Datasheet

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LTC3538

3538fb

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

The LTC

3538 is a highly efﬁ cient, low noise, buck-boost

DC/DC converter that operates from input voltages above,

below, and equal to the output voltage. The topology

incorporated in the IC provides a continuous transfer

function through all operating modes, making the product

ideal for single Lithium Ion or multicell Alkaline or NiMH

applications where the output voltage is within the battery

voltage range.

The LTC3538 is suited for use in Micro Hard Disk Drive

(μHDD) applications with its 800mA current capability. Burst

Mode

operation provides high efﬁ ciency at light loads.

The LTC3538 includes two 0.17Ω N-channel and two

0.2Ω P-channel MOSFET switches. Operating frequency

is internally set to 1MHz to minimize solution footprint

while maximizing efﬁ ciency.

Other features include <5μA shutdown current, internal

soft-start, short circuit protection and thermal shutdown.

The LTC3538 is available in a low proﬁ le (0.75mm), ther-

mally enhanced 8-lead (2mm × 3mm) DFN package.

Li-Ion/Polymer to 3.3V at 800mA

■

Regulated Output with Input Voltages Above,

Below, or Equal to the Output

■

800mA Continuous Output Current from a Single

Lithium-Ion/Polymer Cell

■

Single Inductor

■

1.8V to 5.25V V

OUT

Range

■

2.4V to 5.5V V

Range

■

1MHz Fixed Frequency Operation

■

Output Disconnect in Shutdown

■

35μA Quiesecent Current in Burst Mode Operation

■

<5μA Shutdown Current

■

Internal Soft-Start

■

Small, Thermally Enhanced 8-Lead (2mm x 3mm)

DFN package

■

Miniature Hard Disk Drives

■

MP3 Players

■

Digital Cameras

■

Cellular Handsets

■

PDAs, Handheld PC

■

GPS Receivers

, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology

Corporation. All other trademarks are the property of their respective owners. Protected by

U.S. Patents including 5481178, 6304066, 6580258, 6166527, 6404251.

SW1

200k

464k

15k

10k

330pF

33pF

10μF

BURST

SW2

LTC3538

2.9V TO 4.2V

3.3

μH

GND

OUT

3.3V

800mA

OUT

22μF

PWM BURST

3538 TA01

ON OFF

*μP OPEN DRAIN I/O

Efﬁ ciency vs V

(V)

2.4

3.9 4.9

3538 TA01b

2.9 3.4 4.4 5.4

EFFICIENCY (%)

100

OUT

= 3.3V

LOAD

= 200mA

800mA Synchronous

Buck-Boost

DC/DC Converter

Summary of content (16 pages)

PAGE 1
LTC3538 800mA Synchronous Buck-Boost DC/DC Converter FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTION Regulated Output with Input Voltages Above, Below, or Equal to the Output 800mA Continuous Output Current from a Single Lithium-Ion/Polymer Cell Single Inductor 1.8V to 5.25V VOUT Range 2.4V to 5.
PAGE 2
LTC3538 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) VIN,VOUT Voltage .......................................... –0.3V to 6V SW1,SW2 Voltage DC............................................................ –0.3V to 6V Pulsed < 100ns ........................................ –0.3V to 7V BURST, FB, VC Voltage ................................. –0.3V to 6V Operating Temperature (Note 2)............... –40°C to 85°C Maximum Junction Temperature (Note 3)............. 125°C Storage Temperature Range........
PAGE 3
LTC3538 ELECTRICAL CHARACTERISTICS The ● denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = VOUT = 3.6V, unless otherwise noted. PARAMETER CONDITIONS Minimum Duty Cycle FB = 1.2V MIN TYP MAX ● ● Frequency Accuracy 0 0.8 1 1.2 UNITS % MHz Internal Soft-Start Time 1.5 ms Error Amp AVOL 80 dB Error Amp Source Current VC = 1.5V, FB = OV –13 μA Error Amp Sink Current VC = 1.5V, FB = 1.
PAGE 4
LTC3538 TYPICAL PERFORMANCE CHARACTERISTICS Switch Pins Before Entering Buck Mode TA = 25°C unless otherwise noted VOUT Ripple in Buck, Buck-Boost and Boost Modes at 500mA Load Burst Mode Sleep Current vs Temperature 45 VIN = 2.5V Burst Mode SLEEP CURRENT (μA) SW1 2V/DIV VIN = 3.3V SW2 2V/DIV VIN = 4.2V 3538 G04 50ns/DIV VIN = 3.9V VOUT = 3.3V AT 500mA 3538 G05 1μs/DIV VOUT = 3.
PAGE 5
LTC3538 TYPICAL PERFORMANCE CHARACTERISTICS VC On/Off Threshold vs Temperature TA = 25°C unless otherwise noted Load Transient in Fixed Frequency Mode Current Limit vs Temperature 4.0 0.80 VIN = VOUT = 3.6V VOUT 100mV/DIV 3.5 0.70 0.65 CURRENT LIMIT (A) VC ON/OFF THRESHOLD (V) 0.75 VC ON THRESHOLD 0.60 0.55 VC OFF THRESHOLD 0.50 3.0 ILOAD 200mA/DIV 2.5 100μs/DIV LINEAR CURRENT LIMIT 0 50 TEMPERATURE (°C) 100 1.5 –50 –25 0 25 50 TEMPERATURE (°C) 3538 G10 3538 G13 VIN = 3.3V VOUT = 3.
PAGE 6
LTC3538 PIN FUNCTIONS FB (Pin 1): Feedback Input to Error Amplifer. Connect resistive divider tap from VOUT to this pin to set the output voltage. The output voltage can be adjusted from 1.8V to 5.25V. Referring to the Block Diagram the output voltage is given by: VOUT = 1V • (1 + R1/R2) VC (Pin 2): Error Ampliﬁer Output. A frequency compensation network should be connected between this pin and FB to compensate the loop. See Closing the Feedback Loop section of the datasheet for further information.
PAGE 7
LTC3538 BLOCK DIAGRAM L1 SW1 SW2 7 6 ANTI-RING GATE DRIVERS AND ANTICROSS CONDUCTION CIN B 3.5A + – 2.3V + – C 0.5A – + R1 REVERSE CURRENT LIMIT AVERAGE CURRENT LIMIT PEAK CURRENT LIMIT PWM LOGIC AND OUTPUT PHASING PWM COMPARATORS UVLO CZ1 1V FB CP1 CP2 RZ 2 OSC 1MHz OFF ON BURST 4 1 = BurstMode OPERATION 0 = FIXED FREQUENCY 5μs DELAY R2 VC THERMAL SHUTDOWN BURST MODE CONTROL COUT 1 SOFT-START BURST VOUT 5 + – 2A + – VOUT D + – VIN 2.4V TO 5.
PAGE 8
LTC3538 OPERATION The LTC3538 provides high efﬁciency, low noise power for a wide variety of handheld electronic devices. The LTC proprietary topology allows input voltages above, below and equal to the output voltage through proper phasing of the four on-chip MOSFET switches. The error ampliﬁer output voltage on VC determines the output duty cycle of the switches. Since VC is a ﬁltered signal, it provides rejection of frequencies from well below the switching frequency.
PAGE 9
LTC3538 OPERATION Depending on the VC voltage, the LTC3538 will operate in either buck, buck-boost or boost mode. The four power switches are properly phased so the transfer between operating modes is continuous, smooth and transparent to the user. When VIN approaches VOUT the buck-boost region is entered, where the conduction time of the four-switch region is typically 150ns. Referring to Figures 1 and 2, the various regions of operation will now be described. 88% DMAX BOOST V4 (~2.
PAGE 10
LTC3538 OPERATION During the period when the LTC3538 is delivering energy to the output, the peak inductor current will be equal to 800mA typical and the inductor current will terminate each cycle at zero current. In Burst Mode operation the maximum average output current that can be delivered while maintaining output regulation is given by: IOUT _ BURST(BOOST) = 0.25• VIN A; VOUT > VIN VOUT IOUT _ BURST(BUCK) = 0.
PAGE 11
LTC3538 OPERATION Since the output current is discontinuous in boost mode, the ripple in this mode will generally be much larger than the magnitude of the ripple in buck mode. Minimizing solution size is usually a priority. Please be aware that ceramic capacitors can exhibit a signiﬁcant reduction in effective capacitance when a bias is applied. The capacitors exhibiting the highest reduction are those packaged in the smallest case size. importantly, leakage and parasitic capacitance need to be minimized.
PAGE 12
LTC3538 OPERATION ƒ FILTER _ POLE = VIN 2• VOUT • π • L • COUT Hz (in boost mode) 1 FB 8 VIN 2 VC 7 SW1 3 GND 6 SW2 4 BURST 5 where L is in Henries and COUT is in Farads. The output ﬁlter zero is given by: 1 ƒ FILTER _ ZERO = Hz 2• π •RESR • COUT where RESR is the equivalent series resistance of the output capacitor. VOUT VOUT A troublesome feature in boost mode is the right-half plane zero (RHP), given by: VIN 2 ƒ RHPZ = Hz 2• π •IOUT •L • VOUT 3538 F03 VIA TO GND PLANE Figure 3.
PAGE 13
LTC3538 OPERATION Most applications demand an improved transient response to allow a smaller output ﬁlter capacitor. To achieve a higher bandwidth, Type III compensation is required, providing two zeros to compensate for the double-pole response of the output ﬁlter.
PAGE 14
LTC3538 TYPICAL APPLICATION High Efﬁciency 5V/500mA from USB Input L1 3.3μH VOUT 5V, 500mA LTC3538 USB 4.35V TO 5.
PAGE 15
LTC3538 PACKAGE DESCRIPTION DCB Package 8-Lead Plastic DFN (2mm × 3mm) (Reference LTC DWG # 05-08-1718 Rev A) 0.70 ±0.05 1.35 ±0.05 3.50 ±0.05 1.65 ± 0.05 2.10 ±0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.45 BSC 1.35 REF RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED R = 0.115 TYP R = 0.05 5 TYP 2.00 ±0.10 (2 SIDES) 0.40 ± 0.10 8 1.35 ±0.10 1.65 ± 0.10 3.00 ±0.10 (2 SIDES) PIN 1 NOTCH R = 0.20 OR 0.
PAGE 16
LTC3538 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC3407 600mA (IOUT), 1.5MHz Dual Synchronous Step-Up DC/DC Converter VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V IQ = 40μA, ISD ≤1μA, SC70 Package LTC3410 300mA (ISW), 2.25MHz Synchronous Step-Down DC/DC Converter in SC70 VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V IQ = 26μA, ISD ≤1μA, MS Package LTC3411 1.25A (IOUT), 4MHz Synchronous Step-Down DC/DC Converter VIN: 2.625V to 5.5V, VOUT(MIN) = 0.8V IQ = 62μA, ISD ≤1μA, MS Package LTC3412 2.