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LTC3536

3536fa

Typical applicaTion

FeaTures DescripTion

1A Low Noise, Buck-Boost

DC/DC Converter

The LTC

3536 is an extended V

range, fixed frequency,

synchronous buck-boost DC/DC converter that operates

from input voltages above, below or equal to the regulated

output voltage. The topology incorporated in the LTC3536

provides low noise operation, making it ideal for RF and

precision measurement applications.

The device can produce up to 1A of continuous output

current, and it includes two N-channel and two P-channel

MOSFET switches. Switching frequencies up to 2MHz can

be programmed with an external resistor and the oscilla-

tor can be synchronized to an external clock. Quiescent

current is only 32µA in Burst Mode operation, maximizing

battery life in portable applications. Burst Mode operation

is user controlled and improves efficiency at light loads.

Other features include a 1µA shutdown current, internal

soft-start, overtemperature protection and current limit.

The LTC3536 is available in 12-pin thermally enhanced

MSOP and 10-pin (3mm × 3mm) DFN packages.

Efficiency vs Input Voltage

applicaTions

Regulated Output with Input Voltage Above, Below

or Equal to the Output Voltage

1.8V to 5.5V Input and Output Voltage Range

1A Continuous Output Current for

≥ 3V, V

OUT

= 3.3V

±1% Output Voltage Accuracy

Low Noise Buck-Boost Architecture

Up to 95% Efficiency

Programmable Frequency from 300kHz to 2MHz

Synchronizable Oscillator

Burst Mode

Operation: 32µA I

Internal 1ms Soft-Start

Output Disconnect in Shutdown

Shutdown Current: 1µA

Short-Circuit Protection

Small Thermally Enhanced 12-Pin MSOP

and 10-Pin (3mm × 3mm) DFN Packages

Wireless Inventory Terminals

Handheld Medical Instruments

Wireless Locators, Microphones

Supercapacitor Backup Power Supply

L, LT, LTC, LTM, Burst Mode, LTSpice, Linear Technology and the Linear logo are registered

trademarks and PowerPath and No R

SENSE

are trademarks of Linear Technology Corporation. All

other trademarks are the property of their respective owners.

SW1

SW2

OUT

10µF 22µF

47pF

6.49k

1.8V TO 5.5V

PWM BURST

LTC3536

4.7µH

49.9k

1000k

220pF

MODE/SYNC

SHDN

OUT

3.3V

1A FOR V

≥ 3V

221k

3536 TA01a

PGND SGND

OFF ON

INPUT VOLTAGE (V)

1.5

EFFICIENCY (%)

100

2.5

3.5

3536 TA01b

2 3

4.5

5.5

LOAD

= 1A

LOAD

= 200mA

Summary of content (28 pages)

PAGE 1
LTC3536 1A Low Noise, Buck-Boost DC/DC Converter Features n n n n n n n n n n n n n n Description Regulated Output with Input Voltage Above, Below or Equal to the Output Voltage 1.8V to 5.5V Input and Output Voltage Range 1A Continuous Output Current for VIN ≥ 3V, VOUT = 3.
PAGE 2
LTC3536 Absolute Maximum Ratings (Note 1) VIN, VOUT, (SVIN, PVIN) Voltage..................... –0.3V to 6V SW1, SW2 Voltage DC............................................................. –0.3V to 6V Pulsed (<100ns).........................................–1.0V to 7V VC, RT, FB, SHDN Voltage............................. –0.3V to 6V MODE/SYNC Voltage.................................... –0.3V to 6V Operating Junction Temperature Range (Notes 2, 3).............................................
PAGE 3
LTC3536 Electrical Characteristics The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 3.3V, VOUT = 3.3V, RT = 100kΩ unless otherwise noted. PARAMETER CONDITIONS Quiescent Current, Active (IVIN) VFB = 0.7V, VMODE/SYNC = 0V Input Current Limit VMODE/SYNC = 0V (Note 4) l MIN TYP 2 2.5 3.
PAGE 4
LTC3536 Typical Performance Characteristics Efficiency Li-Ion (3V, 3.7V, 4.2V) to 3.3V Output 90 80 80 70 70 EFFICIENCY (%) 100 90 60 50 VIN = 1.8V VIN = 2.5V VIN = 5.5V VIN = 1.8V BURST VIN = 2.5V BURST VIN = 5.5V BURST 30 20 10 0 0.001 0.01 0.1 LOAD CURRENT (A) 60 60 50 40 VIN = 3V VIN = 3.7V VIN = 4.2V VIN = 3V BURST VIN = 3.7V BURST VIN = 4.2V BURST 30 20 10 0 0.001 1 0.01 0.1 LOAD CURRENT (A) 3536 G01 INPUT CURRENT LIMIT (A) INPUT QUIESCENT CURRENT (mA) 10 8 6 4 2 0 1.5 2 2.
PAGE 5
LTC3536 Typical Performance Characteristics RDS(ON) for P-Channel Switch D 240 220 200 CHANGE IN VOLTAGE FROM 25°C (%) –45°C 0°C 25°C 85°C 125°C 260 180 160 140 120 100 80 60 1.5 2 2.5 3 3.5 4 4.5 5 INPUT VOLTAGE (V) 5.5 Oscillator Frequency vs RT 2.0 0.8 1.8 0.6 0.4 0.2 0 –0.2 –0.4 –0.6 3536 G10 MAXIMUM LOAD CURRENT (mA) MAXIMUM LOAD CURRENT (mA) 500 0 VOUT = 1.8V VOUT = 3.3V VOUT = 5.5V 1.5 2 2.5 3 3.5 4 4.5 5 INPUT VOLTAGE (V) 5.5 6 0.6 0.
PAGE 6
LTC3536 Typical Performance Characteristics Output Voltage Ripple in PWM Mode Output Voltage Ripple in Burst Mode Operation VOUT 20mV/DIV VIN = 2.5V VOUT 20mV/DIV VIN = 3.3V VOUT 20mV/DIV VIN = 5V TA = 25°C, VIN = VOUT = 3.3 V unless otherwise noted. Burst Mode Operation to PWM Mode Transient VOUT 50mV/DIV VIN = 5.5V VOUT 50mV/DIV INDUCTOR CURRENT 200mA/DIV VOUT = 3.3V ILOAD = 0.5A 2µs/DIV VOUT = 3.3V ILOAD = 4mA 3536 G19 50µs/DIV 3536 G20 VOUT 50mV/DIV VIN = 3.3V VOUT 50mV/DIV VIN = 1.
PAGE 7
LTC3536 Pin Functions (DFN/MSOP) RT (Pin 1/Pin 1): Oscillator Frequency Programming Input. Connect a resistor from RT to GND to program the internal oscillator frequency. The frequency is given by: fOSC (MHz) = 100/RT (kΩ) where RT is in kΩ and fOSC is between 0.3MHz and 2MHz. Tying the RT pin to VIN enables the internal 1.2MHz default oscillator frequency. SGND (Pin 2/Pin 2): Ground Connection for the LTC3536. A ground plane is highly recommended.
PAGE 8
LTC3536 Block Diagram L SW1 VIN 1.8V TO 5.5V SW2 SWA VOUT 1.8V TO 5.5V SWD –0.4A SWB – CIN GATE DRIVERS AND ANTICROSS CONDUCTION + + REVERSE CURRENT LIMIT SWC PGND + 3.4A – + 1.75V + PEAK CURRENT LIMIT UVLO ERROR AMP RTOP COUT – 2.5A PWM AND OUTPUT PHASING CURRENT LIMIT + + – SOFT-START 0.
PAGE 9
LTC3536 Operation Introduction The LTC3536 is a monolithic buck-boost converter that can operate with input and output voltages from as low as 1.8V to as high as 5.5V. A proprietary switch control algorithm allows the buck-boost converter to maintain output voltage regulation with input voltages that are above, below or equal to the output voltage. Transitions between these operating modes are seamless and free of transients and subharmonic switching.
PAGE 10
LTC3536 Operation Whether operating from its internal oscillator or when synchronized to an external clock signal, the LTC3536 is able to operate with a switching frequency from 300kHz to 2MHz, providing the ability to minimize the size of the external components and optimize the power conversion efficiency. Error Amplifier The LTC3536 has an internal high gain operational amplifier which provides frequency compensation of the control loop that maintains output voltage regulation.
PAGE 11
LTC3536 Operation Reverse Current Limit Burst Mode OPERATION In PWM mode operation the LTC3536 has the ability to actively conduct current away from the output if that is necessary to maintain regulation. If the output is held above regulation, this could result in large reverse currents. This situation can occur if the output of the LTC3536 is held up momentarily by another supply as may occur during a power-up or power-down sequence.
PAGE 12
LTC3536 Operation soft-start period is reset by thermal shutdown and UVLO events on VIN and the mode of operation is always PWM. In case the output voltage at start -up is already precharged above 90% (typically) of the target value, the internal softstart is skipped and the LTC3536 immediately enters the mode of operation that has been set on the MODE pin. If the MODE pin is tied high and Burst Mode operation is selected, the output voltage is regulated smoothly to the target voltage value.
PAGE 13
LTC3536 Applications Information reducing the peak current to be closer to the average output current and therefore minimize resistive losses due to high RMS currents. However, a larger inductor within any given inductor family will generally have a greater series resistance, thereby counteracting this efficiency advantage. An inductor used in LTC3536 applications should have a saturation current rating that is greater than the worst-case average inductor current plus half the ripple current.
PAGE 14
LTC3536 Applications Information Neglecting the capacitor ESR and ESL, the peak-to-peak output voltage ripple can be calculated by the following formulas, where f is the frequency in MHz, COUT is the capacitance in µF and ILOAD is the output current in amps.
PAGE 15
LTC3536 Applications Information VIN The denominator of the buck mode transfer function exhibits a pair of resonant poles generated by the LCOUT filtering of the power stage. The resonant frequency of the power stage, fO, is given by the following expression where L is the value of the inductor in henries. VOUT COUT A D L VC PWM RC RS B RLOAD C ωO = 3536 F04 Figure 4. Small-Signal Model variety of output voltages, switching frequencies, and external component values supported by the LTC3536.
PAGE 16
LTC3536 Applications Information increasing gain and decreasing phase at higher frequencies. As a result, the crossover frequency in boost mode operation generally must be set lower than in buck mode in order to maintain sufficient phase margin.
PAGE 17
LTC3536 Applications Information Finally, the magnitude of the quality factor of the power stage in buck-boost mode operation is given by the following expression: Q= LCOUT (RLOAD +RC ) RS +RLOAD • ε 2 L + COUTRLOADRC • ε 2 +RSCOUT (RLOAD +RC ) Compensation of the Voltage Loop The small-signal models of the LTC3536 reveal that the transfer function from the error amplifier output, VC, to the output voltage is characterized by a set of resonant poles and a possible zero generated by the ESR of the output
PAGE 18
LTC3536 Applications Information In most applications the compensation network is designed so that the loop crossover frequency is above the resonant frequency of the power stage, but sufficiently below the boost mode right-half plane zero to minimize the additional phase loss. Once the crossover frequency is decided upon, the phase boost provided by the compensation network is centered at that point in order to maximize the phase margin.
PAGE 19
LTC3536 Applications Information Loop Compensation Example This section provides an example illustrating the design of a compensation network for a typical LTC3536 application circuit. In this example a 3.3V regulated output voltage is generated with the ability to supply 300mA load from an input power source ranging from 1.8V to 5.5V. To optimize efficiency 1MHz switching frequency has been chosen. In this application the maximum inductor current ripple will occur at the highest input voltage.
PAGE 20
LTC3536 Applications Information occur at the same frequency, fZ, and both higher order poles (fPOLE2 and fPOLE3) occur at the common frequency, fP . This is a good starting point for determining the compensation network. However the Bode plot for the complete loop should be checked overall operating conditions and for variations in components values to ensure that sufficient phase margin and gain margin exists in all cases.
PAGE 21
LTC3536 Applications Information A Bode plot of the error amplifier with the designed compensation component values is shown in Figure 11. The Bode plot confirms that the peak phase occurs at 37.8kHz and the phase boost at that point is about 60°. In addition, the gain at the peak phase frequency is 2dB which is close to the design target.
PAGE 22
LTC3536 Applications Information In order to improve the stability also in buck-boost mode of operation, the two compensation zeros could be move to different frequency: 1 2πRFBCFB = 5.4kHz The new RFB value is: 1 RFB = = 81.9kΩ ≈ 80.6kΩ 2π • (180pF ) • 10.8kHz As consequence the fPOLE2 will move to higher frequency: fPOLE2 = 2πCPOLERFB = 532kHz In buck mode there is no right-half plane zero and the stability is normally achieved.
PAGE 23
LTC3536 Applications Information Switching Frequency Selection Higher switching frequencies facilitate the use of smaller inductors as well as smaller input and output filter capacitors which results in a smaller solution size and reduced component height. However, higher switching frequencies also generally reduce conversion efficiency due to the increased switching losses.
PAGE 24
LTC3536 Applications Information Figure 17a. Fabrication Layer of Example PCB with 4 Layers Figure 17b. Top Layer of Example PCB Figure 17c.
PAGE 25
LTC3536 Typical Applications 300kHz High Efficiency Li-Ion to 3.6V at 1A, Pulsed with Manual Mode Control Efficiency 3.6V, 300kHz vs Load Current 100 15µH* 10µF SW1 VIN SW2 VOUT 100pF LTC3536 OFF ON 332k 845k MODE/SYNC FB SHDN 1nF 49.9k RT 47µF 15k VC VOUT 3.6V 100µF 1A 80 EFFICIENCY (%) VIN 3V TO 4.2V 90 70 60 50 40 30 20 GND 169k VIN = 3V VIN = 3.6V VIN = 4.2V 10 0 0.001 3536 TA02 *COILCRAFT XAL4040 0.01 0.1 LOAD CURRENT (A) 1 3536 TA02b USB to 5V Converter Load Step 2.
PAGE 26
LTC3536 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. DD Package 10-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1699 Rev C) R = 0.125 TYP 0.40 ± 0.10 6 10 0.70 ±0.05 3.55 ±0.05 1.65 ±0.05 2.15 ±0.05 (2 SIDES) 3.00 ±0.10 (4 SIDES) 1.65 ± 0.10 (2 SIDES) PIN 1 NOTCH R = 0.20 OR 0.35 × 45° CHAMFER PIN 1 PACKAGE TOP MARK OUTLINE (SEE NOTE 6) 0.25 ± 0.05 0.75 ±0.05 0.200 REF 0.50 BSC 2.38 ±0.05 (2 SIDES) 5 0.
PAGE 27
LTC3536 Revision History REV DATE DESCRIPTION A 11/11 Add new bullet Output Disconnect in Shutdown to Features bullet list. PAGE NUMBER 1 In the Absolute Maximum Ratings section change (Notes 1, 2) to (Note 1) and (Note 2) to (Notes 2, 3). 2 In Electrical Characteristics table add conditions for Error Amplifier Sink Current and Error Amplifier Source Current. 3 In Pin Functions add Exposed Pad Pin 13 and remove last sentence to PGND pin description.
PAGE 28
LTC3536 Typical Application Backup Power Supply 4.7µH* VSUPERCAP 1.8V TO 5.5V SW1 VIN 10µF SW2 VOUT 6.49k MODE/SYNC LTC3536 100k 300mA FOR VIN ≥ 1.8V 1A FOR VIN ≥ 3V VSYS 3.3V MAIN POWER 12V DC/DC 22µF 845k 0.1µF R2 47pF SHDN FB RT VC 49.9k VH VL 330pF GND 182k 10pF 20k 20k VCC UV OV LTC2912-2 R1 866k DIS GND 6.