Datasheet

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LTC3521

3521fb

For more information www.linear.com/LTC3521

Typical applicaTion

FeaTures

applicaTions

DescripTion

1A Buck-Boost DC/DC

and Dual 600mA Buck

DC/DC Converters

The LT C

3521 combines a 1A buck-boost DC/DC converter

and dual 600mA synchronous buck DC/DC converters. The

1.1MHz switching frequency minimizes the solution foot

print while maintaining high efficiency. All three converters

feature soft-start and internal compensation to minimize

the solution footprint and simplify the design process.

The buck converters are current mode controlled and

utilize an internal synchronous rectifier to improve ef

ficiency. The

buck converters support 100% duty cycle

operation to extend battery life. If the PWM pin is held

low, the buck converters automatically transition from

Burst Mode operation to PWM mode at high loads. With

the PWM pin held high, the buck converters remain in low

noise, 1.1MHz PWM mode.

The buck-boost converter features continuous conduction

operation to maximize efficiency and minimize noise. At

light loads, the buck-boost converter can be operated in

Burst Mode operation to improve efficiency and reduce

no-load standby current.

The LTC3521 provides a <2μA shutdown mode, over-

temperature shutdown and current limit protection

on all converters. The LTC3521 is available in a 24-pin

0.75mm

× 4mm × 4mm QFN package, and

a 20-pin ther-

mally enhanced TSSOP package.

Three High Efficiency DC/DC Converters:

Buck-Boost (V

OUT

: 1.8V to 5.25V, I

OUT

: 1A)

Dual Buck (V

OUT

: 0.6V to V

, I

OUT

: 600mA)

1.8V to 5.5V Input Voltage Range

Pin-Selectable Burst Mode

Operation

30µA Total Quiescent Current in Burst Mode

Operation

Independent Power Good Indicator Outputs

Integrated Soft-Start

Thermal and Overcurrent Protection

<2µA Current in Shutdown

Small 4mm × 4mm QFN and Thermally Enhanced

TSSOP Packages

Bar Code Readers

Medical Instruments

Handy Terminals

PDAs, Handheld PCs

GPS Receivers

IN1

IN2

SW2

SW3

FB2

FB3

OUT1

LTC3521

SHDN2

SHDN1

1.0M

137k

68.1k

22µF

10µF

4.7µF

Li-Ion

4.7µH 4.7µH

OUT1

3.3V

800mA

(1A, V

> 3.0V)

OUT2

1.8V

600mA

100k

10µF

4.7µH

OUT3

1.2V

600mA

2.4V TO 4.2V

221k

3521 TA01a

SHDN3

PWM

SW1A

SW1B

FB1

PGOOD2

PGOOD1

PGOOD3PGND1A

PGND2GNDPGND1B

OFF

PWM

BURST

(V)

2.4

EFFICIENCY (%)

5.4

3521 TA01b

3.4

4.4

100

OUT1

= 3.3V

OUT

= 500mA

OUT2

= 1.8V

OUT

= 200mA

OUT3

= 1.2V

OUT

= 200mA

Efficiency vs V

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

and PowerPath is a trademark of Linear Technology Corporation. All other trademarks are the

property of their respective owners. Protected by U. S. Patents, including 6404251, 6166527.

Summary of content (22 pages)

PAGE 1
LTC3521 1A Buck-Boost DC/DC and Dual 600mA Buck DC/DC Converters Features n n n n n n n n n Description Three High Efficiency DC/DC Converters: Buck-Boost (VOUT: 1.8V to 5.25V, IOUT: 1A) Dual Buck (VOUT: 0.6V to VIN, IOUT: 600mA) 1.8V to 5.
PAGE 2
LTC3521 Absolute Maximum Ratings (Note 1) PVIN1, PVIN2, VIN Voltage.............................. –0.3V to 6V SW1A, SW1B, SW2, SW3 Voltage DC............................................................. –0.3V to 6V Pulsed < 100ns.............................................–1V to 7V Voltage, All Other Pins.................................. –0.3V to 6V Operating Junction Temperature Range (Notes 2, 5)............................................. –40°C to 125°C Storage Temperature Range...................
PAGE 3
LTC3521 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, PVIN1, PVIN2 = 3.6V, VOUT1 = 3.3V, unless otherwise noted. PARAMETER CONDITIONS MIN Input Voltage l Quiescent Current—Shutdown VSHDN1 = VSHDN2 = VSHDN3 = 0V (Note 6) Burst Mode Quiescent Current VFB1 = 0.66V, VFB2 = 0.66V, VFB3 = 0.66V, VPWM = 0V TYP 1.8 0.01 l MAX 5.
PAGE 4
LTC3521 Electrical Characteristics Note 3: Current measurements are performed when the LTC3521 is not switching. The current limit values in operation will be somewhat higher due to the propagation delay of the comparators. Note 4: The LTC3521 is tested in a proprietary test mode that connects each FB pin to the output of the respective error amplifier. Note 5: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions.
PAGE 5
LTC3521 Typical Performance Characteristics Buck-Boost Switches RDS(ON) vs Temperature Buck Switches RDS(ON) vs Temperature 350 160 VIN = 3.6V 140 VOUT1 = 3.3V 0.8 NMOS (SWITCHES B AND C) 60 0.6 PMOS CHANGE FROM 25°C (%) RDS(ON) (mΩ) NMOS 200 150 100 40 50 20 0 –40 –20 0 20 40 60 80 100 120 0 20 40 60 80 100 120 CHANGE IN FEEDBACK VOLTAGE FROM 25°C (%) 2.0 CHANGE FROM VIN = 3.6V (%) 1.5 1.0 0.5 0 –0.5 –1.0 –1.5 3.3 3.8 4.3 4.8 5.3 0.1 0 –0.1 –0.2 –0.3 –0.4 –0.
PAGE 6
LTC3521 Typical Performance Characteristics QUIESCENT CURRENT (µA) 60 TA = 25°C, unless otherwise noted. Buck-Boost Load Step, 0mA to 750mA No Load Quiescent Current vs VIN VOUT 100mV/DIV 55 VIN = 3.6V, VOUT = 3.3V L = 4.7µH COUT = 22µF 50 INDUCTOR CURRENT 500mA/DIV 45 3521 G15 100µs/DIV 40 1.8 2.3 2.8 3.3 3.8 4.3 4.
PAGE 7
LTC3521 Pin Functions (FE/UF Packages) FB3 (Pin 1/Pin 23): Feedback Voltage for the Buck Converter Derived from a Resistor Divider Connected to the Buck VOUT3 Output Voltage. The buck output voltage is given by the following equation, where R1 is a resistor between FB3 and ground, and R2 is a resistor between FB3 and the buck output voltage: ⎛ R2 ⎞ VOUT3 = 0.
PAGE 8
LTC3521 Pin Functions (FE/UF Packages) PVIN1 (Pin 13/Pin 11): High current power supply connection used to supply switch A of the buck-boost converter. This pin should be bypassed by a 4.7µF, or larger, ceramic cap. The bypass capacitor should be placed as close to the pin as possible and should have a short return path to ground. Pins VIN, PVIN1, and PVIN2 must be connected together in the application circuit. NC (Pin 13, UF Package Only): No Internal Connection.
PAGE 9
LTC3521 11 (UF Package) 15 PVIN1 14 SW1A 16 SW1B VOUT1 D A INTERNAL VCC 5 B VIN C + – REVERSE ILIMIT 0.375A IZERO 0A + – 22 PGOOD1 + – PWM FB1 0.6V SOFT-START RAMP SHDN1 SHDN3 SHDN2 PVIN2 PVIN2 4 PVOUT BUCK-BOOST PWM LOGIC GATE DRIVES 1 – 2.1A PGND1A PGND1B 7 FILTER FORWARD + ILIMIT 0.546V + + – Block Diagram PVIN2 OSCILLATOR 8 10 9 UVLO SW2 GATE DRIVES PGND2 0A + – BUCK PWM LOGIC BUCK PWM LOGIC ZERO CROSSING 1.05A + – ILIMIT 0.
PAGE 10
LTC3521 Operation The LTC3521 combines dual synchronous buck DC/DC converters and a 4-switch buck-boost DC/DC converter in a 4mm × 4mm QFN package and a 20-pin thermally enhanced TSSOP package. The buck-boost converter utilizes a proprietary switching algorithm which allows its output voltage to be regulated above, below or equal to the input voltage. The buck converters provide a high efficiency lower voltage output and support 100% duty cycle operation to extend battery life.
PAGE 11
LTC3521 Operation Soft-Start Buck-Boost Converter Operation The buck converters have an internal voltage mode soft-start circuit with a nominal duration of 800μs. The converters remain in regulation during soft-start and will therefore respond to output load transients which occur during this time. In addition, the output voltage rise time has minimal dependency on the size of the output capacitor or load current.
PAGE 12
LTC3521 Operation modes. These advantages result in increased efficiency and stability in comparison to the traditional 4-switch buck-boost converter. this case, the increased bandwidth created by decreasing R2 is used to counteract the reduced converter bandwidth caused by the large output capacitor. Error Amplifier and Compensation Current Limit Operation The buck-boost converter utilizes a voltage mode error amplifier with an internal compensation network as shown in Figure 2.
PAGE 13
LTC3521 Operation Burst Mode Operation With the PWM pin held low, the buck-boost converter operates utilizing a variable frequency switching algorithm designed to improve efficiency at light load and reduce the standby current at zero load. In Burst Mode operation, the inductor is charged with fixed peak amplitude current pulses. These current pulses are repeated as often as necessary to maintain the output regulation voltage.
PAGE 14
LTC3521 Applications Information The basic LTC3521 application circuit is shown as the Typical Application on the front page of this data sheet. The external component selection is determined by the desired output voltages, output currents and ripple voltage requirements of each particular application. Basic guidelines and considerations for the design process are provided in this section. Table 1 depicts the recommended inductance for several common output voltages. Table 1.
PAGE 15
LTC3521 applications information Buck Input Capacitor Selection Buck-Boost Output Voltage Programming The PVIN2 pin provides current to the buck converter power switch and is the supply pin for the IC’s internal circuitry. It is recommended that a low ESR ceramic capacitor with a value of at least 4.7µF be used to bypass this pin. The capacitor should be placed as close to the pin as possible and have a short return to ground.
PAGE 16
LTC3521 applications information be calculated from the following formulas, where f is the frequency in MHz and L is the inductance in μH: ΔIL,P-P,BUCK = 1 VOUT ( VIN – VOUT ) • fL VIN ΔIL,P-P,BOOST = 1 VIN ( VOUT – VIN ) • fL VOUT In addition to affecting output current ripple, the size of the inductor can also affect 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 17
LTC3521 applications information Capacitor Vendor Information PCB Layout Considerations Both the input and output capacitors used with the LTC3521 must be low ESR and designed to handle the large AC currents generated by switching converters. The vendors in Table 5 provide capacitors that are well suited to LTC3521 application circuits. The LTC3521 switches large currents at high frequencies. Special care should be given to the PCB layout to ensure stable, noise-free operation.
PAGE 18
LTC3521 applications information KELVIN TO VOUT PAD BUCK VOUT VIA TO GROUND PLANE MINIMIZE TRACE LENGTH NC (19) SW2 (20) PGND1A (9) PVIN2 (22) FB3 (23) FB2 (24) MINIMIZE TRACE LENGTH PGOOD1 (4) SW1A (15) VIN (5) SW1B (14) GND (6) NC (13) DIRECT TIE BACK TO GND PIN KELVIN TO VOUT PAD BUCK VOUT BUCK-BOOST VOUT KELVIN TO VOUT PAD PGND1B (12) VOUT1 (16) PVIN (11) PGOOD2 (3) SHDN1 (10) SW3 (17) SHDN3 (9) PGOOD3 (2) FB1 (8) PGND2 (18) PWM (7) SHDN2 (1) MINIMIZE TRACE LENGTH UNINTER
PAGE 19
LTC3521 Typical Application Dual Supercapacitor to 3.3V at 200mA, 1.8V at 50mA and 1.2V at 100mA Backup Power Supply VIN 1.8V TO 5.5V + + VOUT1 3.3V 200mA 1F 1F C4 L1 4.7µF 4.7µH PVIN1 VIN PVIN2 SW2 SW1A L2 4.7µH C1 22µF R2 221k FB2 VOUT1 R1 1.0M LTC3521 OFF BURST ON PWM FB1 SHDN1 SHDN2 SHDN3 SW3 C2 10µF R3 137k SW1B VOUT2 1.8V 50mA R4 68.1k L3 4.7µH VOUT3 1.
PAGE 20
LTC3521 Package Description FE Package 20-Lead Plastic TSSOP (4.4mm) FE Package 20-Lead PlasticLTC TSSOP (Reference DWG(4.4mm) # 05-08-1663 Rev J) (Reference LTC DWG # 05-08-1663 Rev J) Exposed Pad CB Variation Exposed Pad Variation CB 6.40 – 6.60* (.252 – .260) 3.86 (.152) 3.86 (.152) 20 1918 17 16 15 14 13 12 11 6.60 ±0.10 2.74 (.108) 4.50 ±0.10 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 – .
PAGE 21
LTC3521 Revision History REV DATE DESCRIPTION PAGE NUMBER A 11/10 Addition of PGND1A reflected throughout data sheet Addition of VIN to Typical Applications Revised Note 2 3 Changes to Block Diagram Change to Operation Soft-Start section B 08/13 1, 19, 22 Corrected pin numbers on Block Diagram UF package 9 11, 13 9 3521fb Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use.
PAGE 22
LTC3521 Typical Application Li-Ion to 3.3V at 800mA, 1.8V at 600mA and 1.2V at 600mA with Sequenced Start-Up VIN 2.4V TO 4.2V + 4.7µF L1 4.7µH Li-Ion L3 4.7µH VOUT3 1.2V 600mA C3 10µF R5 100k R6 100k 499k PVIN1 VIN PVIN2 SW2 SW1A SW1B FB2 SW3 BURST PWM L2 4.7µH C2 10µF R3 137k LTC3521 FB3 VOUT1 PGOOD3 SHDN1 SHDN2 PGOOD1 PGOOD2 PWM SHDN3 PGND1A PGND1B GND PGND2 VOUT2 1.8V 600mA C1 22µF R1 1.0M VOUT1 3.3V 800mA (1A, VIN > 3.