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LTC1627

Monolithic Synchronous

Step-Down Switching Regulator

Figure 1b. Efficiency vs Output Load Current

Figure 1a. High Efficiency Step-Down Converter

The LTC

1627 is a monolithic current mode synchronous

buck regulator using a fixed frequency architecture. The

operating supply range is from 8.5V down to 2.65V, making

it suitable for one or two 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-operated

systems.

The operating frequency is internally set at 350kHz, allowing

the use of small surface mount inductors. For switching noise

sensitive applications it can be externally synchronized up to

525kHz. The SYNC/FCB control pin guarantees regulation of

secondary windings regardless of load on the main output by

forcing continuous operation. Burst Mode operation is inhib-

ited during synchronization or when the SYNC/FCB pin is

pulled low to reduce noise and RF interference. Soft-start is

provided by an external capacitor.

Optional bootstrapping enhances the internal switch drive for

single lithium-ion cell applications. The internal synchronous

switch increases efficiency and eliminates the need for an

external Schottky diode, saving components and board

space. The LTC1627 comes in an 8-lead SO package.

L1 15µH

OUT

100µF

6.3V

80.6k

249k

47pF

2.8V*

TO 8.5V

OUT

3.3V

1627 F01a

0.1µF

22µF

16V

SYNC/FCB

RUN/SS

GND

LTC1627

OUT

CONNECTED TO V

FOR 2.8V < V

< 3.3V

OUTPUT CURRENT (mA)

EFFICIENCY (%)

100

10 100 1000

1627 F01b

OUT

= 3.3V

= 6V

= 3.6V

= 8.4V

■

High Efficiency: Up to 96%

■

Constant Frequency 350kHz Operation

■

2.65V to 8.5V V

Range

■

OUT

from 0.8V to V

, I

OUT

to 500mA

■

No Schottky Diode Required

■

Synchronizable Up to 525kHz

■

Selectable Burst Mode

Operation

■

Low Dropout Operation: 100% Duty Cycle

■

Precision 2.5V Undervoltage Lockout

■

Secondary Winding Regulation

■

Current Mode Operation for Excellent Line and

Load Transient Response

■

Low Quiescent Current: 200µA

■

Shutdown Mode Draws Only 15µA Supply Current

■

±1.5% Reference Accuracy

■

Available in 8-Lead SO Package

■

Cellular Telephones

■

Portable Instruments

■

Wireless Modems

■

RF Communications

■

Distributed Power Systems

■

Scanners

■

Single and Dual Cell Lithium

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

Burst Mode is a trademark of Linear Technology Corporation.

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO

Summary of content (16 pages)

PAGE 1
LTC1627 Monolithic Synchronous Step-Down Switching Regulator U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO High Efficiency: Up to 96% Constant Frequency 350kHz Operation 2.65V to 8.5V VIN Range VOUT from 0.8V to VIN, IOUT to 500mA No Schottky Diode Required Synchronizable Up to 525kHz Selectable Burst ModeTM Operation Low Dropout Operation: 100% Duty Cycle Precision 2.
PAGE 2
LTC1627 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 Driver Supply Voltage (VIN – VDR) ........... – 0.3V to 10V ITH Voltage .................................................. – 0.3V to 5V Run/SS, VFB Voltages ................................ – 0.3V to VIN Sync/FCB Voltage ...................................... – 0.3V to VIN VDR Voltage (VIN ≤ 5V) ............................... – 5V to 0.
PAGE 3
LTC1627 U W TYPICAL PERFOR A CE CHARACTERISTICS Efficiency vs Input Voltage Efficiency vs Load Current 100 95 95 VOUT = 2.5V L = 15µH VDR = 0V Burst Mode OPERATION 2 0 6 4 INPUT VOLTAGE (V) VDR = – VIN 85 80 75 10 100 OUTPUT CURRENT (mA) UNDERVOLTAGE LOCKOUT THRESHOLD (V) 100 VIN = 2.8V EFFICIENCY (%) 95 VIN = 3.6V 85 VIN = 7.2V 80 VOUT = 2.
PAGE 4
LTC1627 U W TYPICAL PERFOR A CE CHARACTERISTICS Oscillator Frequency vs Temperature 1100 MAXIMUM OUTPUT LOAD CURRENT (mA) VSYNC/FCB = 0V 380 370 360 350 340 330 320 310 370 360 350 340 330 320 310 300 – 50 0 25 50 75 TEMPERATURE (°C) 100 125 0.8 SWITCH RESISTANCE (Ω) SYNCHRONOUS SWITCH LEAKAGE (nA) 0.9 1200 1000 SYNCHRONOUS SWITCH 600 400 MAIN SWITCH 200 0 – 50 – 25 0 25 50 75 TEMPERATURE (°C) 700 600 500 400 100 4.5 5.5 6.5 INPUT VOLTAGE (V) 7.5 2.5 8.5 0.7 0.
PAGE 5
LTC1627 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. VIN (Pin 6): Main Supply Pin. Must be closely decoupled to GND, Pin 4. VDR (Pin 7): Top Driver Return Pin. This pin can be bootstrapped to go below ground to improve efficiency at low VIN (see Applications Information). RUN/SS (Pin 2): Combination of Soft-Start and Run Control Inputs.
PAGE 6
LTC1627 U OPERATIO (Refer to Functional Diagram) Main Control Loop The LTC1627 uses a constant frequency, current mode step-down architecture. Both the main and synchronous switches, consisting of top P-channel and bottom N-channel power MOSFETs, 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
LTC1627 U OPERATIO load slightly allows constant frequency PWM operation to resume. 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. Dropout Operation When the input supply voltage decreases toward the output voltage, the duty cycle increases toward the maximum on-time.
PAGE 8
LTC1627 U W U U APPLICATIO S I FOR ATIO frequency. The actual reduction in average current is less than for peak current. The basic LTC1627 application circuit is shown in Figure 1. External component selection is driven by the load requirement and begins with the selection of L followed by CIN and COUT. Inductor Value Calculation The inductor selection will depend on the operating frequency of the LTC1627. The internal preset frequency is 350kHz, but can be externally synchronized up to 525kHz.
PAGE 9
LTC1627 U W U U APPLICATIO S I FOR ATIO 0.8V ≤ VOUT ≤ 8.5V 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: R2 VFB LTC1627  1  ∆VOUT ≅ ∆IL  ESR +  8 fCOUT   GND 1627 F04 where f = operating frequency, COUT = output capacitance and ∆IL = ripple current in the inductor.
PAGE 10
LTC1627 U W U U APPLICATIO S I FOR ATIO Auxiliary Winding Control Using SYNC/FCB Pin The SYNC/FCB pin can be used as a secondary feedback input to provide a means of regulating a flyback winding output. When this pin drops below its ground referenced 0.8V threshold, continuous mode operation is forced. In continuous mode, the main and synchronous MOSFETs are switched continuously regardless of the load on the main output.
PAGE 11
LTC1627 U W U U APPLICATIO S I FOR ATIO 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. When a load step occurs, VOUT immediately shifts by an amount equal to (∆ILOAD • ESR), where ESR is the effective series resistance of COUT. ∆ILOAD also begins to charge or discharge COUT, which generates a feedback error signal.
PAGE 12
LTC1627 U W U U APPLICATIO S I FOR ATIO 3. Does the (+) plate of CIN connect to VIN as closely as possible? This capacitor provides the AC current to the internal power MOSFETs. L=  2.5V  1 −  = 14.5µH 350kHz 200mA  4.2V  2.5V ( )( ) 4. Keep the switching node SW away from sensitive smallsignal nodes. A 15µH inductor works well for this application. For good efficiency choose a 1A inductor with less than 0.25Ω series resistance.
PAGE 13
LTC1627 U TYPICAL APPLICATIO S 5V Input to 3.3V/0.5A Regulator CITH 47pF 1 2 CSS 0.1µF 3 4 ITH SYNC/FCB RUN/SS VDR LTC1627 VFB VIN GND SW * SUMIDA CD54-150 8 ** AVX TPSC107M006R0150 *** AVX TPSC226M016R0375 7 6 5 VIN = 5V 15µH* R2 249k 1% VOUT 3.3V 0.5A + R1 80.6k 1% + COUT ** 100µF 6.3V CIN*** 22µF 16V 1627 TA03 Double Lithium-Ion to 5V/0.5A Low Dropout Regulator CITH 47pF 1 2 CSS 0.
PAGE 14
LTC1627 U TYPICAL APPLICATIO S 3.3V Input to 2.5V/0.5A Regulator CITH 47pF 1 2 CSS 0.1µF 3 4 ITH SYNC/FCB RUN/SS VDR LTC1627 VFB VIN GND SW 8 7 C1 0.1µF 6 10µH* 5 R2 169k 1% VIN = 3.3V VOUT 2.5V 0.5A + R1 80.6k 1% BAT54S** + D1 COUT† 100µF 6.3V D2 C2 0.1µF * SUMIDA CD54-100 ** ZETEX BAT54S † AVX TPSC107M006R0150 †† AVX TPSC226M016R0375 1627 TA05 Single Lithium-Ion to 1.8V/0.3A Regulator CITH 47pF 1 2 CSS 0.
PAGE 15
LTC1627 U TYPICAL APPLICATIO S Double Lithium-Ion to 2.5V/0.5A Regulator CITH 47pF 1 ITH 2 CSS 0.1µF SYNC/FCB RUN/SS VDR LTC1627 3 VFB 4 GND VIN SW * SUMIDA CD54-250 8 ** AVX TPSC107M006R0150 *** AVX TPSC226M016R0375 7 6 5 VIN ≤ 8.4V 25µH* R2 169k 1% VOUT 2.5V 0.5A + R1 80.6k 1% + CIN*** 22µF 16V COUT ** 100µF 6.3V 1627 TA01 U PACKAGE DESCRIPTIO Dimensions in inches (millimeters) unless otherwise noted. S8 Package 8-Lead Plastic Small Outline (Narrow 0.
PAGE 16
LTC1627 U TYPICAL APPLICATIO S Dual Output 1.8V/300mA and 3.3V/100mA Application CITH 47pF 1 2 CSS 0.1µF 3 4 ITH SYNC/FCB RUN/SS VDR LTC1627 VFB VIN GND SW 8 7 6 R3 249k 1% R4 80.6k 1% VIN ≤ 8.5V ***22µF 6.3V VSEC ††† 3.3V 100mA + D2†† ZENER 1.8V D1 25µH† MBR0520LT1 1:1 VOUT 1.8V 0.3A 5 + CIN* 22µF 16V † COILTRONICS CTX25-1 * AVX TPSC226M016R0375 ** AVX TPSC107M006R0150 †† MMSZ4678T1 ††† A 10mA MIN LOAD CURRENT *** AVX TAJA226M006R IS RECOMMENDED R2 100k 1% + COUT ** 100µF 6.