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FEATURES
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
n
Input Capacitance and Power Supply Induced Noise
Dual N-Channel MOSFET Synchronous Drive
n
Wide V
Range: 4.5V to 38V Operation
n
IN
Output Voltage Range with Low DCR: 0.6V to 3.5V,
n
without Low DCR: 0.6V to 5V
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
n
APPLICATIONS
Servers and Instruments
n
Telecom Systems
n
DC Power Distribution Systems
n
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
OUT
+
470µF
2.5V ×2
SP
Controller with Low Value DCR Sensing
and Temperature Compensation
V
INTV
PHASMD
IN
CC
CLKOUT
PGOOD
RUN1,2
IFAST
ILIM
MODE/PLLIN
ENTMPB
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
OSNS2
1500pF
TK/SS1
TK/SS2
I
TH2
122k
15k
0.1µF
For more information
Dual, 2-Phase, Synchronous
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
All other trademarks are the property of their respective owners. Protected by U.S. Patents
including 5481178, 5705919, 5929620, 6100678, 6144194, 6177787, 6304066, 6580258.
V
IN
6V TO 14V
22µF
16V ×4
(OPTIONAL)
THERMAL
SENSOR
0.3µH
(0.32m DCR)
V
20k
OUT
1.2V
60A
+
470µF
20k
2.5V ×2
SP
3875 TA01a
www.linear.com/LTC3875
LTC3875
is a trademark of Linear Technology Corporation.
Efficiency and Power Loss
vs Load Current
100
12V
IN
1.8V
O
95
~400kHz
CCM
90
85
80
0.32m
1.5m
75
0.32m PLOSS
1.5m PLOSS
70
0
10
20
30
40
50
LOAD CURRENT (A)
3875 TA01b
14
12
10
8
6
4
2
0
60
3875fb
1
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Table of Contents
Related Manuals for Linear LTC3875
Summary of Contents for Linear LTC3875
- Page 1 40-lead 6mm × 6mm (0.5mm pitch) and 40-lead 5mm × 5mm (0.4mm pitch) QFN packages. Servers and Instruments L, LT, LTC, LTM, Linear Technology, the Linear logo OPTI-LOOP, Burst Mode and PolyPhase Telecom Systems are registered trademarks and No R is a trademark of Linear Technology Corporation.
-
Page 2: Absolute Maximum Ratings
LTC3875 ABSOLUTE MAXIMUM RATINGS (Note 1) Input Supply Voltage (V ) ......40V to –0.3V MODE/PLLIN, ILIM, FREQ, IFAST, ENTMPB – Topside Driver Voltages Voltages ....INTV to –0.3V OSNS(s) OSNS(s) (BOOST1, BOOST2)........46V to –0.3V , PHASMD, TRSET1, TRSET2, Switch Voltage (SW1, SW2) ...... -
Page 3: Order Information
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... - Page 4 LTC3875 ELECTRICAL CHARACTERISTICS denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at T = 25°C (Note 2). V = 15V, V = 5V unless otherwise noted. RUN1,2 SYMBOL PARAMETER CONDITIONS UNITS Maximum Current Sense Threshold 0°C to 85°C...
- Page 5 Note 2: The LTC3875 is tested under pulsed load conditions such that the 6mm × 6mm QFN. ≈ T . The LTC3875E is guaranteed to meet specifications from Note 4: The LTC3875 is tested in a feedback loop that servos V to a ITH1,2 0°C to 85°C junction temperature. Specifications over the –40°C to...
-
Page 6: Typical Performance Characteristics
LTC3875 TYPICAL PERFORMANCE CHARACTERISTICS Load Step Efficiency vs Output Current and Efficiency vs Output Current and (Figure 16 Application Circuit) Mode (Figure 16 Application Circuit) Mode (Figure 16 Application Circuit) (Burst Mode Operation) Burst Mode LOAD OPERATION 40A/DIV 5A TO 30A... - Page 7 LTC3875 TYPICAL PERFORMANCE CHARACTERISTICS Current Sense Threshold Maximum Current Sense Threshold INTV Line Regulation vs I Voltage vs Common Mode Voltage ILIM = 0 ILIM = INTV ILIM = 1/4 INTV ILIM = 1/2 INTV ILIM = 3/4 INTV ILIM = 3/4 INTV...
- Page 8 LTC3875 TYPICAL PERFORMANCE CHARACTERISTICS Oscillator Frequency Shutdown Current Undervoltage Lockout Threshold vs Input Voltage vs Input Voltage (INTV ) vs Temperature = INTV FREQ RISING = 1.22V FREQ FALLING = GND FREQ –50 INPUT VOLTAGE (V) TEMPERATURE (°C) INPUT VOLTAGE (V)
-
Page 9: Pin Functions
LTC3875 PIN FUNCTIONS ENTMPB (Pin 18): Enable Pin for Temperature Balanc- TK/SS1, TK/SS2 (Pin 1, Pin 8): Output Voltage Tracking and Soft-Start Inputs. When one channel is configured to ing Function. Ground this pin to enable the temperature be the master, a capacitor to ground at this pin sets the balancing function. - Page 10 LTC3875 PIN FUNCTIONS PHASMD (Pin 32): Phase Programmable Pin. This pin can TCOMP1/ITEMP1, TCOMP2/ITEMP2 (Pin 37, Pin 12): be tied to SGND, INTV or left floating. It determines the Input of the Temperature Balancing Circuitries. Connect relative phases between the internal controllers as well these pins to external NTC resistors or temperature sensing as the phasing of the CLKOUT signal.
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Page 11: Block Diagram
LTC3875 BLOCK DIAGRAM (Functional diagram shows one channel only) EXTV MODE/PLLIN PHASMD TCOMP/ITEMP 4.7V – FREQ TEMPSNS 0.6V MODE/SYNC DETECT 5.5V – INTV PLL-SYNC BOOST BURST EN CLKOUT FCNT IFAST (CHANNEL 2 ONLY) SNSA SWITCH LOGIC – – ANTISHOOT- THROUGH –... -
Page 12: Operation
However, it is recommended that a load be present or the IC operates The LTC3875 is a constant frequency, current mode step- at low frequency during the drop-out transition to ensure down controller with two channels operating 180° or 240°... - Page 13 (discontinuous operation). This mode, like forced Pulse-Skipping, or Continuous Conduction) continuous operation, exhibits low output ripple as well as The LTC3875 can be enabled to enter high efficiency Burst low audio noise and reduced RF interference as compared Mode operation, constant frequency pulse-skipping mode, to Burst Mode operation.
- Page 14 The LTC3875 can sense a DCR value as low as 0.2mΩ with amplifier inside the controller. Therefore, its differential careful PCB layout. The LTC3875 uses two positive sense output signal (V ) is not accessible from outside the IC.
- Page 15 Thermal Balancing For Multiphase Operation The sensed voltage from each channel is converted to a current, which is programmable with resistor, R When LTC3875 is used as a single output multiphase TCOMP at the TRSET pin. The current from each channel is then converter, the temperature of the whole system can be summed together at the TAVG pin.
- Page 16 SENSE(MAX) the above equation. The current limit can be adjusted LTC3875 offers a method to counter this inaccuracy by using this method either with a sense resistor or DCR allowing the user to place an NTC temperature sensing sensing.
- Page 17 The PGOOD pin is also pulled low when the RUN pins are for the worst case transient steps. For this application, below 1.14V or when the LTC3875 is in the soft-start, IFAST pin voltage is programmed to be around 0.62V and UVLO or tracking phase.
-
Page 18: Applications Information
(Figure 4a). The ILIM pin is a 5-level logic input which sets the maximum Because the LTC3875 is designed to be used with a very current limit of the controller. The input impedance of the low DCR value to sense inductor current, without proper ILIM pin is 250kΩ. - Page 19 For applications where the inductor DCR is large, the sense resistor will cause a significant reduction in power LTC3875 could also be used like any typical current efficiency. For a specific output requirement and induc- mode controller with conventional DCR sensing by...
- Page 20 DCR resistance, approximately 0.4%/°C, should be taken ITEMP maintain the current limit threshold constant over a wide into account. The LTC3875 features a DCR temperature operating temperature. The TCOMP/ITEMP voltage range compensation circuit that uses an NTC temperature sensing that activates the correction is from 0.7V or less. If this resistor for this purpose.
- Page 21 LTC3875 APPLICATIONS INFORMATION 1. Set the TCOMP/ITEMP pin resistance to 23.33k at 25°C. from the following equation: With 30µA flowing out of the TCOMP/ITEMP pin, the voltage on the TCOMP/ITEMP pin will be 0.7V at room R = R •...
- Page 22 Take care to keep the TCOMP/ITEMP pins away from the sults in a reduction of maximum inductor peak current for switch nodes. duty cycles > 40%. However, the LTC3875 uses a scheme that counteracts this compensating ramp, which allows Slope Compensation and Inductor Peak Current the maximum inductor peak current to remain unaffected throughout all duty cycles.
- Page 23 LTC3875 APPLICATIONS INFORMATION A reasonable starting point is to choose a ripple current The peak-to-peak MOSFET gate drive levels are set by the that is about 40% of I . Note that the largest ripple internal regulator voltage, V , requiring the use of...
- Page 24 • Soft-Start and Tracking MILLER The LTC3875 has the ability to either soft-start by itself with a capacitor or track the output of another channel or • f – V ...
- Page 25 OUT2 the voltage ramp rate on this pin is determined by the refers to the LTC3875’s output 2 as a slave channel. In ramp rate of the other supply’s voltage. Note that the small practice, though, either phase can be used as the master.
- Page 26 LTC3875 APPLICATIONS INFORMATION the slave channel output will be affected as well. For bet- mum junction temperature rating for the LTC3875 to be ter output regulation, use the coincident tracking mode exceeded. The INTV current, which is dominated by instead of ratiometric.
- Page 27 Undervoltage Lockout than 4.7V. The LTC3875 has two functions that help protect the For applications where the main input power is below 5V, controller in case of undervoltage conditions. A precision tie the V...
- Page 28 A small (0.1µF to 1µF) bypass capacitor between the chip The out-of-phase technique typically reduces the input pin and ground, placed close to the LTC3875, is also capacitor’s RMS ripple current by a factor of 30% to 70% suggested. A 2.2Ω to 10Ω resistor placed between C when compared to a single phase power supply solution.
- Page 29 LTC3875 APPLICATIONS INFORMATION with very low duty cycles, the LTC3875 will begin cycle The internal thermal shutdown is set for approximately skipping in order to limit the short-circuit current. In this 160°C with 10°C of hysteresis. When the chip reaches situation the bottom MOSFET will be dissipating most of 160°C, both TG and BG are disabled until the chip cools...
- Page 30 ON(MIN) driver and control currents. V current typically results that the LTC3875 is capable of turning on the top MOSFET. in a small (<0.1%) loss. It is determined by internal timing delays and the gate charge required to turn on the top MOSFET. Low duty...
- Page 31 ESR at the switching frequency. The MOSFETs. Each time a MOSFET gate is switched from LTC3875 2-phase architecture typically halves this input low to high to low again, a packet of charge dQ moves capacitance requirement over competing solutions. Other from INTV to ground.
- Page 32 LTC3875 APPLICATIONS INFORMATION 2. Are the signal and power grounds kept separate? The a power MOSFET directly across the output capacitor and combined IC signal ground pin and the ground return driving the gate with an appropriate signal generator is a...
- Page 33 LTC3875 APPLICATIONS INFORMATION TK/SS1 CLKOUT PULL-UP PGOOD PGOOD LTC3875 PHASMD OSNS1 IFAST – OSNS OUT1 SNSA1 – SNS1 SNSD1 BOOST1 (OPT) FREQ 10µF ×2 MODE/PLLIN CERAMIC RUN1 OUT1 RUN2 1µF SGND PGND 1µF – EXTV INTVCC SNSA2 4.7µF – SNS2...
- Page 34 LTC3875 APPLICATIONS INFORMATION 8. Use a low impedance source such as a logic gate to not required. Only after each controller is checked for drive the MODE/PLLIN pin and keep the lead as short its individual performance should both controllers be as possible.
- Page 35 LTC3875 APPLICATIONS INFORMATION INTV 4.5V TO 20V 10µF 270µF INTV ×4 4.7µF INTV PHASMD CLKOUT CMDSH-3 CMDSH-3 PGOOD RUN1,2 THERMAL THERMAL IFAST ILIM MODE/PLLIN SENSOR ENTMPB SENSOR BSC050NE2LS BSC050NE2LS 0.1µF 0.1µF LTC3875 0.33µH 0.33µH BOOST1 BOOST2 (0.32m DCR) (0.32m DCR)
- Page 36 LTC3875 APPLICATIONS INFORMATION An Infineon BSC010NE2LS, R = 1.1mΩ, is chosen Choose R1 = 4.64k and R2 = 931Ω. DS(ON) for the bottom FET. The resulting power loss is: The maximum DCR of the inductor is 0.34mΩ. The 20V – 1.5V...
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Page 37: Typical Applications
LTC3875 TYPICAL APPLICATIONS 3875fb For more information www.linear.com/LTC3875... - Page 38 LTC3875 TYPICAL APPLICATIONS 3875fb For more information www.linear.com/LTC3875...
- Page 39 LTC3875 TYPICAL APPLICATIONS 3875fb For more information www.linear.com/LTC3875...
- Page 40 LTC3875 TYPICAL APPLICATIONS 3875fb For more information www.linear.com/LTC3875...
- Page 41 LTC3875 TYPICAL APPLICATIONS 3875fb For more information www.linear.com/LTC3875...
- Page 42 LTC3875 TYPICAL APPLICATIONS 3875fb For more information www.linear.com/LTC3875...
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Page 43: Package Description
LTC3875 PACKAGE DESCRIPTION Please refer to http://www.linear.com/product/LTC3875#packaging for the most recent package drawings. UH Package 40-Lead Plastic QFN (5mm × 5mm) (Reference LTC DWG # 05-08-1746 Rev B) 0.70 ±0.05 5.50 ±0.05 4.10 ±0.05 3.50 ±0.05 3.60 REF (4 SIDES) 3.50 ±0.05... - Page 44 LTC3875 PACKAGE DESCRIPTION Please refer to http://www.linear.com/product/LTC3875#packaging for the most recent package drawings. UJ Package 40-Lead Plastic QFN (6mm × 6mm) (Reference LTC DWG # 05-08-1728 Rev Ø) 0.70 ±0.05 6.50 ±0.05 5.10 ±0.05 4.42 ±0.05 4.50 ±0.05 (4 SIDES) 4.42 ±0.05...
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Page 45: Revision History
SENSE(AC) 3875fb Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa- For more information www.linear.com/LTC3875 tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. -
Page 46: Typical Application
≤ 24V, I = 50µA/170µA 3875fb Linear Technology Corporation LT 1115 REV B • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 For more information www.linear.com/LTC3875 (408) 432-1900 FAX: (408) 434-0507 www.linear.com/LTC3875 LINEAR TECHNOLOGY CORPORATION 2013 ● ●...