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LTC1707

High Efficiency Monolithic

Synchronous Step-Down

Switching Regulator

Figure 1b. Efficiency vs Output Load Current

22µF

16V

47pF

3V TO

8.5V

1707 F01a

REF

RUN

SYNC/MODE

LTC1707

15µH

100µF

6.3V

80.6k

OUT

FOLLOWS V

FOR

3V < VIN < 3.3V

249k

OUT

3.3V

GND

OUTPUT CURRENT (mA)

EFFICIENCY (%)

100

10 100 1000

1707 F01b

OUT

= 3.3V

= 6V

= 3.6V

= 8.4V

Figure 1a. High Efficiency Low Dropout Step-Down Converter

The LTC

1707 is a high efficiency monolithic current

mode synchronous buck regulator using a fixed frequency

architecture. The operating supply range is from 8.5V

down to 2.85V, making it suitable for both single and dual

lithium-ion battery-powered applications. Burst Mode op-

eration provides high efficiency at low load currents.

100% duty cycle provides low dropout operation, extend-

ing operating time in battery-powered systems.

The switching frequency is internally set at 350kHz,

allowing the use of small surface mount inductors. For

noise sensitive applications it can be externally synchro-

nized up to 550kHz. Burst Mode operation is inhibited

during synchronization or when the SYNC/MODE pin is

pulled low preventing low frequency ripple from interfer-

ing with audio circuitry. Soft-start is provided by an

external capacitor.

The internal synchronous MOSFET switch increases effi-

ciency and eliminates the need for an external Schottky

diode, saving components and board space. Low output

voltages down to 0.8V are easily achieved due to the 0.8V

internal reference. The LTC1707 comes in an 8-lead SO

package.

■

600mA Output Current (V

≥ 4V)

■

High Efficiency: Up to 96%

■

Constant Frequency: 350kHz Synchronizable

to 550kHz

■

2.85V to 8.5V V

Range

■

0.8V Feedback Reference Allows Low Voltage

Outputs: 0.8V ≤ V

OUT

≤ V

■

No Schottky Diode Required

■

1.19V ±1% Reference Output Pin

■

Selectable Burst Mode

Operation/Pulse

Skipping Mode

■

Low Dropout Operation: 100% Duty Cycle

■

Precision 2.7V Undervoltage Lockout

■

Current Mode Control for Excellent Line and

Load Transient Response

■

Low Quiescent Current: 200µA

■

Shutdown Mode Draws Only 11µA Supply Current

■

Available in 8-Lead SO Package

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

Burst Mode is a trademark of Linear Technology Corporation.

■

Cellular Telephones

■

Portable Instruments

■

Wireless Modems

■

RF Communications

■

Distributed Power Systems

■

Single and Dual Cell Lithium

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO

Summary of content (16 pages)

PAGE 1
LTC1707 High Efficiency Monolithic Synchronous Step-Down Switching Regulator U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO 600mA Output Current (VIN ≥ 4V) High Efficiency: Up to 96% Constant Frequency: 350kHz Synchronizable to 550kHz 2.85V to 8.5V VIN Range 0.8V Feedback Reference Allows Low Voltage Outputs: 0.8V ≤ VOUT ≤ VIN No Schottky Diode Required 1.19V ±1% Reference Output Pin Selectable Burst ModeTM Operation/Pulse Skipping Mode Low Dropout Operation: 100% Duty Cycle Precision 2.
PAGE 2
LTC1707 W W W AXI U U ABSOLUTE RATI GS U U W PACKAGE/ORDER I FOR ATIO (Note 1) Input Supply Voltage ................................ – 0.3V to 10V ITH Voltage ................................................. – 0.3V to 5V RUN/SS, VFB Voltages ............................... – 0.3V to VIN SYNC/MODE Voltage ................................. – 0.3V to VIN P-Channel Switch Source Current (DC) .............. 800mA N-Channel Switch Sink Current (DC) ..................
PAGE 3
LTC1707 U W TYPICAL PERFOR A CE CHARACTERISTICS Efficiency vs Load Current Efficiency vs Input Voltage Efficiency vs Load Current 100 100 100 VIN = 2.8V 95 Burst Mode 90 OPERATION ILOAD = 300mA 90 ILOAD = 10mA 85 95 85 EFFICIENCY (%) ILOAD = 100mA EFFICIENCY (%) EFFICIENCY (%) 95 PULSE SKIPPING MODE 80 75 70 65 80 75 60 VOUT = 2.5V L = 15µH Burst Mode OPERATION 2 0 6 4 INPUT VOLTAGE (V) VIN = 3.6V VOUT = 2.5V L = 15µH 55 10 100 OUTPUT CURRENT (mA) VIN RAMPING DOWN 2.70 2.65 2.
PAGE 4
LTC1707 U W TYPICAL PERFOR A CE CHARACTERISTICS Maximum Output Current vs Input Voltage Switch Leakage Current vs Temperature 1800 VOUT = 1.8V VOUT = 1.5V SWITCH LEAKAGE (nA) OUTPUT CURRENT (mA) 800 600 VOUT = 5V 400 VOUT = 3.3V VOUT = 2.5V VOUT = 2.9V 200 TJ = 85°C L = 15µH 3.5 4.5 5.5 6.5 INPUT VOLTAGE (V) 0.8 1400 0.7 1200 1000 SYNCHRONOUS SWITCH 800 600 400 0 2.5 0.9 VIN = 8.4V 1600 SWITCH RESISTANCE (Ω) 1000 Switch Resistance vs Temperature 8.5 7.
PAGE 5
LTC1707 U U U PI FU CTIO S 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. 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 0.5s/µF. Forcing this pin below 0.4V shuts down the LTC1707. VFB (Pin 3): Feedback Pin.
PAGE 6
LTC1707 U OPERATIO (Refer to Functional Diagram) Main Control Loop The LTC1707 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 7
LTC1707 U OPERATIO 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 shortcircuit protection. switch is on continuously. Hence, the I2R loss is due mainly to the RDS(ON) of the P-channel MOSFET. See Efficiency Considerations in the Applications Information section. Dropout Operation Below VIN = 4V, the output current must be derated as shown in Figures 2a and 2b.
PAGE 8
LTC1707 U W U U APPLICATIO S I FOR ATIO with the oscillator synchronized at its minimum frequency, i.e., to a clock just above the oscillator free-running frequency. The actual reduction in average current is less than for peak current. The basic LTC1707 application circuit is shown in Figure␣ 1a. External component selection is driven by the load requirement and begins with the selection of L followed by CIN and COUT.
PAGE 9
LTC1707 U W U U APPLICATIO S I FOR ATIO size or height requirements in the design. Always consult the manufacturer if there is any question. The selection of COUT is driven by the required effective series resistance (ESR). Typically, once the ESR requirement is satisfied, the capacitance is adequate for filtering. The output ripple ∆VOUT is determined by:  1  ∆VOUT ≅ ∆IL  ESR +  8 fCOUT   where f = operating frequency, COUT = output capacitance and ∆IL = ripple current in the inductor.
PAGE 10
LTC1707 U W U U APPLICATIO S I FOR ATIO D1 in Figure 5 reduces the start delay but allows CSS to ramp up slowly providing the soft-start function. This diode can be deleted if soft-start is not needed. both top and bottom MOSFET RDS(ON) and the duty cycle (DC) as follows: Efficiency Considerations The RDS(ON) for both the top and bottom MOSFETs can be obtained from the Typical Performance Characteristics curves.
PAGE 11
LTC1707 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 12
LTC1707 U W U U APPLICATIO S I FOR ATIO Design Example A 22µH 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 LTC1707 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 2.85V. 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 13
LTC1707 U TYPICAL APPLICATIO S 5V Input to 3.3V/0.6A Regulator CITH 47pF 1 ITH VREF * SUMIDA CD54-150 ** AVX TPSC107M006R0150 *** TAIYO YUDEN LMK325BJ106K-T 8 2 CSS 0.1µF 7 RUN/SS SYNC/MODE LTC1707 3 6 VFB VIN 4 GND SW 5 VIN = 5V 15µH* VOUT 3.3V 0.6A R2 249k 1% + R1 80.6k 1% CIN*** 10µF CERAMIC COUT ** 100µF 6.3V 1707 TA01 Double Lithium-Ion Battery to 5V/0.5A Low Dropout Regulator CITH 47pF 1 ITH VREF * SUMIDA CD54-330 8 ** AVX TPSD107M010R0100 *** AVX TPSC226M016R0375 2 CSS 0.
PAGE 14
LTC1707 U TYPICAL APPLICATIO S 3.3V Input to 2.5V/0.4A Regulator CITH 47pF 1 2 CSS 0.1µF 3 4 ITH VREF 8 7 SYNC/MODE LTC1707 6 VIN RUN/SS VFB GND SW VIN = 3.3V 10µH* 5 VOUT 2.5V 0.4A R2 169k 1% + R1 80.6k 1% COUT† 100µF 6.3V CIN** 10µF CERAMIC 1707 TA03 * SUMIDA CD54-100 ** TAIYO YUDEN LMK325BJ106K-T † AVX TPSC107M006R0150 Double Lithium-Ion to 2.5V/0.5A Regulator CITH 47pF 1 ITH VREF 8 * SUMIDA CD54-250 ** AVX TPSC107M006R0150 *** AVX TPSC226M016R0375 2 CSS 0.
PAGE 15
LTC1707 U PACKAGE DESCRIPTIO Dimensions in inches (millimeters) unless otherwise noted. S8 Package 8-Lead Plastic Small Outline (Narrow 0.150) (LTC DWG # 05-08-1610) 0.189 – 0.197* (4.801 – 5.004) 8 7 6 5 0.150 – 0.157** (3.810 – 3.988) 0.228 – 0.244 (5.791 – 6.197) 1 0.010 – 0.020 × 45° (0.254 – 0.508) 0.008 – 0.010 (0.203 – 0.254) 0.053 – 0.069 (1.346 – 1.752) 0°– 8° TYP 0.016 – 0.050 (0.406 – 1.270) 0.014 – 0.019 (0.355 – 0.483) TYP *DIMENSION DOES NOT INCLUDE MOLD FLASH.
PAGE 16
LTC1707 U TYPICAL APPLICATIO Single Lithium-Ion to 1.8V/0.3A Regulator CITH 47pF 1 ITH VREF * SUMIDA CD54-150 ** AVX TPSC107M006R0150 *** TAIYO YUDEN LMK325BJ106K-T 8 2 CSS 0.1µF 7 RUN/SS SYNC/MODE LTC1707 3 6 VFB VIN 4 GND SW 5 VIN ≤ 4.2V 15µH* R2 100k 1% VOUT 1.8V 0.3A + R1 80.6k 1% COUT ** 100µF 6.3V CIN*** 10µF CERAMIC 1707 TA04 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC1174/LTC1174-3.