FEATURES
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LTC3875
Dual, 2-Phase, Synchronous
Controller with Low Value DCR Sensing
and Temperature Compensation
DESCRIPTION
The
LTC
®
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 differ-
ential 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 cur-
rent 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, LTC, LTM, Linear Technology, the Linear logo OPTI-LOOP, Burst Mode and PolyPhase
are registered trademarks and No R
SENSE
is a trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners. Protected by U.S. Patents
including 5481178, 5705919, 5929620, 6100678, 6144194, 6177787, 6304066, 6580258.
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Low Value DCR Current Sensing
Programmable DCR Temperature Compensation
±0.5% 0.6V Output Voltage Accuracy
Dual True Remote Sensing Differential Amplifiers
Optional Fast Transient Operation
Phase-Lockable Fixed Frequency 250kHz to 720kHz
Dual, 180° Phased Controllers Reduce Required
Input Capacitance and Power Supply Induced Noise
Dual N-Channel MOSFET Synchronous Drive
Wide V
IN
Range: 4.5V to 38V Operation
Output Voltage Range with Low DCR: 0.6V to 3.5V,
without Low DCR: 0.6V to 5V
Adjustable Soft-Start Current Ramping or Tracking
Foldback Output Current Limiting
Clock Input and Output for Up to 12-Phase Operation
Short-Circuit Soft Recovery
Output Overvoltage Protection
Power Good Output Voltage Monitor
40-Lead QFN Packages
APPLICATIONS
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Servers and Instruments
Telecom Systems
DC Power Distribution Systems
TYPICAL APPLICATION
High Efficiency Dual Phase 1.2V/60A Step-Down Converter
INTV
CC
4.7µF
(OPTIONAL)
THERMAL
SENSOR
0.3µH
(0.32m DCR)
V
IN
INTV
CC
PHASMD
CLKOUT
PGOOD
RUN1,2
IFAST
ILIM
ENTMPB MODE/PLLIN
TG2
TG1
BOOST1
BOOST2
SW1 LTC3875 SW2
EXTV
CC
BG2
BG1
TAVG
PGND
TRSET1
TRSET2
SNSA1
+
SNSA2
+
SNS1
–
SNS2
–
SNSD1
+
SNSD2
+
TCOMP1
TCOMP2
V
OSNS1+
FREQ
–
V
OSNS1
V
OSNS2+
–
I
TH1
V
0.1µF
122k
V
IN
22µF 6V TO 14V
16V
×4
(OPTIONAL)
THERMAL
SENSOR
0.3µH
(0.32m DCR)
EFFICIENCY (%)
100
95
90
85
6
80
75
20k
1500pF
15k
20k
V
OUT
1.2V
60A
70
4
0.32m
1.5m
2
0.32m PLOSS
1.5m PLOSS
0
20
30
40
50
60
LOAD CURRENT (A)
3875 TA01b
Efficiency and Power Loss
vs Load Current
12V
IN
1.8V
O
~400kHz
CCM
14
12
10
8
POWER LOSS (W)
V
OUT
+
470µF
2.5V
×2
SP
OSNS2
TK/SS1 TK/SS2 I
TH2
0
10
+
470µF
2.5V
×2
SP
3875 TA01a
3875fb
For more information
www.linear.com/LTC3875
1
LTC3875
ABSOLUTE MAXIMUM RATINGS
(Note 1)
Input Supply Voltage (V
IN
) ......................... 40V to –0.3V
Topside Driver Voltages
(BOOST1, BOOST2).................................... 46V to –0.3V
Switch Voltage (SW1, SW2).......................... 40V to –5V
INTV
CC
, RUN(s), PGOOD, EXTV
CC
(BOOST-SW1), (BOOST2-SW2).................... 6V to –0.3V
SNSA
+
(s), SNSD
+
(s),
SNS
–
(s) Voltages .................................. INTV
CC
to –0.3V
MODE/PLLIN, ILIM, FREQ, IFAST, ENTMPB
V
OSNS(s)+
, V
OSNS(s)–
Voltages ............... INTV
CC
to –0.3V
I
TH1
, I
TH2
, PHASMD, TRSET1, TRSET2,
TCOMP1, TCOMP2, TAVG Voltages ....... INTV
CC
to –0.3V
INTV
CC
Peak Output Current ................................100mA
Operating Junction Temperature Range
(Notes 2, 3) ............................................ –40°C to 125°C
Storage Temperature Range .................. –65°C to 125°C
PIN CONFIGURATION
TCOMP1/ITEMP1
TCOMP1/ITEMP1
TOP VIEW
MODE/PLLIN
TOP VIEW
MODE/PLLIN
PHASMD
PHASMD
PGOOD
CLKOUT
40 39 38 37 36 35 34 33 32 31
TK/SS1 1
V
OSNS1
+
40 39 38 37 36 35 34 33 32 31
30 SW1
29 TG1
28 BOOST1
27 BG1
26 V
IN
25 INTV
CC
24 EXTV
CC
23 BG2
22 BOOST2
21 TG2
TK/SS1 1
V
OSNS1
+
CLKOUT
30 SW1
29 TG1
28 BOOST1
27 BG1
26 V
IN
25 INTV
CC
24 EXTV
CC
23 BG2
22 BOOST2
21 TG2
SW2
SNSD1
+
SNSD1
+
SNSA1
+
SNSA1
+
TRSET1
TRSET1
FREQ
SNS1
–
SNS1
–
RUN1
2
2
V
OSNS1–
3
I
TH1
4
I
TH2
5
V
OSNS2+
6
V
OSNS2
–
V
OSNS1–
3
I
TH1
4
I
TH2
5
V
OSNS2+
6
V
OSNS2
–
41
SGND/PGND
41
SGND/PGND
7
7
TK/SS2 8
SNSA2
+
9
SNS2
–
TK/SS2 8
SNSA2
+
9
SNS2
–
10
11 12 13 14 15 16 17 18 19 20
SNSD2
+
TCOMP2/ITEMP2
TAVG
TRSET2
FREQ
RUN2
IFAST
ENTMPB
PGOOD
SW2
10
11 12 13 14 15 16 17 18 19 20
SNSD2
+
TCOMP2/ITEMP2
TAVG
TRSET2
RUN2
IFAST
ENTMPB
UJ PACKAGE
40-LEAD (6mm
×
6mm) PLASTIC QFN
T
JMAX
= 125°C,
θ
JA
= 33°C/W,
θ
JC
= 2.0°C/W
EXPOSED PAD (PIN 41) IS SGND/PGND, MUST BE SOLDERED TO PCB
UH PACKAGE
40-LEAD (5mm
×
5mm) PLASTIC QFN
T
JMAX
= 125°C,
θ
JA
= 44°C/W,
θ
JC
= 7.3°C/W
EXPOSED PAD (PIN 41) IS SGND/PGND, MUST BE SOLDERED TO PCB
RUN1
ILIM
ILIM
2
3875fb
For more information
www.linear.com/LTC3875
LTC3875
ORDER INFORMATION
LEAD FREE FINISH
LTC3875EUH#PBF
LTC3875IUH#PBF
LTC3875EUJ#PBF
LTC3875IUJ#PBF
TAPE AND REEL
LTC3875EUH#TRPBF
LTC3875IUH#TRPBF
LTC3875EUJ#TRPBF
LTC3875IUJ#TRPBF
PART MARKING
3875
3875
LTC3875
LTC3875
PACKAGE DESCRIPTION
40-Lead (5mm
×
5mm) Plastic QFN
40-Lead (5mm
×
5mm) Plastic QFN
40-Lead (6mm
×
6mm) Plastic QFN
40-Lead (6mm
×
6mm) Plastic QFN
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
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
junction temperature range, otherwise specifications are at T
A
= 25°C (Note 2). V
IN
= 15V, V
RUN1,2
= 5V unless otherwise noted.
SYMBOL
PARAMETER
Main Control Loops
Input Voltage Range
V
IN
Output Voltage Range
V
OUT
V
OSNS1,2+
I
OSNS1,2+
V
REFLNREG
V
LOADREG
Regulated V
OUT
Feedback Voltage
Including Diffamp Error
Feedback Current
Reference Voltage Line Regulation
Output Voltage Load Regulation
CONDITIONS
MIN
TYP
MAX
UNITS
V
V
V
V
V
nA
%/V
%
%
mmho
µA
°C
°C
µA
µA
nA
V/V
ELECTRICAL CHARACTERISTICS
Transconductance Amplifier g
m
g
m1,2
Thermal Functions
Thermal Sensor Current
I
TCOMP1,2
Internal Thermal Shutdown
T
SHDN
Internal TS Hysteresis
T
HYS
Fast Transient Functions
Fast Transient Program Current
I
FAST
Current Sensing Functions
AC Sense Pins Bias Current
I
SENSE(AC)
DC Sense Pins Bias Current
I
SENSE(DC)
Total Sense Gain to Current Comp
A
VT(SNS)
SNSD
+
Pin to V
OUT
SNSD
+
Pin to GND
(Note 4); I
TH1,2
Voltage = 1.2V, –40°C to 85°C
(Note 4); I
TH1,2
Voltage = 1.2V,–40°C to 125°C
(Note 4)
V
IN
= 4.5V to 38V (Note 4)
(Note 4)
Measured in Servo Loop;
∆I
TH
Voltage = 1.2V to 0.7V
Measured in Servo Loop;
∆I
TH
Voltage = 1.2V to 1.6V
I
TH1,2
= 1.2V; Sink/Source 5µA (Note 4)
4.5
38
0.6
3.5
0.6
5
0.597 0.600 0.603
l
0.5965 0.600 0.6045
–30
–100
0.002 0.005
l
l
0.01
–0.01
2.2
29
30
160
10
10
±0.5
±30
5
0.1
–0.1
31
(Note 8)
(Note 8)
l
9
11
±2
±50
Each Channel; V
SNSA+(S)
= 3.3V
Each Channel; V
SNSD+(S)
= 3.3V
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3875fb
For more information
www.linear.com/LTC3875
3
LTC3875
The
l
denotes the specifications which apply over the specified operating
junction temperature range, otherwise specifications are at T
A
= 25°C (Note 2). V
IN
= 15V, V
RUN1,2
= 5V unless otherwise noted.
SYMBOL
V
SENSE(MAX)(DC)
PARAMETER
CONDITIONS
Maximum Current Sense Threshold 0°C to 85°C
with SNSD
+
Pin to V
OUT
V
SNS–(s)
= 1.2V, ILIM = 0V
V
SNS–(s)
= 1.2V, ILIM = 1/4 INTV
CC
V
SNS–(s)
= 1.2V, ILIM = 1/2 INTV
CC
V
SNS–(s)
= 1.2V, ILIM = 3/4 INTV
CC
V
SNS–(s)
= 1.2V, ILIM = INTV
CC
–40°C to 125°C
V
SNS–(s)
= 1.2V, ILIM = 0V
V
SNS–(s)
= 1.2V, ILIM = 1/4 INTV
CC
V
SNS–(s)
= 1.2V, ILIM = 1/2 INTV
CC
V
SNS–(s)
= 1.2V, ILIM = 3/4 INTV
CC
V
SNS–(s)
= 1.2V, ILIM = INTV
CC
V
SENSE(MAX)(NODE)
Maximum Current Sense Threshold V
SNS–(s)
= 1.2V, ILIM = 0V
with SNSD
+
Pin to GND
V
SNS–(s)
= 1.2V, ILIM = 1/4 INTV
CC
V
SNS–(s)
= 1.2V, ILIM = 1/2 INTV
CC
V
SNS–(s)
= 1.2V, ILIM = 3/4 INTV
CC
V
SNS–(s)
= 1.2V, ILIM = INTV
CC
Channel-to-Channel Current
ILIM = Float, ENTMPB = Float
I
MISMATCH
Mismatch
(Thermal Balance Disabled)
Input DC Supply Current
(Note 5)
I
Q
Normal Mode
V
IN
= 15V (without EXTV
CC
Enabled)
Shutdown
V
RUN1,2
= 0V
UVLO
Undervoltage Lockout
V
INTVCC
Ramping Down
UVLO Hyst
UVLO Hysteresis
Feedback Overvoltage Lockout
Measured at V
OSNS1,2+
V
OVL
Soft-Start Charge Current
V
TK/SS1,2
= 0V
I
TK/SS1,2
RUN Pin On Threshold
V
RUN1
, V
RUN2
Rising
V
RUN1,2
RUN Pin On Hysteresis
V
RUN1,2HYS
Driver Functions
TG Transition Time
(Note 6)
Rise Time
C
LOAD
= 3300pF
TG1,2 t
r
Fall Time
TG1,2 t
f
C
LOAD
= 3300pF
BG Transition Time
(Note 6)
Rise Time
C
LOAD
= 3300pF
BG1,2 t
r
Fall Time
BG1,2 t
f
C
LOAD
= 3300pF
Top Gate Off to Bottom Gate On
C
LOAD
= 3300pF Each Driver
TG/BG t
1D
Delay Synchronous Switch-On
Delay Time
Bottom Gate Off to Top Gate On
C
LOAD
= 3300pF Each Driver
BG/TG t
2D
Delay Synchronous Switch-On
Delay Time
Minimum On-Time
(Note 7)
t
ON(MIN)
MIN
9
14
19
23.5
28.5
l
l
l
l
l
l
l
l
l
l
ELECTRICAL CHARACTERISTICS
TYP
10
15
20
25
30
10
15
20
25
30
50
75
100
125
150
MAX
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
4.0
0.665
1.5
1.35
UNITS
mV
mV
mV
mV
mV
mV
mV
mV
mV
mV
mV
mV
mV
mV
mV
%
mA
µA
V
V
V
µA
V
mV
8.5
13.5
17.5
22
26.5
45
70
95
117.5
142.5
3.5
l
l
l
0.625
1.0
1.1
7
40
3.7
0.5
0.645
1.25
1.22
80
25
25
25
25
30
30
90
ns
ns
ns
ns
ns
ns
ns
4
3875fb
For more information
www.linear.com/LTC3875
LTC3875
The
l
denotes the specifications which apply over the specified operating
junction temperature range, otherwise specifications are at T
A
= 25°C (Note 2). V
IN
= 15V, V
RUN1,2
= 5V unless otherwise noted.
SYMBOL
PARAMETER
INTV
CC
Linear Regulator
Internal V
CC
Voltage
V
INTVCC
INTV
CC
Load Regulation
V
LDO
INT
EXTV
CC
Switchover Voltage
V
EXTVCC
EXTV
CC
Voltage Drop
V
LDO
EXT
EXTV
CC
Hysteresis
V
LDOHYS
Oscillator and Phase-Locked Loop
Nominal Frequency
f
NOM
Lowest Frequency
f
LOW
Highest Frequency
f
HIGH
MODE/PLLIN Input Resistance
R
MODE/PLLIN
Frequency Setting Current
I
FREQ
CLKOUT
Phase (Relative to Controller 1)
CLK High
CLK Low
V
PGL
I
PGOOD
V
PG
On-Chip Driver
TG R
UP
TG R
DOWN
BG R
UP
BG R
DOWN
CONDITIONS
6V < V
IN
< 38V
I
CC
= 0mA to 20mA
EXTV
CC
Ramping Positive
I
CC
= 20mA, V
EXTVCC
= 5.5V
MIN
5.3
l
ELECTRICAL CHARACTERISTICS
TYP
5.5
0.5
4.7
50
200
500
250
720
250
10
60
90
120
5.5
0.1
MAX
5.7
2.0
100
UNITS
V
%
V
mV
mV
kHz
kHz
kHz
kΩ
µA
Deg
Deg
Deg
V
V
V
µA
%
%
Ω
Ω
Ω
Ω
4.5
V
FREQ
= 1.2V
V
FREQ
= 0V
V
FREQ
≥ 2.4V
450
220
650
9.5
550
270
790
10.5
Clock Output High Voltage
Clock Output Low Voltage
PGOOD Voltage Low
I
PGOOD
= 2mA
PGOOD Leakage Current
V
PGOOD
= 5.5V
PGOOD Trip Level, Either Controller V
OSNS+
with Respect to Set Output Voltage
V
OSNS+
Ramping Negative
V
OSNS+
Ramping Positive
TG Pull-Up R
DS(ON)
TG Pull-Down R
DS(ON)
BG Pull-Up R
DS(ON)
BG Pull-Down R
DS(ON)
TG High
TG Low
BG High
BG Low
PHASMD = GND
PHASMD = FLOAT
PHASMD = INTV
CC
V
INTVCC
= 5.5V
4.5
0.2
0.3
±2
–7.5
7.5
2.6
1.5
2.4
1.1
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.
Note 2:
The LTC3875 is tested under pulsed load conditions such that
T
J
≈ T
A
. The LTC3875E is guaranteed to meet specifications from
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.
Note 3:
T
J
is calculated from the ambient temperature, T
A
, and power
dissipation, P
D
, according to the formula:
T
J
= T
A
+ (P
D
•
θ
JA
°C/W)
where
θ
JA
= 44°C/W for the 5mm × 5mm QFN and
θ
JA
= 33°C/W for
the 6mm × 6mm QFN.
Note 4:
The LTC3875 is tested in a feedback loop that servos V
ITH1,2
to a
specified voltage and measures the resultant V
OSNS1,2+
.
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
MAX
(see Minimum On-Time
Considerations in the Applications Information section).
Note 8:
Guaranteed by design.
3875fb
For more information
www.linear.com/LTC3875
5
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