Datasheet

ManualsBrandsLINEAR TECHNOLOGY ManualsVoltage RegulatorsLTC1878EMS8#PBF

LTC1878

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High Efficiency: Up to 95%

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Very Low Quiescent Current: Only 10

During Operation

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600mA Output Current at V

= 3.3V

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2.65V to 6V Input Voltage Range

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550kHz Constant Frequency Operation

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Synchronizable from 400kHz to 700kHz

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Selectable Burst Mode

Operation or

Pulse Skipping Mode

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No Schottky Diode Required

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Low Dropout Operation: 100% Duty Cycle

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0.8V Reference Allows Low Output Voltages

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Shutdown Mode Draws < 1µA Supply Current

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±2% Output Voltage Accuracy

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Current Mode Control for Excellent Line and

Load Transient Response

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Overcurrent and Overtemperature Protected

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Available in 8-Lead MSOP Package

The LTC

1878 is a high efficiency monolithic synchro-

nous 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

6V input voltage range makes the LTC1878 ideally suited

for single Li-Ion battery-powered applications. 100% duty

cycle provides low dropout operation, extending battery

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 LTC1878 can be

externally 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 efficiency and

eliminates the need for an external Schottky diode. Low

output voltages are easily supported with the 0.8V feed-

back reference voltage. The LTC1878 is available in a

space saving 8-lead MSOP package.

For higher input voltage (11V abs max) applications, refer

to the LTC1877 data sheet.

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Cellular Telephones

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Wireless Modems

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Personal Information Appliances

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Portable Instruments

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Distributed Power Systems

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Battery-Powered Equipment

High Efficiency Step-Down Converter

High Efficiency

Monolithic Synchronous

Step-Down Regulator

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

Burst Mode is a trademark of Linear Technology Corporation.

Efficiency vs Output Load Current

47µF***

LTC1878

GND

10µH*

22µF**

CER

220pF

OUT

†

3.3V

***

†

TOKO D62CB A920CY-100M

TAIYO-YUDEN CERAMIC JMK325BJ226MM

SANYO POSCAP 6TPA47M

OUT

CONNECTED TO V

FOR 2.65V < V

< 3.3V

887k

20pF

280k

1878 TA01

SYNC

RUN

2.65V

TO 6V

OUTPUT CURRENT (mA)

EFFICIENCY (%)

100

0.1 10 100 1000

1878 TA02

= 6V

= 4.2V

= 3.6V

Burst Mode OPERATION

OUT

= 3.3V

L = 10µH

APPLICATIO S

FEATURES

TYPICAL APPLICATIO

DESCRIPTIO

Summary of content (16 pages)

PAGE 1
LTC1878 High Efficiency Monolithic Synchronous Step-Down Regulator U DESCRIPTIO FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ The LTC ®1878 is a high efficiency 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 6V input voltage range makes the LTC1878 ideally suited for single Li-Ion battery-powered applications.
PAGE 2
LTC1878 W W W AXI U U ABSOLUTE RATI GS U U W PACKAGE/ORDER I FOR ATIO (Note 1) Input Supply Voltage (VIN)...........................– 0.3V to 7V 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) ...........
PAGE 3
LTC1878 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. V IN = 3.6V unless otherwise specified. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS IPK Peak Inductor Current VIN = 3.3V, VFB = 0.7V, Duty Cycle < 35% 0.8 1.0 1.25 A ILSW SW Leakage ±0.01 ±1 µA 1.0 1.5 V ±0.01 ±1 µA 0.7 1.5 V ±0.
PAGE 4
LTC1878 U W TYPICAL PERFOR A CE CHARACTERISTICS Reference Voltage vs Temperature Efficiency vs Output Current 95 0.814 VIN = 3V VIN = 3.6V VIN = 4.2V 80 VIN = 6V 75 585 0.804 0.799 0.794 10 100 1 OUTPUT CURRENT (mA) 1000 50 25 75 0 TEMPERATURE (°C) 100 –25 25 50 75 0 TEMPERATURE (°C) Output Voltage vs Load Current RDS(ON) vs Input Voltage 0.8 SYNCHRONOUS SWITCH 0.7 565 555 545 535 525 515 4 6 1.80 1.79 8 SUPPLY VOLTAGE (V) 250 0.9 VIN = 3V 0.8 0.7 VIN = 5V 0.6 0.
PAGE 5
LTC1878 U W TYPICAL PERFOR A CE CHARACTERISTICS Switch Leakage vs Temperature 2.5 Switch Leakage vs Input Voltage 1.2 VIN = 7V RUN = 0V RUN = 0V SW 5V/DIV 1.0 SWITCH LEAKAGE (nA) 2.0 SWITCH LEAKAGE (µA) Burst Mode Operation 1.5 MAIN SWITCH 1.0 SYNCHRONOUS SWITCH 0.5 0 – 50 – 25 SYNCHRONOUS SWITCH 0.8 VOUT 50mV/DIV AC COUPLED 0.6 0.4 MAIN SWITCH 0.
PAGE 6
LTC1878 U U U PI FU CTIO S RUN (Pin 1): Run Control Input. Forcing this pin below 0.4V shuts down the LTC1878. In shutdown all functions are disabled drawing < 1µA supply current. Forcing this pin above 1.2V enables the LTC1878. 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
LTC1878 U OPERATIO Main Control Loop The LTC1878 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 8
LTC1878 U OPERATIO 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 9
LTC1878 U W U U APPLICATIO S I FOR ATIO ∆IL =  V  1 VOUT  1 − OUT  ( f)(L)  VIN  (1) Accepting larger values of ∆IL 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 ∆IL = 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 10
LTC1878 U W U U APPLICATIO S I FOR ATIO ESR is a direct function of the volume of the capacitor. Manufacturers such as Taiyo-Yuden, AVX, Kemet, Sprague and Sanyo should be considered for high performance capacitors. The POSCAP solid electrolytic chip 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.
PAGE 11
LTC1878 U W U U APPLICATIO S I FOR ATIO 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.
PAGE 12
LTC1878 U W U U APPLICATIO S I FOR ATIO Thermal Considerations P-channel switch at 70°C is approximately 0.7Ω. Therefore, power dissipated by the part is: In most applications the LTC1878 does not dissipate much heat due to its high efficiency. But, in applications where the LTC1878 is running at high ambient temperature with low supply voltage and high duty cycles, such as in dropout, the heat dissipated may exceed the maximum junction temperature of the part.
PAGE 13
LTC1878 U W U U APPLICATIO S I FOR ATIO A second, more severe transient is caused by switching in loads with large (>1µF) supply bypass capacitors. The discharged bypass capacitors are effectively put in parallel with COUT, causing a rapid drop in VOUT. No regulator can deliver enough current to prevent this problem if the load switch resistance is low and it is driven quickly.
PAGE 14
LTC1878 U W U U APPLICATIO S I FOR ATIO 95 VIN = 3V VIN 2.65V TO 4.2V LTC1878 2 3 4 RUN PLL LPF ITH SYNC/MODE VFB VIN GND SW VIN = 3.6V 22µF** CER 8 EFFICIENCY (%) 220pF 1 90 7 6 15µH* 5 VOUT 2.5V + 80 VOUT = 2.5V L = 15µH 70 0.1 1 10 100 OUTPUT CURRENT (mA) 887k 1878 F08a 20pF VIN = 4.2V 75 47µF*** 412k 85 *SUMIDA CD54-150 **TAIYO-YUDEN CERAMIC JMK325BJ226MM ***SANYO POSCAP 6TPA47M 1878 F08b Figure 8. Single Lithium-Ion to 2.5V/0.
PAGE 15
LTC1878 U TYPICAL APPLICATIO S Externally Synchronized 2.5V/0.6A Regulator Using All Ceramic Capacitors LTC1878 1 RUN 2 220pF ITH 3 0.01µF PLL LPF SYNC/MODE VFB 4 VIN GND SW 10k 8 7 EXT CLOCK 700kHz 6 10µH* 5 20pF *TOKO D62CB A920CY-100M **TAIYO-YUDEN CERAMIC JMK325BJ226MM VOUT 2.5V COUT** 0.6A 22µF CER CIN** 22µF CER VOUT 2.5V COUT*** 0.3A 47µF 6.3V CIN** 22µF CER 887k VIN 3V TO 6V 412k 1878 TA04 Low Noise 2.5V/0.
PAGE 16
LTC1878 U TYPICAL APPLICATIO Single Li-Ion to 2.5V/0.5A Regulator with Precision 2.7V Undervoltage Lockout 0.1µF LTC1540 1.58M 1% 1 2 3 4 1.18M 1% GND V– IN+ IN – OUT V+ REF HYS 10k 8 7 2 220pF 6 5 LTC1878 1 44.2k 1% 3 0.01µF 4 RUN PLL LPF ITH SYNC/MODE VFB VIN GND SW 8 7 6 5 10µH* 20pF 2.37M 1% 887k COUT** 22µF CER VOUT 2.5V 0.6A CIN** 22µF CER VIN 2.7V TO 4.