Datasheet

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LTC1779

250mA Current Mode

Step-Down DC/DC Converter

in ThinSOT

The LTC

1779 is a constant frequency current mode step-

down DC/DC converter in a 6-lead ThinSOT package. The

part operates with a 2.5V to 9.8V input and can provide up

to 250mA of output current. Current mode control pro-

vides excellent AC and DC load and line regulation. The

device incorporates an accurate undervoltage lockout fea-

ture that shuts down the LTC1779 when the input voltage

falls below 2V.

The LTC1779 boasts a ±2.5% output voltage accuracy and

consumes only 135µA of quiescent current. For applica-

tions where efficiency is a prime consideration, the LTC1779

is configured for Burst Mode operation, which enhances

efficiency at low output current.

To further maximize the life of a battery source, the

internal P-channel MOSFET is turned on continuously in

dropout (100% duty cycle). In shutdown, the device draws

a mere 8µA. High constant operating frequency of 550kHz

allows the use of a small external inductor.

The LTC1779 is available in a low profile (1mm) ThinSOT

package.

Figure 1. LTC1779 High Efficiency 2.5V/100mA Step-Down Converter

■

1- or 2-Cell Lithium-Ion-Powered Applications

■

Cellular Telephones

■

Wireless Modems

■

Portable Computers

■

Distributed 3.3V, 2.5V or 1.8V Power Systems

■

Scanners

■

High Efficiency: Up to 94%

■

250mA Output Current

■

Wide V

Range: 2.5V to 9.8V

■

550kHz Constant Frequency Operation

■

Burst Mode

Operation at Light Load

■

Low Dropout: 100% Duty Cycle

■

0.8V Reference Allows Low Output Voltages

■

±2.5% Reference Accuracy

■

Current Mode Operation for Excellent Line and Load

Transient Response

■

Low Quiescent Current: 135µA

■

Shutdown Mode Draws Only 8µA Supply Current

■

Low Profile (1mm) ThinSOT

Package

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

Burst Mode and ThinSOT are trademarks of Linear Technology Corporation.

Efficiency vs Load Current

/RUN

LTC1779

20k

169k

78.7k

10Ω

22µH

100pF

GND

1779 F01a

SENSE

–

10µF

16V

0.1µF

C1: TAIYO YUDEN CERAMIC EMK325BJ106MNT

C2: SANYO POSCAP 6TPA47M

D1: IR10BQ015

L1: COILTRONICS UP1B220

2.5V

TO 9.8V

OUT

2.5V

100mA

47µF

LOAD CURRENT (mA)

EFFICIENCY (%)

100

0.1 10 100 1000

1779 F01b

= 3.3V

= 6V

= 9.8V

OUT

= 2.5V

SENSE

= 10Ω

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO

Summary of content (12 pages)

PAGE 1
LTC1779 250mA Current Mode Step-Down DC/DC Converter in ThinSOT U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO High Efficiency: Up to 94% 250mA Output Current Wide VIN Range: 2.5V to 9.8V 550kHz Constant Frequency Operation Burst ModeTM Operation at Light Load Low Dropout: 100% Duty Cycle 0.8V Reference Allows Low Output Voltages ±2.
PAGE 2
LTC1779 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 10V SENSE –, SW Voltages .................. – 0.3V to (VIN + 0.3V) VFB, ITH/RUN Voltages ..............................– 0.3V to 2.4V SW Peak Output Current (< 10µs) .......................... 0.5A 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
LTC1779 U W TYPICAL PERFOR A CE CHARACTERISTICS Reference Voltage vs Temperature 15 VIN = 4.2V 810 805 800 795 790 785 9 2.12 6 2.08 3 0 –3 –6 780 775 –55 –35 –15 2.16 1.88 5 25 45 65 85 105 125 TEMPERATURE (°C) 1.80 –55 –35 –15 1779 G03 RDS(ON) of Internal P-Channel FET vs Input Voltage 1.85 VIN = 4.2V RDS(ON) of Internal P-Channel FET vs Temperature 1.85 ISW = 100mA SENSE – = VIN 1.70 ISW = 100mA 1.70 SENSE – = V IN 520 1.55 1.55 480 1.40 1.40 440 1.25 400 0.95 320 0.
PAGE 4
LTC1779 W FU CTIO AL DIAGRA U U SENSE – VIN 4 5 + OVERTEMP DETECT ICMP – 2Ω VIN RS1 SLOPE COMP OSC 1× SWITCHING LOGIC AND BLANKING CIRCUIT R Q S 24× SW 6 – FREQ FOLDBACK BURST CMP + 0.3V + SHORT-CIRCUIT DETECT SLEEP – 0.15V OVP + – VREF + 60mV + VREF 0.8V VIN EAMP 0.5µA VFB + – 1 ITH/RUN 3 VIN VIN 0.3V – 0.325V VOLTAGE REFERENCE + SHDN CMP VREF 0.8V – GND SHDN UV 2 UNDERVOLTAGE LOCKOUT 1.
PAGE 5
LTC1779 (Refer to Functional Diagram) up, the corresponding output current trip level follows, allowing normal operation. Comparator OVP guards against transient overshoots > 7.5% by turning off the internal P-channel power MOSFET and keeping it off until the fault is removed. Burst Mode Operation The LTC1779 enters Burst Mode operation at low load currents.
PAGE 6
LTC1779 U OPERATIO (Refer to Functional Diagram) The variable M is the ratio of the total switch current to that portion of the switch current that flows through RSENSE. M is a function of both RSENSE and ROUT of the internal power switch, which in turn, is a strong function of supply voltage. For values of M refer to Figure 3.
PAGE 7
LTC1779 U W U U APPLICATIO S I FOR ATIO A smaller value than L MIN could be used in the circuit; however, the inductor current will not be continuous during burst periods. understand how it is going to work over the entire input voltage range. Inductor Core Selection RSENSE Selection for Output Current The selection of RSENSE determines the output current limit, the maximum possible output current before the internal current limit threshold is reached.
PAGE 8
LTC1779 U W U U APPLICATIO S I FOR ATIO Under normal load conditions, the average current conducted by the diode is: V −V  ID =  IN OUT  IOUT  VIN + VD  The allowable forward voltage drop in the diode is calculated from the maximum short-circuit current as: VF ≈ PD ICL (MAX) where PD is the allowable power dissipation and will be determined by efficiency and/or thermal requirements. A fast switching diode must also be used to optimize efficiency.
PAGE 9
LTC1779 U W U U APPLICATIO S I FOR ATIO Although all dissipative elements in the circuit produce losses, four main sources usually account for most of the losses in LTC1779 circuits: 1) LTC1779 DC bias current, 2) MOSFET gate charge current, 3) I2R losses and 4) voltage drop of the output diode. NORMALIZED VOLTAGE (%) 105 VREF 100 VITH 95 90 1. The VIN current is the DC supply current, given in the electrical characteristics, that excludes MOSFET driver and control currents.
PAGE 10
LTC1779 U W U U APPLICATIO S I FOR ATIO Foldback Current Limiting PC Board Layout Checklist As described in the Output Diode Selection, the worstcase dissipation occurs with a short-circuited output when the diode conducts the current limit value almost continuously. To prevent excessive heating in the diode, foldback current limiting can be added to reduce the current in proportion to the severity of the fault.
PAGE 11
LTC1779 U PACKAGE DESCRIPTION S6 Package 6-Lead Plastic SOT-23 (LTC DWG # 05-08-1634) (LTC DWG # 05-08-1636) 2.80 – 3.10 (.110 – .118) (NOTE 3) SOT-23 (Original) SOT-23 (ThinSOT) A .90 – 1.45 (.035 – .057) 1.00 MAX (.039 MAX) A1 .00 – 0.15 (.00 – .006) .01 – .10 (.0004 – .004) A2 .90 – 1.30 (.035 – .051) .80 – .90 (.031 – .035) L .35 – .55 (.014 – .021) .30 – .50 REF (.012 – .019 REF) 2.60 – 3.00 (.102 – .118) 1.50 – 1.75 (.059 – .069) (NOTE 3) PIN ONE ID .95 (.037) REF .25 – .50 (.
PAGE 12
LTC1779 U TYPICAL APPLICATIONS Efficiency vs Load Current LTC1779 Minimal Component Count, Single Li-Ion to 1.8V/250mA Step-Down Converter 20k 2 SW ITH/RUN LTC1779 VIN GND 6 + D1 5 85 C2 47µF 6V 100k SENSE – VFB VIN = 3.6V 80 VIN = 4.2V 75 70 65 100pF 3 VOUT 1.8V 250mA VIN = 2.7V 90 EFFICIENCY (%) 1 95 VIN 2.7V TO 4.2V C1 10µF 16V L1 10µH 100 80.6k 4 60 VOUT = 1.