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LTC1877

1877fb

TYPICAL APPLICATION

FEATURES DESCRIPTION

High Efﬁ ciency

Monolithic Synchronous

Step-Down Regulator

The LTC

1877 s a high efﬁ ciency monolithic synchronous

buck regulator using a constant-frequency, current mode

architecture. Supply current during operation is only 10μA

and drops to < 1μA in shutdown. The 2.65V to 10V input

voltage range makes the LTC1877 ideally suited for both

single and dual Li-Ion battery-powered applications. 100%

duty cycle provides low dropout operation, extending bat-

tery life in portable systems.

Switching frequency is internally set at 550kHz, allowing

the use of small surface mount inductors and capacitors.

For noise sensitive applications the LTC1877 can be exter-

nally synchronized from 400kHz to 700kHz. Burst Mode

operation is inhibited during synchronization or when the

SYNC/MODE pin is pulled low, preventing low frequency

ripple from interfering with audio circuitry.

The internal synchronous switch increases efﬁ ciency

and eliminates the need for an external Schottky diode.

Low output voltages are easily supported with the 0.8V

feedback reference voltage. The LTC1877 is available in a

space saving 8-lead MSOP package. Lower input voltage

applications (less than 7V abs max) should refer to the

LTC1878 data sheet.

High Efﬁ ciency Step-Down Converter

APPLICATIONS

High Efﬁ ciency: Up to 95%

Very Low Quiescent Current: Only 10μA

During Operation

600mA Output Current at V

= 5V

2.65V to 10V Input Voltage Range

550kHz Constant-Frequency Operation

No Schottky Diode Required

Low Dropout Operation: 100% Duty Cycle

Synchronizable from 400kHz to 700kHz

Selectable Burst Mode

Operation or

Pulse-Skipping Mode

0.8V Reference Allows Low Output Voltages

Shutdown Mode Draws < 1μA Supply Current

±2% Output Voltage Accuracy

Current Mode Control for Excellent Line and Load

Transient Response

Overcurrent and Overtemperature Protected

Available in 8-Lead MSOP Package

Cellular Telephones

Wireless Modems

Personal Information Appliances

Portable Instruments

Distributed Power Systems

Battery-Powered Equipment

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

of Linear Technology Corporation. All other trademarks are the property of their respective

owners.

Efﬁ ciency vs Output Current

47μF***

LTC1877

GND

10μH*

10μF**

CER

220pF

OUT

†

3.3V

887k

1877 TA01

SYNC

RUN

2.65V

TO 10V

20pF

*TOKO D62CB A920CY-100M

**TAIYO-YUDEN CERAMIC LMK325BJ106MN

***SANYO POSCAP 6TPA47M

†

OUT

CONNECTED TO V

FOR 2.65V < V

< 3.3V

280k

OUTPUT CURRENT (mA)

0.1

EFFICIENCY (%)

1000

100

1877 TA02

1.0 100

= 3.6V

= 10V

OUT

= 3.3V

L = 10μH

Burst Mode OPERATION

= 7.2V

= 5V

Summary of content (18 pages)

PAGE 1
LTC1877 High Efficiency Monolithic Synchronous Step-Down Regulator DESCRIPTION FEATURES n n n n n n n n n n n n n n n High Efﬁciency: Up to 95% Very Low Quiescent Current: Only 10μA During Operation 600mA Output Current at VIN = 5V 2.65V to 10V Input Voltage Range 550kHz Constant-Frequency Operation No Schottky Diode Required Low Dropout Operation: 100% Duty Cycle Synchronizable from 400kHz to 700kHz Selectable Burst Mode® Operation or Pulse-Skipping Mode 0.
PAGE 2
LTC1877 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) Input Supply Voltage (VIN) ..........................–0.3V to 11V ITH, PLL LPF Voltage ................................. –0.3V to 2.7V RUN, VFB Voltages ...................................... –0.3V to VIN SYNC/MODE Voltage .................................. –0.3V to VIN SW Voltage ................................... –0.3V to (VIN + 0.3V) P-Channel MOSFET Source Current (DC)............ 800mA N-Channel MOSFET Sink Current (DC)................
PAGE 3
LTC1877 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 5V unless otherwise speciﬁed. SYMBOL PARAMETER CONDITIONS IPLL LPF Phase Detector Output Current Sinking Capability Sourcing Capability fPLLIN < fOSC fPLLIN > fOSC l l MIN TYP MAX UNITS 3 –3 10 –10 20 –20 μA μA RPFET RDS(ON) of P-Channel MOSFET ISW = 100mA 0.65 0.
PAGE 4
LTC1877 TYPICAL PERFORMANCE CHARACTERISTICS Efﬁciency vs Output Current Oscillator Frequency vs Temperature Reference Voltage vs Temperature 605 0.814 95 VIN = 5V VIN = 3.6V 90 EFFICIENCY (%) VIN = 10V VIN = 5V 75 70 65 60 50 0.1 0.799 0.794 545 535 50 25 75 0 TEMPERATURE (°C) 100 495 –50 125 2.53 595 2.52 585 2.51 555 545 535 525 100 RDS(ON) vs Input Voltage PULSE-SKIPPING MODE VIN = 4.2V L = 10μH 1.0 2.50 SYNCHRONOUS SWITCH 0.8 2.49 2.48 2.47 0.6 MAIN SWITCH 0.4 2.46 2.
PAGE 5
LTC1877 TYPICAL PERFORMANCE CHARACTERISTICS Switch Leakage vs Temperature Switch Leakage vs Input Voltage 1400 VIN = 10V RUN = 0V 1000 MAIN SWITCH 800 600 400 SYNCHRONOUS SWITCH 200 RUN = 0V SW 5V/DIV 12 10 50 25 75 0 TEMPERATURE (°C) 100 6 4 0 125 VOUT 20mV/DIV AC COUPLED SYNCHRONOUS SWITCH 8 IL 200mA/DIV 2 0 –50 –25 Burst Mode Operation 14 SWITCH LEAKAGE (nA) SWITCH LEAKAGE (nA) 1200 16 MAIN SWITCH 0 2 4 6 INPUT VOLTAGE (V) 8 1877 G12 10 1877 G13 Start-Up from Shutdown VIN =
PAGE 6
LTC1877 PIN FUNCTIONS RUN (Pin 1): Run Control Input. Forcing this pin below 0.4V shuts down the LTC1877. In shutdown, all functions are disabled drawing <1μA supply current. Forcing this pin above 1.2V enables the LTC1877. Do not leave RUN ﬂoating. ITH (Pin 2): Error Ampliﬁer Compensation Point. The current comparator threshold increases with this control voltage. Nominal voltage range for this pin is from 0.5V to 1.9V. VFB (Pin 3): Feedback Pin.
PAGE 7
LTC1877 FUNCTIONAL DIAGRAM VIN BURST DEFEAT PLL LPF Y Y = “0” ONLY WHEN X IS A CONSTANT “1” X 8 SLOPE COMP SYNC/MODE 7 – 6 VIN FREQ SHIFT + – – EA gm = 0.5m 7 – 0.55V 0.8V REF EN SLEEP – + VIN S Q R Q RS LATCH 6Ω + ICOMP BURST VIN SLEEP 2 ITH VIN RUN 1 + + VREF 0.8V SWITCHING LOGIC AND BLANKING CIRCUIT ANTI SHOOTTHRU 5 SW – OVDET 0.85V SHUTDOWN + IRCMP – 3 VFB OSC + 0.6V 0.
PAGE 8
LTC1877 OPERATION (Refer to Functional Diagram) The LTC1877 uses a constant-frequency, current mode step-down architecture. Both the main (P-channel MOSFET) and synchronous (N-channel MOSFET) switches are internal. During normal operation, the internal top power MOSFET is turned on each cycle when the oscillator sets the RS latch, and turned off when the current comparator, ICOMP, resets the RS latch.
PAGE 9
LTC1877 OPERATION Dropout Operation When the input supply voltage decreases toward the output voltage, the duty cycle increases toward the maximum on-time. Further reduction of the supply voltage forces the main switch to remain on for more than one cycle until it reaches 100% duty cycle. The output voltage will then be determined by the input voltage minus the voltage drop across the internal P-channel MOSFET and the inductor.
PAGE 10
LTC1877 APPLICATIONS INFORMATION Accepting larger values of ΔIL allows the use of low inductance, but results in higher output voltage ripple and greater core losses. A reasonable starting point for setting ripple current is ΔIL = 0.4(IMAX). The inductor value also has an effect on Burst Mode operation. The transition to low current operation begins when the inductor current peaks fall to approximately 250mA.
PAGE 11
LTC1877 APPLICATIONS INFORMATION pacitors. The POSCAP solid electrolytic capacitor available from Sanyo is an excellent choice for output bulk capacitors due to its low ESR/size ratio. Once the ESR requirement for COUT has been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement. When using tantalum capacitors, it is critical that they are surge tested for use in switching power supplies.
PAGE 12
LTC1877 APPLICATIONS INFORMATION The loop ﬁlter components CLP and RLP smooth out the current pulses from the phase detector and provide a stable input to the voltage controlled oscillator. The ﬁlter component’s CLP and RLP determine how fast the loop acquires lock. Typically RLP = 10k and CLP is 2200pF to 0.01μF. When not synchronized to an external clock, the internal connection to the VCO is disconnected. This disallows setting the internal oscillator frequency by a DC voltage on the VPLL LPF pin.
PAGE 13
LTC1877 APPLICATIONS INFORMATION temperature of the part. If the junction temperature reaches approximately 150°C, both power switches will be turned off and the SW node will become high impedance. To avoid the LTC1877 from exceeding the maximum junction temperature, the user will need to do some thermal analysis. The goal of the thermal analysis is to determine whether the power dissipated exceeds the maximum junction temperature of the part.
PAGE 14
LTC1877 APPLICATIONS INFORMATION Design Example A 15μH inductor works well for this application. For best efﬁciency choose a 1A inductor with less than 0.25Ω series resistance. As a design example, assume the LTC1877 is used in a single lithium-ion battery-powered cellular phone application. The input voltage will be operating from a maximum of 4.2V down to about 2.7V. The load current requirement is a maximum of 0.3A but most of the time it will be in standby mode, requiring only 2mA.
PAGE 15
LTC1877 TYPICAL APPLICATIONS Dual Lithium-Ion to 2.5V/0.6A Regulator Using All Ceramic Capacitors 1 2 RUN ITH 220pF PLL LPF SYNC/MODE 1 8 CIN** 10μF CER 4 VFB VIN GND SW 6 2 VIN b 8.4V 7 LTC1877 3 4-to 6-Cell NiCd/NiMH to 1.8V/0.6A Regulator Using All Ceramic Capacitors 4 VOUT 2.5V/0.6A 887k VFB VIN GND SW 1877 TA03 2 ITH 220pF PLL LPF SYNC/MODE 8 7 LTC1877 3 4 VFB GND VIN SW EXT CLOCK 700kHz 5 VOUT 1.8V/0.
PAGE 16
LTC1877 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. MS8 Package 8-Lead Plastic MSOP (Reference LTC DWG # 05-08-1660 Rev F) 0.889 ± 0.127 (.035 ± .005) 5.23 (.206) MIN 3.20 – 3.45 (.126 – .136) 3.00 ± 0.102 (.118 ± .004) (NOTE 3) 0.65 (.0256) BSC 0.42 ± 0.038 (.0165 ± .0015) TYP 8 7 6 5 0.52 (.0205) REF RECOMMENDED SOLDER PAD LAYOUT 0.254 (.010) 3.00 ± 0.102 (.118 ± .004) (NOTE 4) 4.90 ± 0.152 (.193 ± .
PAGE 17
LTC1877 REVISION HISTORY (Revision history begins at Rev B) REV DATE DESCRIPTION PAGE NUMBER B 11/11 Part marking for LTC1877IMS8 corrected from LTLU to LTLV 2 1877fb Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
PAGE 18
LTC1877 TYPICAL APPLICATION Single Lithium-Ion to 3.3V/0.3A Regulator 1 2 RUN ITH 220pF PLL LPF SYNC/MODE 10μF** 8 + – 7 VIN Li-Ion BATTERY 3V TO 4.2V LTC1877 3 4 VFB VIN GND SW 6 5 VOUT 3.3V/0.25A 10μH* 20pF 887k 47μF*** 280k *TOKO D62CB A920CY-100M **TAIYO YUDEN CERAMIC LMK325BJ106MN ***SANYO POSCAP 6TPA47M 1877 TA07 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC1174/LTC1174-3.