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LTC3112

3112fc

For more information www.linear.com/LTC3112

Typical applicaTion

DescripTion

15V, 2.5A Synchronous

Buck-Boost DC/DC

Converter

The LT C

3112 is a fixed frequency synchronous buck-boost

DC/DC converter with an extended input and output range.

The unique 4-switch, single inductor architecture provides

low noise and seamless operation from input voltages

above, below or equal to the output voltage.

With an input range of 2.7V to 15V, the LTC3112 is well-

suited for a wide variety of single or multiple cell battery,

backup capacitor or wall adapter source applications. Low

DS(ON)

internal N-Channel MOSFET switches provide

highly efficient operation in applications with higher load

current requirements.

The LTC3112 features selectable PWM or Burst Mode

operation, an easily synchronized oscillator and output

disconnect in shutdown. An output current monitor circuit

allows the load current to be controlled or measured. Other

features include <1µA shutdown current, short circuit

protection, soft-start, current limit and thermal shutdown.

The LTC3112 is offered in both a 16-pin (4mm × 5mm ×

0.75mm) DFN and 20-pin TSSOP packages.

Efficiency at 5V

OUT

FeaTures

applicaTions

Regulated Output with V

Above, Below or Equal to V

OUT

2.7V to 15V Input Voltage Range

2.5V to 14V Output Voltage Range

2.5A Continuous Output Current: V

≥ 5V,

OUT

= 5V, PWM Mode

Output Current Monitor

Up to 95% Efficiency

750kHz Switching Frequency, Synchronizable

Between 300kHz and 1.5MHz

Internal N-Channel MOSFETs

Selectable Burst Mode

Operation, I

= 50µA

Shutdown Current < 1µA

Overvoltage Protection

Output Disconnect in Shutdown

Internal Soft-Start

Small, Thermally Enhanced 16-Lead (4mm × 5mm ×

0.75mm) DFN and 20-Lead TSSOP Package

3.3V or 5V from 1, 2 or 3 Li-Ion, Backup Capacitor Stack

Hand Held Inventory Terminals

RF Transmitters

12V Synchronous Boost Converter

Multiple Power Input Systems

LED Lighting with Current Regulation

12V Lead Acid Battery to 12V

L, LT , LT C , LT M , Linear Technology Burst Mode, LTSpice and the Linear logo are registered

trademarks and No R

SENSE

and PowerPath are trademarks of Linear Technology Corporation.

All other trademarks are the property of their respective owners. Protected by U.S. Patents,

including 5481178.

4.7µH

0.1µF 0.1µF

680pF

22pF

33k

845k

158k

47pF

47µF

1µF

10µF

10k

42.2k100pF

TO ADC

1V PER AMP

5V/2.5A, V

> 5V

ONOFF

PWMBURST

2.7V TO 15V

SW1

BST1

RUN

GND

PWM/SYNC

SW2

BST2

OUT

COMP

OUT

OVP

LTC3112

OUT

3112 TA01

LOAD CURRENT (A)

0.0001

EFFICIENCY (%)

100

0.01 10.1

3112 TA02

0.001

2.7V

5.0V

12V

PWM

BURST

5V, 750kHz Wide Input Voltage Range Buck-Boost Regulator

Summary of content (32 pages)

PAGE 1
LTC3112 15V, 2.5A Synchronous Buck-Boost DC/DC Converter Description Features n n n n n n n n n n n n n n Regulated Output with VIN Above, Below or Equal to VOUT 2.7V to 15V Input Voltage Range 2.5V to 14V Output Voltage Range 2.5A Continuous Output Current: VIN ≥ 5V, VOUT = 5V, PWM Mode Output Current Monitor Up to 95% Efficiency 750kHz Switching Frequency, Synchronizable Between 300kHz and 1.
PAGE 2
LTC3112 Absolute Maximum Ratings (Notes 1, 3) VIN Voltage.................................................. – 0.3V to 16V VOUT Voltage............................................... – 0.3V to 15V SW1 Voltage (Note 4).................... – 0.3V to (VIN + 0.3V) SW2 Voltage (Note 4)................. – 0.3V to (VOUT + 0.3V) VBST1 Voltage.................... (VSW1 – 0.3V) to (VSW1 + 6V) VBST2 Voltage....................(VSW2 – 0.3V) to (VSW2 + 6V) RUN Voltage............................................... – 0.
PAGE 3
LTC3112 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 = VOUT = PWM/SYNC = RUN = 5V unless otherwise noted. PARAMETER CONDITIONS MIN Input Operating Range 0°C to 150°C –55°C to 0°C 2.7 2.85 VIN UVLO Threshold Rising 2.0 VIN UVLO Hysteresis VCC UVLO Threshold TYP MAX 15 15 2.3 2.7 300 Rising l 2.2 VCC UVLO Hysteresis 2.
PAGE 4
LTC3112 Electrical Characteristics Note 3: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 150°C when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability.
PAGE 5
LTC3112 Typical Performance Characteristics TA = 25°C, VIN = 5.0V, VOUT = 5.0V unless otherwise specified Maximum Output Current PWM Mode 3.2 2.4 1.6 3.3VOUT 5.0VOUT 12VOUT 2 3 4 100 480 90 400 80 320 240 160 0 300 10 5 0 200 150 4.3 4.2 4.1 4.0 3.9 0 20 40 60 80 100 120 CURRENT FROM VCC (mA) 140 3112 G10 3.5 3.0 100 275 VIN = 5V MINIMUM SW1 LOW TIME (ns) 4.4 VCC (V) 4.0 250 0 VCC Voltage vs VCC Current 10 3112 G06 4.5 2.
PAGE 6
LTC3112 Typical Performance Characteristics TA = 25°C, VIN = 5.0V, VOUT = 5.0V unless otherwise specified Feedback Pin Program Voltage vs Temperature 1.2 1.1 1.0 0.9 0.8 0.7 –60 –40 –20 0 20 40 60 80 100 120 140 160 TEMPERATURE (°C) 3112 G13 2.7 0.820 2.6 0.815 2.5 0.810 0.805 0.800 0.795 0.790 0.785 0.780 RUN and PWM/SYNC Threshold Voltage vs Temperature 1.0 12 PWM/SYNC RISING PWM/SYNC FALLING RUN RISING RUN FALLING 0.9 0.8 0.7 0.
PAGE 7
LTC3112 Typical Performance Characteristics TA = 25°C, VIN = 5.0V, VOUT = 5.0V unless otherwise specified 3.3VOUT Die Temperature Rise vs Continuous Load Current 4 Layer Demo Board at 25°C IOUT Voltage vs VIN and Switching Frequency IOUT Voltage vs VOUT 60 2500 2500 IOUT = 2A IOUT = 1A 1000 IOUT = 500mA 500 2 3 4 5 6 1500 1000 0 7 8 9 10 11 12 13 14 VOUT (V) 3112 G22 60 60 50 50 40 30 20 0 VIN = 2.7V VIN = 5V VIN = 12V 0 0.5 1 1.5 2 2.
PAGE 8
LTC3112 Typical Performance Characteristics TA = 25°C, VIN = 5.0V, VOUT = 5.0V unless otherwise specified 12VIN to 5VOUT Burst Mode Operation Waveforms 7.5VIN to 5.0VOUT Soft-Start Waveforms PWM VOUT Ripple VIN 2V/DIV VOUT 50mV/DIV VOUT 100mV/DIV VOUT 2V/DIV INDUCTOR CURRENT 1A/DIV INDUCTOR CURRENT 500mA/DIV 1µs/DIV 12.0VIN TO 5.0VOUT 1A LOAD COUT = 47µF 20µs/DIV 100mA LOAD COUT = 47µF INDUCTOR CURRENT 1A/DIV 3112 G31 1ms/DIV IL 1A/DIV 3112 G32 3112 G33 12VIN to 5.
PAGE 9
LTC3112 Pin Functions (DFN/TSSOP) COMP (Pin 1/Pin 2): Error Amp Output. An R-C network connected from this pin to FB sets the loop compensation for the voltage converter. FB (Pin 2/Pin 3): Feedback Voltage Input. Connect VOUT resistor divider tap to this pin. The output voltage can be adjusted from 2.5V to 14V by the following equation:  R1  VOUT = 0.8V • 1+   R2  where R1 is the resistor between VOUT and FB and R2 is the resistor between FB and GND. OVP (Pin 3/Pin 4): Overvoltage Protection Input.
PAGE 10
LTC3112 Block Diagram 4.7µH 2.7V TO 15V VIN BST1 VIN SW1 5V VOUT SW2 VOUT BST2 VCC VCC ADRV VCC A D B C VCC CDRV VCC DRIVERS REVERSE ILIM + – IZERO IPEAK 10A + – ILIMIT 6A + – LOGIC RUN OFF ON VCC 2.3V + – UVLO + – VCC PLL 0.8V SOFT-START RAMP COMP 750kHz OSCILLATOR 4.2V REGULATOR /CLAMP Burst Mode OPERATION REFERENCE 10 FB OVERVOLTAGE PROTECTION VIN VCC VCC 1.2V – + OVP + – –1A + + – 300mA GND CDRV DDRV ADRV BDRV – + 24µA/A DDRV BDRV GND IOUT 0.
PAGE 11
LTC3112 Operation INTRODUCTION The LTC3112 is an extended input and output range, high current synchronous buck-boost DC/DC converter optimized for a variety of demanding applications. The LTC3112 utilizes a proprietary switching algorithm, which allows its output voltage to be regulated above, below or equal to the input voltage. The error amplifier output on COMP determines the output duty cycle of the switches.
PAGE 12
LTC3112 Operation THERMAL CONSIDERATIONS UNDERVOLTAGE LOCKOUTS For the LTC3112 to provide maximum output power, it is imperative that a good thermal path be provided to dissipate the heat generated within the package. This can be accomplished by taking advantage of the large thermal pad on the underside of the IC. It is recommended that multiple vias in the printed circuit board be used to conduct the heat away from the IC and into a copper plane with as much area as possible.
PAGE 13
LTC3112 Operation are pulse width modulated to produce the required duty cycle to support the output regulation voltage. shown in the Typical Performance Characteristics curves (compared to VCC powered from VIN). As the input voltage nears the output voltage, switches A and D are on for a greater portion of the switching period, providing a direct current path from VIN to VOUT. Switches B and C are turned on only enough to ensure proper regulation and/or provide charging of the BST1 and BST2 capacitors.
PAGE 14
LTC3112 Operation Burst Mode operation, the error amplifier is configured for low power operation and used to hold the compensation pin COMP, to reduce transients that may occur during transitions from and to burst and PWM mode. Output Current Monitor The LTC3112 includes a circuit that sources an approximate 24µA/A current replica of the VOUT (or SWD) current. This current is typically passed through a resistor from IOUT to GND and filtered to produce a DC voltage proportional to average load current.
PAGE 15
LTC3112 Applications Information Where f is the switching frequency in Hz and L is the inductor value in Henries. In addition to affecting output current ripple, the size of the inductor can also impact the stability of the feedback loop. In boost mode, the converter transfer function has a right half plane zero at a frequency that is inversely proportional to the value of the inductor.
PAGE 16
LTC3112 Applications Information CAPACITOR VENDOR INFORMATION Both the input bypass capacitors and output capacitors used with the LTC3112 must be low ESR and designed to handle the large AC currents generated by switching converters. This is important to maintain proper functioning of the IC and to reduce ripple on both the input and output. Many modern low voltage ceramic capacitors experience significant loss in capacitance from their rated value with increased DC bias voltages.
PAGE 17
LTC3112 Applications Information Top Layer 2nd Layer L CIN COUT 3rd Layer Bottom Layer (Top View) Figure 3. Example PCB Layout 3112fc For more information www.linear.
PAGE 18
LTC3112 Applications Information Buck Mode Small Signal Model The LTC3112 uses a voltage mode control loop to maintain regulation of the output voltage. An externally compensated error amplifier drives the COMP pin to generate the appropriate duty cycle of the power switches. Use of an external compensation network provides the flexibility for optimization of closed loop performance over the wide variety of output voltages, switching frequencies, and external component values supported by the LTC3112.
PAGE 19
LTC3112 Applications Information Boost Mode Small Signal Model When stepping up from a lower input voltage to a higher output voltage, the buck-boost converter will operate in boost mode where the small signal transfer function from control voltage, VCOMP, to the output voltage is given by the following expression.
PAGE 20
LTC3112 Applications Information For charging, LED lighting, or other applications that do not require an optimized output voltage transient re-sponse, a simple Type I compensation network as shown in Figure 5 can be used to stabilize the voltage loop. To ensure sufficient phase margin, the gain of the error am-plifier must be low enough that the resultant crossover frequency of the control loop is well below the resonant frequency. VOUT LTC3112 0.
PAGE 21
LTC3112 Applications Information fZERO1 = fZERO2 = 1 2πRFB CFB 1 1 ≅ 2π (RTOP +RFF ) CFF 2πRTOP CFF CFB + CPOLE 1 fPOLE2 = ≅ 2πCFBCPOLE RFB 2πCPOLE RFB fPOLE3 = the phase contributed by this additional pole is negligible. However, for loops with higher crossover frequencies this additional phase loss should be taken into account when designing the compensation network. LTC3112 FB 0.8V VCOMP + – RFILT CFILT INTERNAL VCOMP 1 3112 F08 2πCFF RFF Figure 8. Internal Loop Filter.
PAGE 22
LTC3112 Applications Information Phase Margin = fBUCK-BOOST + fERRORAMPLIFIER + 180° = –180° + 60° + 180° = 60° Similarly, if a phase margin of 45° is required, the target crossover frequency should be picked as the frequency at which the buck-boost converter phase reaches −195° so that the combined phase at the crossover frequency yields the desired 45° of phase margin.
PAGE 23
LTC3112 Applications Information both higher order poles (fPOLE2 and fPOLE3) occur at the common frequency, fP. In most cases this is a reasonable assumption since the zeros are typically located between 1kHz and 10kHz and the poles are typically located near each other at much higher frequencies. Given this assumption, the maximum phase boost, fMAX, provided by the compensated error amplifier is determined simply by the amount of separation between the poles and zeros as shown by the following equation.
PAGE 24
LTC3112 Applications Information 1 2π (33kΩ) (250kHz) ≅ 22pF Next, CFF can be chosen to set the second zero, fZERO2, to the common zero frequency of 5kHz. CFF = 1 ≅ 40pF 2π (845kΩ) (5kHz) In this case CFF was selected at 47pF giving a lower fre quency of 4kHz for the second zero. Finally, the resistor value RFF can be chosen to place the second pole.
PAGE 25
LTC3112 Typical Applications 1,2 or 3 Li-Ion to 5V 4.7µH VIN 3V TO 12.6V 1-3 CELL Li-ION SW1 SW2 BST1 VIN BST2 VOUT VCC + LTC3112 COMP BURST PWM 10µF OFF ON 1µF 680pF 33k FB RUN IOUT GND OVP VOUT 5V/1.5A VIN > 4V 845k 47pF 22pF PWM/SYNC – 90 0.1µF 47µF TO ADC 1V PER AMP BURST 70 60 50 10k 100pF 42.2k PWM 80 EFFICIENCY (%) 0.1µF 100 3.6VIN 7.2VIN 10.8VIN 40 158k 30 0.0001 3112 TA03 0.001 0.01 0.1 LOAD CURRENT (A) 1 3112 TA03a LTC3112 Synchronized to 1.
PAGE 26
LTC3112 Typical Applications 5V Backup Supply from Supercap Runs Down to VIN = 2V with 250mA Load VOUT 4.7µH 499k 0.1µF VIN 15V TO 2V + SW1 SW2 BST1 VIN BST2 VOUT VCC 1M LTC3112 0.1µF 680pF COMP VOUT 5V/250mA 33k 845k 22pF 220µF TANT PWM/SYNC 22mF SUPERCAP 499k FB RUN IOUT GND OVP TO ADC 1V PER AMP 1µF 100pF 47µF 47pF 42.2k 10k 158k 3112 TA05 VIN 5V/DIV VOUT 5V/DIV RUN 5V/DIV ILOAD 500mA/DIV 500ms/DIV 3112 TA05a 26 3112fc For more information www.linear.
PAGE 27
LTC3112 Typical Applications Stepped Response from 1 or 2 Li-Ion to 12V Adapter Source with VIN Feedforward Network 12V ADAPTER 1-OR 2-SERIES Li-ION CELLS 4.7µH LTC4352 IDEAL DIODE MBR735 0.1µF VIN VOUT 33pF SW1 SW2 BST1 VIN BST2 VOUT LTC3112 VCC COMP OPTIONAL 680pF VOUT 5V/2.5A, VIN > 5V 33k 845k 1µF OFF ON PWM/SYNC FB RUN IOUT GND OVP 1050k 47pF 22pF BURST PWM 100µF 1000k 0.1µF 10k TO ADC 1V PER AMP 100pF 42.
PAGE 28
LTC3112 Typical Applications Regulated 12V Output from Wide Input Supply Range 10µH 0.1µF VIN 4.5V TO 15V SW1 SW2 BST1 VIN BST2 VOUT VCC LTC3112 COMP BURST PWM OFF ON 2210k 47pF FB 1µF RUN IOUT GND OVP VOUT 12V 1A VIN > 5V 2A VIN > 9V 33k 820pF 22pF PWM/SYNC 22µF 0.1µF 10k TO ADC 1V PER AMP 100pF 42.2k 47µF 158k 3112 TA07 100 90 PWM BURST EFFICIENCY (%) 80 70 60 50 40 30 0.0001 28 5.0VIN 12VIN 0.001 0.01 0.1 LOAD CURRENT (A) 1 3112 TA07a 3112fc For more information www.
PAGE 29
LTC3112 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. DHD Package 16-Lead Plastic DFN (5mm × 4mm) (Reference LTC DWG # 05-08-1707 Rev A) 0.70 ±0.05 4.50 ±0.05 3.10 ±0.05 2.44 ±0.05 (2 SIDES) PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 4.34 ±0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 5.00 ±0.10 (2 SIDES) 9 4.00 ±0.10 (2 SIDES) R = 0.115 TYP 0.40 ±0.10 16 2.44 ±0.
PAGE 30
LTC3112 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. FE Package 20-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663 Rev J) Exposed Pad Variation CA 6.40 – 6.60* (.252 – .260) 4.95 (.195) 4.95 (.195) 20 1918 17 16 15 14 13 12 11 6.60 ±0.10 4.50 ±0.10 2.74 (.108) 6.40 2.74 (.252) (.108) BSC SEE NOTE 4 0.45 ±0.05 1.05 ±0.10 0.65 BSC 1 2 3 4 5 6 7 8 9 10 RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50* (.169 – .177) 0.
PAGE 31
LTC3112 Revision History REV DATE DESCRIPTION A 06/13 Clarified Absolute Maximum Rating: IOUT voltage spec. B C 10/13 06/14 PAGE NUMBER 2 Clarified RUN threshold specification. 3 Clarified thermal considerations last paragraph. 12 Clarified LTC4352 part designator. 28 Clarified Related Parts list.
PAGE 32
LTC3112 Typical Application 10W, 10V High Intensity LED Driver with Programmable Current and Low-Loss Sensing LED Current vs VIN and DAC Voltage 6.8µH VIN 5V TO 15V 22µF SW1 SW2 BST1 VIN BST2 VOUT VCC LTC3112 PWMSYNC 1µF COMP 0.1µF 68.1k 3300pF VOUT 200k 2210k FB RUN IOUT GND OVP DAC PROGRAMS LED CURRENT 1.0A AT 0V, 500mA AT 0.8V IOUT = (1-0.625 • VDAC)A 68.1k 22µF 158k VF = 3.6V PER LED AT 1A OPEN LAMP AT 12V 1000 750 500 250 47pF 3300pF 1250 LED CURRENT (mA) 0.