Datasheet

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LTC1773

1773fb

APPLICATIO S

DESCRIPTIO

FEATURES

TYPICAL APPLICATIO

■

High Efficiency: Up to 95%

■

Constant Frequency 550kHz Operation

■

from 2.65V to 8.5V

■

OUT

from 0.8V to V

■

OPTI-LOOP

Compensation Minimizes C

OUT

■

Synchronizable up to 750kHz

■

Selectable Burst Mode Operation

■

Low Quiescent Current: 80

µµ

µA

■

Low Dropout Operation: 100% Duty Cycle

■

Secondary Winding Regulation

■

Soft-Start

■

Current Mode Operation for Excellent Line and

Load Transient Response

■

Low Shutdown I

= 10µA

■

±1.5% Reference Accuracy

■

Precision 2.5V Undervoltage Lockout

■

Available in 10-Lead MSOP

The LTC

1773 is a current mode synchronous buck regu-

lator controller that drives external complementary power

MOSFETs using a fixed frequency architecture. The oper-

ating supply range is from 2.65V to 8.5V, making it

suitable for 1- or 2-cell lithium-ion battery powered appli-

cations. Burst Mode

operation provides high efficiency at

low load currents. 100% duty cycle provides low dropout

operation which extends operating time in battery-oper-

ated systems.

The operating frequency is internally set at 550kHz, allow-

ing the use of small surface mount inductors. For switch-

ing-noise sensitive applications, it can be synchronized up

to 750kHz. Peak current limit is user programmable with

an external high side sense resistor. A SYNC/FCB control

pin guarantees regulation of secondary windings regard-

less of load on the main output by forcing continuous

operation. Burst Mode operation is inhibited during syn-

chronization or when the SYNC/FCB pin is pulled low to

reduce noise and RF interference. Soft-start is provided by

an external capacitor.

Synchronous rectification increases efficiency and elimi-

nates the need for a Schottky diode, saving components

and board space. The LTC1773 comes in a 10-lead MSOP

package.

■

Cellular Telephones

■

RF PA Supplies

■

Portable Instruments

■

Wireless MODEMS

■

Distributed Power Systems

■

Notebook and Palm Top Computers, PDAs

■

Single and Dual Cell Lithium-Ion Powered Devices

Synchronous Step-Down

DC/DC Controller

Figure 1. Step-Down Converter

High Efficiency

OUTPUT CURRENT (mA)

EFFICIENCY (%)

100

1 100 1000 5000

1773 F1b

L = SUMIDA CDRH6D28-3R0

= 3.3V

= 5V

= 8V

30k

220pF

0.1µF

OUT

180µF

68µF

3µH

2.65V TO 8.5V

OUT

2.5V

SENSE

0.025Ω

1773 F01

LTC1773

RUN/SS

SYNC/FCB

GND

SENSE

–

Si9801DY

80.6k

169k

47pF

Protected by U.S. Patents, including 5481178, 6580258, 6304066, 6127815.

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.

All other trademarks are the property of their respective owners.

OPTI-LOOP and Burst Mode are registered trademarks of Linear Technology Corporation.

Summary of content (20 pages)

PAGE 1
LTC1773 Synchronous Step-Down DC/DC Controller U FEATURES DESCRIPTIO ■ The LTC®1773 is a current mode synchronous buck regulator controller that drives external complementary power MOSFETs using a fixed frequency architecture. The operating supply range is from 2.65V to 8.5V, making it suitable for 1- or 2-cell lithium-ion battery powered applications. Burst Mode® operation provides high efficiency at low load currents.
PAGE 2
LTC1773 U W W W ABSOLUTE AXI U RATI GS U W U PACKAGE/ORDER I FOR ATIO (Note 1) Input Supply Voltage .............................. –0.3V to 10.0V ITH Voltage ................................................ –0.3V to 2.5V RUN/SS, VFB, SENSE– Voltages .................. –0.3V to VIN SYNC/FCB Voltage ...................................... –0.3V to VIN BG, TG Voltages...........................................–0.3V to VIN SW Voltage ...................................................
PAGE 3
LTC1773 ELECTRICAL CHARACTERISTICS Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LTC1773 is guaranteed to meet performance specifications from 0°C to 70°C. Specifications over the –40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls.
PAGE 4
LTC1773 U W TYPICAL PERFOR A CE CHARACTERISTICS 120 100 80 60 40 20 0 0.5 1.0 1.5 2.0 2.5 3.0 VRUN/SS (V) 3.5 4.0 105 100 95 85 – 60 – 40 – 20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) 1773 G07 100 80 60 Burst Mode OPERATION 40 20 0 560 2.5 550 2.0 540 530 520 – 60 –40 –20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) FORCED CONTINUOUS 0 0.2 0.4 0.6 0.8 1.0 VITH (V) 1.2 1.4 1.
PAGE 5
LTC1773 U U U PIN FUNCTIONS ITH (Pin 1): Error Amplifier Compensation Point. The current comparator threshold increases with this control voltage. Nominal voltage range for this pin is 0V to 1.2V. Under high duty cycle and nearing current limit, ITH can swing up to 2.4V. RUN/SS (Pin 2): Combination of Soft-Start and Run Control Inputs. A capacitor to ground at this pin sets the ramp time to full current output. The time is approximately 0.8s/µF. Forcing this pin below 0.4V shuts down all the circuitry.
PAGE 6
LTC1773 U OPERATIO (Refer to Functional Diagram) Main Control Loop The LTC1773 uses a constant frequency, current mode step- down architecture to drive an external pair of complementary power MOSFETs. During normal operation, the external top P-channel power MOSFET turns on each cycle when the oscillator sets the RS latch, and turns off when the current comparator ICOMP resets the RS latch.
PAGE 7
LTC1773 U OPERATIO (Refer to Functional Diagram) and forces the main switch to stay off for the same number of cycles. Increasing the output load current slightly, above the minimum required for discontinuous conduction mode, allows constant frequency PWM. Frequency synchronization is inhibited when the feedback voltage, VFB, is below 0.6V. This prevents the external clock from interfering with the frequency foldback for short-circuit protection.
PAGE 8
LTC1773 U W U U APPLICATIONS INFORMATION The operating frequency and inductor selection are interrelated in that higher operating frequencies allow the use of smaller inductor and capacitor values. However, operating at a higher frequency generally results in lower efficiency because of external MOSFET gate charge losses. The inductor value has a direct effect on ripple current. The ripple current, ∆IL, decreases with higher inductance or frequency and increases with higher VIN or VOUT.
PAGE 9
LTC1773 U W U U APPLICATIONS INFORMATION PSYNC = VIN – VOUT 2 IMAX ) (1 + δ )RDS(ON) ( VIN where δ is the temperature dependency of RDS(ON) and K is a constant inversely related to the gate drive current. Both MOSFETs have I2R losses while the topside P-channel equation includes an additional term for transition losses, which are highest at high input voltages. The synchronous MOSFET losses are greatest at high input voltage or during a short-circuit when the duty cycle in this switch is nearly 100%.
PAGE 10
LTC1773 U W U U APPLICATIONS INFORMATION The ITH pin OPTI-LOOP compensation components can be optimized to provide stable, high performance transient response regardless of the output capacitors selected. Manufacturers such as Nichicon, United Chemicon and Sanyo should be considered for high performance throughhole capacitors. The OS-CON semiconductor dielectric capacitor available from Sanyo has the lowest ESR/size ratio of any aluminum electrolytic at a somewhat higher price.
PAGE 11
LTC1773 U W U U APPLICATIONS INFORMATION Synchronous switching removes the normal limitation that power must be drawn from the inductor primary winding in order to extract power from auxiliary windings. With continuous synchronous operation, power can be drawn from the auxiliary windings without regard to the primary output load. The secondary output voltage is set by the turns ratio of the transformer in conjunction with a pair of external resistors returned to the SYNC/FCB pin as shown in Figure 5.
PAGE 12
LTC1773 U W U U APPLICATIONS INFORMATION Other losses including CIN and COUT ESR dissipative losses, and inductor core losses, generally account for less than 2% total additional loss. Checking Transient Response The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in load current.
PAGE 13
LTC1773 U U W U APPLICATIONS INFORMATION PC Board Layout Checklist Design Example When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC1773. These items are also illustrated graphically in the layout diagram of Figure 6. Check the following in your layout: As a design example, assume the LTC1773 is used in a single lithium-ion battery powered cellular phone application. The VIN will be operating from a maximum of 4.2V down to about 2.
PAGE 14
LTC1773 U W U U APPLICATIONS INFORMATION For the selection of the external MOSFETs, the RDS(ON) must be guaranteed at 2.5V since the LTC1773 has to operate down to 2.7V. This requirement can be met by the Si9801DY. For the feedback resistors, choose R1 = 80.6k. R2 can then be calculated from equation (2) to be: ⎞ ⎛V R2 = ⎜ OUT – 1⎟ R1 = 171k; use 169k ⎠ ⎝ 0.8 Figure 7 shows the complete circuit along with its efficiency curve. 33pF 2.7V ≤ VIN ≤ 4.2V 30k 200pF 1 ITH SW 2 0.1µF – 10 9 RSENSE 0.
PAGE 15
LTC1773 U TYPICAL APPLICATIONS 33pF 2.7V ≤ VIN ≤ 8.4V 200pF 1 30k 2 0.1µF 10 SW SYNC/FCB VIN VFB TG 5 + L1 5µH 1:1 8 VOUT2 5V 100mA 22µF 6.2V 422k VOUT1 2.5V 2A 7 6 BG GND RSENSE 0.033Ω 9 RUN/SS SENSE– 3 4 MBR0530LT1 LTC1773 ITH 169k 1% + Si9801DY 80.6k 1% + 80.6k COUT 220µF 6.3V L1: COILTRONICS CTX5-4/BH ELECTRONICS 511-0033 RSENSE: IRC LR1206-01-R033-F CIN: SANYO POSCAP 10TPA100M COUT: AVX TPSD227M006R0100 CIN 100µF 10V 1773 TA01 Figure 8. Dual Output 2.
PAGE 16
LTC1773 U TYPICAL APPLICATIONS 33pF 2.7V ≤ VIN ≤ 4.2V 30k 200pF 1 2 0.1µF VIN 3 4 5 LTC1773 ITH SW RUN/SS SENSE – SYNC/FCB VIN VFB TG BG GND 10 RSENSE 0.025Ω 9 Si9803DY 8 L1 2µH 47µF 7 VOUT 3.3V 1A + + 6 Si9804DY + 249k 1% 80.6k 1% CIN 150µF 6.3V COUT 220µF 6.3V L1 2µH CIN: SANYO POSCAP 6TPA150M COUT: AVX TPSD227M006R0100 L1: COILTRONICS CTX2-4/BH ELECTRONICS 511-1010 RSENSE: IRC LR1206-01-R033-F 1773 TA03 Figure 10. Single Lithium-Ion to 3.
PAGE 17
LTC1773 U TYPICAL APPLICATIONS 2.7V ≤ VIN ≤ 4.2V 47pF 30k 1 220pF LTC1773 ITH 2 RUN/SS SENSE 4 SYNC/FCB 8 VIN VFB TG GND BG 5 100pF RSENSE 0.05Ω 9 – 0.1µF 3 750kHz CLK 10 SW VOUT 2.5V 1A L1 3µH 7 CIN 47µF 6.3V 6 169k 1% 4.7µF 6.3V Si6803DQ + 100pF 80.6k 1% + COUT 47µF 6.3V 0.1µF CIN, COUT: SANYO POSCAP 6TPA47M L1: SUMIDA CDRH5D28 3R0 RSENSE: IRC LR1206-01-R050-J 1773 TA06 Figure 11. 750kHz Single Lithium-Ion to 2.5V/1A Regulator VOUT3 2.5V 150mA 8 LT1762-2.
PAGE 18
LTC1773 U TYPICAL APPLICATIONS 2.7V ≤ VIN ≤ 6V 47pF 30k 1 220pF 2 0.1µF 3 4 5 100pF LTC1773 ITH 10 SW RUN/SS SENSE– VFB TG GND BG D1 MMSD914T1 8 VIN SYNC/FCB RSENSE 0.068Ω 9 VOUT 2V 800mA L1 4.2µH 7 CIN 47µF 6.3V 6 118k 1% 4.7µF 6.3V Si9801DY 100pF 80.6k 1% + D2* MBR0530LT1 0.1µF + COUT 47µF 6.3V *NOTE: D2 NOT NECESSARY. IF REMOVED, EFFICIENCY DROPS BY 1% CIN, COUT: SANYO POSCAP 6TPA47M L1: SUMIDA CDRH5D28 4R2 RSENSE: IRC LR1206-01-R068-F 1773 TA05 Figure 13.
PAGE 19
LTC1773 U TYPICAL APPLICATIONS 33pF 4.5V ≤ VIN ≤ 5.5V 200pF 1 30k 2 0.1µF VIN 3 4 5 LTC1773 ITH SW – RUN/SS SENSE SYNC/FCB VIN VFB TG BG GND 10 9 RSENSE 0.04Ω 8 L1 2.5µH 7 VOUT 1.8V 2A CIN 47µF 10V 6 COUT 47µF 10V Si9942DY 100k 1% 80.6k 1% CIN, COUT: TAIYO YUDEN LMK550BJ476MM L1: CDRH5D28 RSENSE: IRC LR1206-01-R040-F 1773 TA09 Figure 15. 5V to 1.8V/2A Regulator with Ceramic Capacitors U PACKAGE DESCRIPTIO MS Package 10-Lead Plastic MSOP (Reference LTC DWG # 05-08-1661) 0.889 ± 0.
PAGE 20
LTC1773 U TYPICAL APPLICATIO 33pF 30k 4.5V ≤ VIN ≤ 5.5V 200pF 1 2 0.1µF VIN 3 4 5 LTC1773 ITH SW RUN/SS SENSE– SYNC/FCB VIN VFB TG GND BG 10 9 L1 10µH 3:1 8 + Si2302DS 6 COUT2 150µF 6.3V D1 + 169k 1% CIN 150µF 6.3V VOUT1 2.5V 1A 7 Si6801DY 40.2k 1% + VOUT2 3.3V 500mA RSENSE 0.050Ω 249k 1% 47k COUT1 150µF 6.3V 0.1µF CIN, COUT1, COUT2: SANYO POSCAP 6TPA150M RSENSE: IRC LR1206-01-R050-F D1: BAS16 1773 TA04 Figure 16.