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Linear Technology LT8611 Manual

42v, 2.5a synchronous step-down regulator with current sense
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FEATURES
Rail-to-Rail Current Sense Amplifier with Monitor
n
Wide Input Voltage Range: 3.4V to 42V
n
Ultralow Quiescent Current Burst Mode
n
2.5μA I
Regulating 12V
n
Q
Output Ripple < 10mV
n
High Efficiency Synchronous Operation:
n
96% Efficiency at 1A, 5V
n
94% Efficiency at 1A, 3.3V
n
Fast Minimum Switch-On Time: 50ns
n
Low Dropout Under All Conditions: 200mV at 1A
n
Allows Use of Small Inductors
n
Low EMI
n
Adjustable and Synchronizable: 200kHz to 2.2MHz
n
Current Mode Operation
n
Accurate 1V Enable Pin Threshold
n
Internal Compensation
n
Output Soft-Start and Tracking
n
Small Thermally Enhanced 3mm × 5mm 24-Lead
n
QFN Package
APPLICATIONS
Automotive and Industrial Supplies
n
General Purpose Step-Down
n
CCCV Power Supplies
n
TYPICAL APPLICATION
5V Step-Down Converter with 2.5A Output Current Limit
V
IN
V
IN
5.5V TO 42V
4.7µF
ON OFF
EN/UV
SYNC
IMON
ICTRL
INTV
TR/SS
0.1µF
RT
1µF
60.4k
f
= 700kHz
SW
Operation:
®
to 3.3V
IN
OUT
P-P
from 12V
OUT
IN
from 12V
OUT
IN
BST
0.1µF
4.7µH
0.02
SW
LT8611
1µF
ISP
ISN
BIAS
10pF
PG
CC
1M
FB
PGND
GND
243k
8611 TA01a
For more information
42V, 2.5A Synchronous
Step-Down Regulator with
Current Sense and 2.5µA
DESCRIPTION
The
LT
8611
is a compact, high efficiency, high speed
®
synchronous monolithic step-down switching regulator
that consumes only 2.5µA of quiescent current. Top and
bottom power switches are included with all necessary
circuitry to minimize the need for external components.
The built-in current sense amplifier with monitor and
control pins allows accurate input or output current
regulation and limiting. Low ripple Burst Mode operation
enables high efficiency down to very low output currents
while keeping the output ripple below 10mV
pin allows synchronization to an external clock. Internal
compensation with peak current mode topology allows
the use of small inductors and results in fast transient
response and good loop stability. The EN/UV pin has an
accurate 1V threshold and can be used to program V
undervoltage lockout or to shut down the LT8611 reduc-
ing the input supply current to 1µA. A capacitor on the
TR/SS pin programs the output voltage ramp rate during
start-up. The PG flag signals when V
the programmed output voltage as well as fault conditions.
The LT8611 is available in a small 24-lead 3mm × 5mm
QFN package with exposed pad for low thermal resistance.
L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
100
95
90
V
OUT
85
5V
2.5A
80
75
70
65
60
55
50
0
47µF
www.linear.com/LT8611
Quiescent Current
is within ±9% of
OUT
12V
to 5V
Efficiency
IN
OUT
f
= 700kHz
SW
V
= 12V
IN
V
= 24V
IN
0.5
1
1.5
2
2.5
LOAD CURRENT (A)
8611 G01
LT8611
. A SYNC
P-P
IN
8611fa
1

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Summary of Contents for Linear Technology LT8611

  • Page 1 QFN package with exposed pad for low thermal resistance. Automotive and Industrial Supplies General Purpose Step-Down L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective CCCV Power Supplies owners.

  • Page 2: Absolute Maximum Ratings

    LT8611 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) , EN/UV, PG, ISP, ISN ...........42V TOP VIEW BIAS ................30V BST Pin Above SW Pin..........4V 24 23 22 21 FB, TR/SS, RT, INTV , IMON, ICTRL......4V SYNC SYNC Voltage .............6V...
  • Page 3 LT8611 ELECTRICAL CHARACTERISTICS denotes the specifications which apply over the full operating temperature range, otherwise specifications are at T = 25°C. PARAMETER CONDITIONS UNITS INTV Undervoltage Lockout BIAS Pin Current Consumption = 3.3V, I = 1A, 2MHz BIAS LOAD Minimum On-Time...

  • Page 4: Typical Performance Characteristics

    LT8611 TYPICAL PERFORMANCE CHARACTERISTICS Efficiency at 5V Efficiency at 3.3V Efficiency at 5V = 12V = 24V = 700kHz = 700kHz = 12V = 12V = 24V = 24V = 700MHz 0.001 0.01 1000 10000 LOAD CURRENT (A) LOAD CURRENT (A)
  • Page 5 LT8611 TYPICAL PERFORMANCE CHARACTERISTICS No Load Supply Current No Load Supply Current Top FET Current Limit vs Duty Cycle = 3.3V = 3.3V IN REGULATION = 12V IN REGULATION –55 –25 INPUT VOLTAGE (V) TEMPERATURE (°C) DUTY CYCLE 8611 G10...
  • Page 6 LT8611 TYPICAL PERFORMANCE CHARACTERISTICS Dropout Voltage Switching Frequency Burst Frequency = 60.4k = 12V = 3.3V –55 –25 TEMPERATURE (°C) LOAD CURRENT (mA) LOAD CURRENT (A) 8611 G20 8611 G21 8611 G19 Minimum Load to Full Frequency (SYNC DC High)
  • Page 7 LT8611 TYPICAL PERFORMANCE CHARACTERISTICS RT Programmed Switching Frequency UVLO Bias Pin Current 5.00 = 5V BIAS = 5V 4.75 = 1A LOAD = 700kHz 4.50 4.25 4.00 3.75 3.50 3.25 3.00 –25 –55 SWITCHING FREQUENCY (MHz) INPUT VOLTAGE (V) TEMPERATURE (°C)
  • Page 8 LT8611 TYPICAL PERFORMANCE CHARACTERISTICS Transient Response Start-Up Dropout Performance Start-Up Dropout Performance 1A/DIV 2V/DIV 2V/DIV 200mV/DIV 2V/DIV 2V/DIV 50µs/DIV 100ms/DIV 100ms/DIV 8611 G37 8611 G38 8611 G39 2.5 LOAD 20 LOAD 50mA TO 1A TRANSIENT (2A IN REGULATION) (250mA IN REGULATION) , 5V = 47µF...

  • Page 9: Pin Functions

    IC. EN/UV (Pin 4): The LT8611 is shut down when this pin BIAS (Pin 18): The internal regulator will draw current from is low and active when this pin is high. The hysteretic...

  • Page 10: Block Diagram

    ISN (Pin 22): Current Sense (–) Pin. This is the inverting ICTRL (Pin 24): Current Adjustment Pin. ICTRL adjusts input to the current sense amplifier. the maximum ISP-ISN drop before the LT8611 reduces output current. Connect directly to INTV or float for IMON (Pin 23): Proportional-to-Current Monitor Output.

  • Page 11: Operation

    ISP , ISN pin to a lower set point for the rent conditions. current control loop. If the EN/UV pin is low, the LT8611 is shut down and draws 1µA from the input. When the EN/UV pin is above 1V, the switching regulator will become active.

  • Page 12: Applications Information

    INTV (1a) 8611 F01a pin. When a clock is applied to the SYNC pin the LT8611 will also operate in pulse-skipping mode. Minimum Load to Full Frequency (SYNC DC High)
  • Page 13 R1+R2   15.8 where 1.7µA is the quiescent current of the LT8611 and Operating Frequency Selection and Trade-Offs the second term is the current in the feedback divider Selection of the operating frequency is a trade-off between reflected to the input of the buck operating at its light efficiency, component size, and input voltage range.
  • Page 14 LT8611 APPLICATIONS INFORMATION where ∆I is the inductor ripple current as calculated in The LT8611 is capable of a maximum duty cycle of greater Equation 9 and I is the maximum output load than 99%, and the V -to-V dropout is limited by the LOAD(MAX) for a given application.
  • Page 15 (under damped) tank cir- aware that low inductance may result in discontinuous cuit. If the LT8611 circuit is plugged into a live supply, the mode operation, which further reduces maximum load input voltage can ring to twice its nominal value, possibly current.
  • Page 16 V bypass capacitor and the input The LT8611 is in shutdown when the EN pin is low and power source to sense input current. The current loop active when the pin is high. The rising threshold of the EN modulates the internal cycle-by-cycle switch current limit comparator is 1.0V, with 40mV of hysteresis.
  • Page 17 BIAS pin is at 3.1V or higher. Typically the resistor. Otherwise, the internal pull-down device will pull BIAS pin can be tied to the output of the LT8611, or can the PG pin low. To prevent glitching both the upper and be tied to an external supply of 3.3V or above.
  • Page 18 There is another situation to consider in systems where The slope compensation is set by the R value, while the the output will be held high when the input to the LT8611 minimum slope compensation required to avoid subhar- is absent. This may occur in battery charging applications...
  • Page 19 PCB to ensure good heat sinking of the by calculating the total power loss from an efficiency LT8611. The exposed pad on the bottom of the package measurement and subtracting the inductor loss. The must be soldered to a ground plane. This ground should be die temperature is calculated by multiplying the LT8611 tied to large copper layers below with thermal vias;...

  • Page 20: Typical Applications

    LT8611 TYPICAL APPLICATIONS 5V Step-Down with 1A Output Current Limit 5.5V TO 42V 0.1µF 4.7µF ON OFF EN/UV 4.7µH 0.050 SYNC LT8611 1µF IMON ICTRL BIAS 10pF INTV TR/SS 0.1µF PGND 1µF 52.3k 243k 47µF 8611 TA02 = 800kHz 3.3V Step-Down with 1A Input Current Limit 1µF...
  • Page 21 LT8611 TYPICAL APPLICATIONS Digitally Controlled Current/Voltage Source 3.8V TO 42V 0.1µF 4.7µF ON OFF EN/UV 4.7µH 0.025 SYNC 3.3V LT8611 1µF IMON µC ICTRL BIAS INTV 4.7pF TR/SS PGND 1µF 60.4k 412k 47µF 8611 TA05 = 700kHz CCCV Battery Charger 3.8V TO 42V...
  • Page 22 LT8611 TYPICAL APPLICATIONS 2MHz, 3.3V Step-Down with Power Good without Current Sense 3.8V TO 42V 0.1µF 4.7µF ON OFF EN/UV 2.2µH 3.3V SYNC 2.5A IMON LT8611 150k ICTRL BIAS PGOOD 4.7pF INTV TR/SS 0.1µF PGND 1µF 18.2k 412k 47µF f = 2MHz...
  • Page 23 LT8611 TYPICAL APPLICATIONS 12V Step-Down with 1A Output Current Limit 12.5V TO 42V 0.1µF 10µF ON OFF EN/UV 10µH 0.05 SYNC IMON LT8611 1µF ICTRL BIAS 10pF INTV TR/SS 0.1µF PGND 1µF 60.4k 88.7k 22µF f = 700kHz 8611 TA10 2A LED Driver 3.8V TO 42V...

  • Page 24: Package Description

    LT8611 PACKAGE DESCRIPTION Please refer to http://www.linear.com/product/LT8611#packaging for the most recent package drawings. UDD Package 24-Lead Plastic QFN (3mm × 5mm) (Reference LTC DWG # 05-08-1833 Rev Ø) 0.70 ±0.05 3.50 ±0.05 2.10 ±0.05 3.65 ±0.05 1.50 REF 1.65 ±0.05 PACKAGE OUTLINE 0.25 ±0.05...

  • Page 25: Revision History

    Clarified Current Sense Voltage and IMON Monitor Pin Voltage Parameters 8611fa 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/LT8611...

  • Page 26: Typical Application

    < 1µA, 3mm × 4mm DFN-16 and MSOP-16E Packages 8611fa Linear Technology Corporation LT 0117 REV A • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 For more information www.linear.com/LT8611 (408) 432-1900 FAX: (408) 434-0507 www.linear.com/LT8611  LINEAR TECHNOLOGY CORPORATION 2012 ● ●...

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