resistor divider to an internal voltage (V
SGND. The V

signal is then compared to the internal

FB

) referred to

FB

0.6V reference voltage by the EA. When the load current

increases, it causes

the 0.6V reference, which in turn causes the I

a slight decrease in VFB relative to

voltage

TH

to increase until the average inductor current matches the
new load current. After the top MOSFET has turned off,

the bottom MOSFET is turned on until either the inductor

current starts to reverse, as indicated by the reverse cur-

rent comparator, I

INTV

/EXTVCC Power

CC

, or the beginning of the next cycle.

REV

Power for the top and bottom MOSFET drivers and most

other internal circuitry is derived from the INTV

When the EXTV

pin is left open or tied to a voltage less

CC

than 4.5V, an internal 5.5V linear regulator supplies INTV

power from V

. If EXTVCC is taken above 4.7V, the 5.5V

IN

regulator is turned off and an internal switch is

connecting EXTV
voltage has to be higher than EXTV
has to come before EXTV
current will flow back to V

to INTVCC. When using EXTVCC, the VIN

CC

voltage at all time and

CC

is applied. Otherwise, EXTVCC

CC

through the internal switch’s

IN

pin.

CC

CC

turned on

body diode and potentially damage the device. Using the
EXTV

pin allows the INTVCC power to be derived from

CC

a high efficiency external source.

Each top MOSFET driver is biased from the floating
bootstrap capacitor, C

, which normally recharges dur-

B

ing each off cycle through an external diode when the top

MOSFET turns off. If the input voltage, V
a voltage close to V

, the loop may enter dropout and

OUT

, decreases to

IN

attempt to turn on the top MOSFET continuously. The
dropout detector detects

this and forces the top MOSFET

off for about one-twelfth of the clock period plus 100ns

every third cycle to allow C

to recharge. However, it is

B

recommended that a load be present or the IC operates
at low frequency during the drop-out transition to ensure
that C

S

Shutdown and Start-Up

is recharged.

B

(RUN1, RUN2 and TK/SS1, TK/SS2 Pins)

two channels of the LTC3875 can be independently

The
shut down using the RUN1 and RUN2 pins. Pulling either
of these pins below 1.14V shuts down the main control
loop for that channel. Pulling both pins low disables both
channels and most internal circuits, including the INTV
regulator. Releasing either RUN pin allows an internal
1µA current to pull up the pin and enable the controller.
Alternatively

, the RUN pins may be externally pulled up
or driven directly by logic. Be careful not to exceed the
absolute maximum rating of 6V on these pins.

The start-up of each channel’s output voltage, V
controlled by the voltage on its TK/SS pin. When the
voltage on the TK/SS pin is less than the 0.6V internal
reference, the LTC3875 regulates the

VFB voltage to the
TK/SS pin voltage instead of the 0.6V reference. This al­lows the TK/SS pin to be used to program the soft-start
period by connecting an external capacitor from the TK/SS
pin to SGND. An internal 1.25µA pull-up current charges
this capacitor, creating a voltage ramp on the TK/SS pin.
As the TK/SS voltage rises linearly
beyond), the output voltage V

from 0V to 0.6V (and

rises smoothly from zero

OUT

to its final value. Alternatively the TK/SS pin can be used
to cause the start-up of V

to “track” that of another

OUT

supply. Typically, this requires connecting to the TK/SS
pin an external resistor divider from the other supply to
ground (see the Applications Information section). When
the corresponding
controller, or when INTV

RUN pin is pulled low to disable a

drops below its undervoltage

CC

lockout threshold of 3.7V, the TK/SS pin is pulled low
by an internal MOSFET. When in undervoltage lockout,
both controllers are disabled and the external MOSFETs
are held off.

Internal Soft-Start

By default, the start-up of the output voltage is normally
controlled by an internal soft-start ramp.

The internal

soft-start ramp represents one of the noninverting inputs

OUT

, is

3875fb

CC

12

For more information www.linear.com/LTC3875

OPERATION

LTC3875

to the error amplifier. The VFB signal is regulated to the
lower of the error amplifier’s three noninverting inputs
(the internal soft-start ramp, the TK/SS pin or the internal

600mV reference). As the ramp voltage rises from 0V to

0.6V, over approximately 600µs, the output voltage rises
smoothly from its pre-biased value to its final set value.
Certain applications can require

the start-up of the con­verter into a non-zero load voltage, where residual charge
is stored on the output capacitor at the onset of converter
switching. In order to prevent the output from discharging
under these conditions, the top and bottom MOSFETs are
disabled until soft-start is greater than V

FB

.

Light Load Current Operation (Burst Mode Operation,
Pulse-Skipping, or Continuous Conduction)

The LTC

3875 can be enabled to enter high efficiency Burst
Mode operation, constant frequency pulse-skipping mode,
or forced continuous conduction mode. To select forced
continuous operation, tie the MODE/PLLIN pin to a DC
voltage below 0.6V (e.g., SGND). To select pulse-skipping
mode of operation, tie the MODE/PLLIN pin to INTV

CC

. To

select Burst Mode operation, float the MODE/PLLIN pin.

When a controller

is enabled for Burst Mode operation,
the peak current in the inductor is set to approximately
one-third of the maximum sense voltage even though the
voltage on the I

pin indicates a lower value. If the aver-

TH

age inductor current is higher than the load current, the
error amplifier, EA, will decrease the voltage on the I
pin. When the I

voltage drops below 0.5V, the internal

TH

TH

sleep signal goes high (enabling sleep mode) and both
external MOSFETs are turned off.

In forced continuous operation, the inductor current is
allowed to

reverse at light loads or under large transient
conditions. The peak inductor current is determined by
the voltage on the I

pin. In this mode, the efficiency at

TH

light loads is lower than in Burst Mode operation. However,
continuous mode has the advantages of lower output ripple
and less interference with audio circuitry.

When the MODE/PLLIN pin is connected to INTV
LTC3875 operates in

PWM pulse-skipping mode at light
loads. At very light loads, the current comparator, I
may remain tripped for several cycles and force the external
top MOSFET to stay off for the same number of cycles (i.e.,
skipping pulses). The inductor current is not allowed to
reverse (discontinuous operation). This mode, like forced
continuous operation, exhibits low output ripple as well as
low audio noise

and reduced RF interference as compared
to Burst Mode operation. It provides higher low current
efficiency than forced continuous mode, but not nearly as
high as Burst Mode operation.

Multichip Operations (PHASMD and CLKOUT Pins)

The PHASMD pin determines the relative phases between
the internal channels as well as the CLKOUT signal as shown
in Table 1. The phases tabulated are relative to zero phase
being defined as

Table 1

PHASMD GND FLOAT INTV

Phase 1 0° 0° 0°

Phase 2 180° 180° 240°

CLKOUT 60° 90° 120°

the rising edge of the clock of phase 1.

CC

CC

, the

CMP

,

In sleep mode, the load current is supplied by the output
capacitor. As the output voltage decreases, the EA’s output

begins to rise. When the output voltage drops enough, the
sleep signal goes low, and the controller resumes normal
operation by turning on the top external MOSFET on the

next cycle of the internal oscillator. When a controller is
enabled for Burst Mode operation, the inductor current is

not allowed to reverse. The reverse current comparator

) turns off the bottom external MOSFET just before

(I

REV

the inductor current reaches zero, preventing it from re­versing and going negative. Thus, the controller operates

in discontinuous operation.

For more information www.linear.com/LTC3875

The CLKOUT signal can be used to synchronize additional
power stages in a multiphase power supply solution feeding
a single, high current output or separate outputs. Input
capacitance ESR requirements and efficiency losses are
substantially reduced because the peak current drawn from
the input capacitor is effectively divided by the number of
phases used and power loss

is proportional to the RMS
current squared. A 2-stage, single output voltage imple­mentation can reduce input path power loss by 75% and
radically reduce the required RMS current rating of the
input capacitor(s).

3875fb

13

LTC3875

OPERATION

Single Output Multiphase Operation

The LTC3875 can be used for single output multiphase

converters by making these connections

• Tie all of the ITH pins together;

• Tie all of the V

+

pins together;

OSNS

• Tie all of the TK/SS pins together;

• Tie all of the RUN pins together.

Examples of single output multiphase converters are shown
in the Typical Applications section.

Sensing the Output Voltage

The LTC3875 includes two low offset, high input imped­ance, unity gain, high bandwidth differential amplifier for
applications that require true remote sensing. Differentially
sensing the load greatly improves regulation in high cur­rent, low voltage applications, where board interconnec­tion losses can

be a significant portion of the total error

budget. The LTC3875 differential amplifier’s positive

+

terminal V

sistor divider and its negative terminal V

senses the divided output through a re-

OSNS

OSNS

–

senses the

remote ground of the load. The differential amplifier output

is connected to the negative terminal of the internal error
amplifier inside the controller. Therefore, its differential
output signal (V

) is not accessible from outside the IC. In

FB

a typical application where differential sensing is desired,

+

connect the V
divider across the output load, and the V

pin to the center tap of the feedback

OSNS

OSNS

–

pin to the

load ground. When differential sensing is not used, the

–

V

The LTC3875 differential amplifier has a typical output

pin can be connected to local ground. See Figure 1.

OSNS

slew
rate of 2V/µs. The amplifier is configured for unity gain,
meaning that the difference between V

OSNS

+

and V

OSNS

–

is

translated to its output, relative to SGND. Care should be
taken to route the V

OSNS

+

and V

–

PCB traces parallel

OSNS

to each other all the way to the remote sensing points on
the board. In addition, avoid routing these sensitive traces

any high speed switching nodes in the circuit. Ideally,

near
the V

OSNS

+

and V

–

traces should be shielded by a

OSNS

low impedance ground plane to maintain signal integrity.

Current Sensing with Very Low Inductor DCR

For low output voltage, high current applications, it’s
common to use low winding resistance (DCR) inductors
to minimize the winding conduction loss and maximize the
supply efficiency. Inductor DCR current sensing

is also used
to eliminate the current sensing resistor and its conduction
loss. Unfortunately, with a very low inductor DCR value,
1mΩ or less, the AC current sensing signal ripple can be
less than 10mV

. This makes the current loop sensitive

P-P

to PCB switching noise and causes switching jitter.

The LTC3875 employs a unique and proprietary current
sensing architecture to enhance its signal-to-

noise ratio
in these situations. This enables it to operate with a small
sense signal of a very low value inductor DCR, 1mΩ or
less. The result is improved power efficiency, and reduced
jitter due to switching noise which could corrupt the signal.
The LTC3875 can sense a DCR value as low as 0.2mΩ with
careful PCB layout. The LTC3875 uses two positive sense

+

pins, SNSD

and SNSA+ to acquire signals. It processes

them internally to provide the response as with a DCR sense
signal that has a 14dB (5×) signal-to-noise ratio improve­ment without affecting output voltage feedback loop. In
the meantime, the current limit threshold is still a function
of the inductor peak current times its DCR value and its
accuracy is also improved five times and can

be accurately

set from 10mV to 30mV in a 5mV steps with the ILIM pin

14

V

OUT

C

OUT2

10Ω

C

OUT1

10Ω

FEEDBACK DIVIDER

Figure 1. Differential Amplifier Connection

C

R

R

For more information www.linear.com/LTC3875

FF

D1

D2

V

V

OSNS

OSNS

+

–

+

DIFFAMP

–

0.6V

INTSS

TK/SS

LTC3875

–
+

+

EA

+

I

TH

3875 F01

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