Datasheet

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LTC1772B

1772bfa

TYPICAL APPLICATION

DESCRIPTION

Constant Frequency

Current Mode Step-Down

DC/DC Controller in SOT-23

The LTC

1772B is a constant frequency current mode

step-down DC/DC controller providing excellent AC and

DC load and line regulation. The device incorporates an

accurate undervoltage lockout feature that shuts down the

LTC1772B when the input voltage falls below 2.0V.

The LTC1772B provides a ±2.5% output voltage accuracy

and consumes only 270μA of quiescent current. In shut-

down, the device draws a mere 8μA.

To further maximize the life of a battery source, the

external P-channel MOSFET is turned on continuously

in dropout (100% duty cycle). High constant operating

frequency of 550kHz allows the use of a small external

inductor.

The LTC1772B is available in a small footprint 6-lead

TSOT-23.

For a Burst Mode operation enabled version of the

LTC1772B, please refer to the LTC1772 data sheet.

Figure 1. High Efﬁ ciency, High Output Current 2.5V/2A Regulator

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

All other trademarks are the property of their respective owners.

FEATURES

APPLICATIONS

Burst Mode

Operation Disabled for Lower Output

Ripple at Light Loads

High Efﬁ ciency: Up to 94%

High Output Currents Easily Achieved

Wide V

Range: 2.5V to 9.8V

Constant Frequency 550kHz Operation

Low Dropout: 100% Duty Cycle

Output Voltage Down to 0.8V

Current Mode Operation for Excellent Line and Load

Transient Response

Shutdown Mode Draws Only 8µA Supply Current

Tiny 6-Lead TSOT-23 Package

One or Two Lithium-Ion-Powered Applications

Cellular Telephones

Wireless Devices

Portable Computers

Distributed 3.3V, 2.5V or 1.8V Power Systems

/RUN

LTC1772B

10k

0.03Ω

174k

80.6k

4.7μH

220pF

C1: TAIYO YUDEN LMK325BJ106K-T

C2A: SANYO 6TPA47M

C2B: AVX 0805ZC105KAT1A

D1: MOTOROLA MBRM120T3

L1: MURATA LQN6C-4R7

M1: FAIRCHILD FDC638P

R1: IRC LRC-LR1206-01-R030F

GND

1772 F01a

PGATE

SENSE

–

10μF

10V

C2B

1μF

10V

2.5V

TO 9.8V

OUT

2.5V

C2A

47μF

LOAD CURRENT (mA)

EFFICIENCY (%)

100

100 1000 1000010

= 3.6V

OUT

= 2.5V

LTC1772

Burst Mode

OPERATION

LTC1772B

NON-Burst Mode

OPERATION

*OUTPUT RIPPLE WAVEFORMS FOR THE CIRCUIT

OF FIGURE 1 APPEAR IN FIGURE 2.

1772 F01b

Efﬁ ciency vs Load Current*

Summary of content (16 pages)

PAGE 1
LTC1772B Constant Frequency Current Mode Step-Down DC/DC Controller in SOT-23 FEATURES DESCRIPTION n The LTC®1772B is a constant frequency current mode step-down DC/DC controller providing excellent AC and DC load and line regulation. The device incorporates an accurate undervoltage lockout feature that shuts down the LTC1772B when the input voltage falls below 2.0V.
PAGE 2
LTC1772B ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) Input Supply Voltage (VIN) .........................– 0.3V to 10V SENSE–, PGATE Voltages ...............–0.3V to (VIN + 0.3V) VFB, ITH/RUN Voltages............................... –0.3V to 2.4V PGATE Peak Output Current (< 10µs) ........................ 1A Storage Ambient Temperature Range.....– 65°C to 150°C Operating Temperature Range (Note 2).... –40°C to 85°C Junction Temperature (Note 3) ............................
PAGE 3
LTC1772B ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 4.2V unless otherwise speciﬁed. (Note 2) PARAMETER CONDITIONS VFB Input Current (Note 5) Overvoltage Protect Threshold Measured at VFB MIN 0.820 Overvoltage Protect Hysteresis TYP MAX 10 50 nA 0.860 0.895 V 20 UNITS mV Oscillator Frequency VFB = 0.
PAGE 4
LTC1772B TYPICAL PERFORMANCE CHARACTERISTICS Reference Voltage vs Temperature 10 VIN = 4.2V 8 VFB VOLTAGE (mV) 815 810 805 800 795 790 785 2.20 6 2.16 4 2.12 2 0 –2 –4 –6 780 775 –55 –35 –15 2.24 VIN = 4.2V TRIP VOLTAGE (V) 820 NORMALIZED FREQUENCY (%) 825 Undervoltage Lockout Trip Voltage vs Temperature Normalized Oscillator Frequency vs Temperature 5 25 45 65 85 105 125 TEMPERATURE (°C) 2.00 1.96 1.
PAGE 5
LTC1772B PIN FUNCTIONS ITH/RUN (Pin 1): This pin performs two functions. It serves as the error ampliﬁer compensation point as well as the run control input. Nominal voltage range for this pin is 0.85V to 1.9V. Forcing this pin below 0.35V causes the device to be shut down. In shutdown all functions are disabled and the PGATE pin is held high. GND (Pin 2): Ground Pin. SENSE – (Pin 4): The Negative Input to the Current Comparator. VIN (Pin 5): Supply Pin. Must be closely decoupled to GND Pin 2.
PAGE 6
LTC1772B OPERATION (Refer to Functional Diagram) Main Control Loop Low Load Current Operation The LTC1772B is a constant frequency current mode switching regulator. During normal operation, the external P-channel power MOSFET is turned on each cycle when the oscillator sets the RS latch (RS1) and turned off when the current comparator (ICMP) resets the latch.
PAGE 7
LTC1772B OPERATION (Refer to Functional Diagram) that the external P-channel MOSFET will remain on for more than one oscillator cycle since the inductor current has not ramped up to the threshold set by EAMP. Further reduction in input supply voltage will eventually cause the P-channel MOSFET to be turned on 100%, i.e., DC. The output voltage will then be determined by the input voltage minus the voltage drop across the MOSFET, the sense resistor and the inductor.
PAGE 8
LTC1772B APPLICATIONS INFORMATION The basic LTC1772B application circuit is shown in Figure 1. External component selection is driven by the load requirement and begins with the selection of L1 and RSENSE (= R1). Next, the power MOSFET, M1 and the output diode D1 is selected followed by CIN (= C1)and COUT(= C2). RSENSE Selection for Output Current The inductance value also has a direct effect on ripple current.
PAGE 9
LTC1772B APPLICATIONS INFORMATION Since the LTC1772B is designed for operation down to low input voltages, a logic level threshold MOSFET (RDS(ON) guaranteed at VGS = 2.5V) is required for applications that work close to this voltage. When these MOSFETs are used, make sure that the input supply to the LTC1772B is less than the absolute maximum VGS rating, typically 8V. The required minimum RDS(ON) of the MOSFET is governed by its allowable power dissipation.
PAGE 10
LTC1772B APPLICATIONS INFORMATION 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 (ΔVOUT) is approximated by: 1 VOUT IRIPPLE ESR + 4fC OUT where f is the operating frequency, COUT is the output capacitance and IRIPPLE is the ripple current in the inductor. The output ripple is highest at maximum input voltage since ΔIL increases with input voltage.
PAGE 11
LTC1772B APPLICATIONS INFORMATION current, 2) MOSFET gate charge current, 3) I2R losses and 4) voltage drop of the output diode. 1. The VIN current is the DC supply current, given in the electrical characteristics, that excludes MOSFET driver and control currents. VIN current results in a small loss which increases with VIN. 2. MOSFET gate charge current results from switching the gate capacitance of the power MOSFET.
PAGE 12
LTC1772B APPLICATIONS INFORMATION PC Board Layout Checklist 4. Connect the end of RSENSE as close to VIN (Pin 5) as possible. The VIN pin is the SENSE + of the current comparator. When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC1772B. These items are illustrated graphically in the layout diagram in Figure 7. Check the following in your layout: 5.
PAGE 13
LTC1772B TYPICAL APPLICATIONS LTC1772B High Efﬁciency, Small Footprint 3.3V to 1.8V/0.5A Regulator 1 R4 10k C3 220pF ITH/RUN PGATE 6 L1 M1 10μH LTC1772B 2 3 GND VFB VIN 5 4 SENSE– VIN 3.3V C1 10μF 10V R1 0.15Ω D1 + C2 47μF 6V C1: TAIYO YUDEN CERAMIC L1: COILTRONICS UP1B-100 LMK325BJ106K-T M1: Si3443DV C2: SANYO POSCAP 6TPA47M R1: DALE 0.25W D1: MOTOROLA MBRM120T3 VOUT 1.8V 0.5A R2 100k R3 80.
PAGE 14
LTC1772B TYPICAL APPLICATIONS LTC1772B 5V/0.5A Flyback Regulator R1 0.033Ω 1 R4 10k C3 220pF ITH/RUN PGATE 6 C1 10μF 10V VIN 2.5V TO 9.8V M1 LTC1772B 2 3 GND VFB VIN SENSE– 5 4 D1 VOUT 5V 0.5A T1 • 10μH + 10μH • C1: TAIYO YUDEN CERAMIC LMK325BJ106K-T C2: AVXTPSE107M010R0100 D1: IR10BQ015 M1: Si9803 R1: DALE 0.25W T1: COILTRONICS CTX10-4 C2 100μF 10V ×2 R2 52.
PAGE 15
LTC1772B PACKAGE DESCRIPTION S6 Package 6-Lead Plastic TSOT-23 (Reference LTC DWG # 05-08-1636) 0.62 MAX 2.90 BSC (NOTE 4) 0.95 REF 1.22 REF 3.85 MAX 2.62 REF 1.4 MIN 2.80 BSC 1.50 – 1.75 (NOTE 4) PIN ONE ID RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR 0.30 – 0.45 6 PLCS (NOTE 3) 0.95 BSC 0.80 – 0.90 0.20 BSC 0.01 – 0.10 1.00 MAX DATUM ‘A’ 0.30 – 0.50 REF 0.09 – 0.20 (NOTE 3) 1.90 BSC S6 TSOT-23 0302 REV B NOTE: 1. DIMENSIONS ARE IN MILLIMETERS 2. DRAWING NOT TO SCALE 3.
PAGE 16
LTC1772B TYPICAL APPLICATION LTC1772B 3.3V to 5V/1A Boost Regulator R1 0.033Ω VIN 3.3V C1 47μF 16V ×2 L1 4.7μH D1 U1 1 R4 10k C3 220pF ITH/RUN PGATE 6 3 VIN GND VFB C1: AVXTPSE476M016R0047 C2: AVXTPSE107M010R0100 D1: IR10BQ015 SENSE– 5 4 M1 + 3 LTC1772B 2 2 5 VOUT 5V 1A C2 100μF 10V ×2 4 L1: MURATA LQN6C-4R7 M1: Si9804 R1: DALE 0.25W R2 422k U1: FAIRCHILD NC7SZ04 ALSO SEE LTC1872 FOR THIS APPLICATION R3 80.