SGS Thomson Microelectronics ST779ACN, ST779ACD-TR, ST779ACD, ST778ACN, ST778ACD-TR Datasheet

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1/11October 2002

■ 1V TO 6V INPUT GUARANTEES START-UP

UNDER LOAD

■ MAXIMUM OUTPUT CURRENT OF 300mA

(778OR 779 ADJUSTED TO 3V)

■ LOAD FULLY DISCONNECTEDIN

SHUTDOWN

■ TYPICAL EFFICIENCY OF 82%

■ INTERNAL 1A PO WER SWITCH AND

SYNCHRONOUS RECTIFIER

■ ADJUSTABLE CURRENT LIMIT ALLOWS

LOW-COST INDUCTORS

■ SUPPLY CURRENT OF 270µA (NO LOAD )

■ SHUTDOWN SUPPLY CURRENT 20µA

■ PACKAGE AVAILABLE: DIP-8 AND SO-8

DESCRIPTION

The ST777/778/779 are dc-dc converters that step-up from low voltage inputs requiring only three external components, an inductor (typically 22µH) and two capacitors. The device includ e a Sinchronous Rectifier that elimina tes the need for an external catch diode, and allows regulation even when the input is greater than the output.

Unlike others step-up DC-DC converters the ST777/778/779’s Sinchronous Rectifierturns off in the shu tdown mode, fully disconnecting the output from the source. This eliminates the current drain associated with conventional step-up c onverters when off or in shutdown. Supply current is 270µA under no load and only 20µA in stand by mode.

ST777/778/779

LOW VOLTAG E INPUT, 3-3.3V/5V/ADJUSTABLE OUTPUT

DC-DC CONVERTER WITH SYNCHRONOUS RECTIFIER

22µH

100µF

RLIM

ILIM LX

OUT

PGND

SEL

AGND

SHDN

1.25V

VREF

SHUTDOWN

CONTROL

DELAY TIMER

OFF

tON

SWITCH DRIVER

ACTIVE RECTIFIER

RECTIFIER

CONTROL

1:N

SCHEMATIC DIAGRAM

SO-8DIP-8

ST777/778/779

2/11

ABSOLUTE MAXIMUM RATINGS

Absolute Maximum Ratings are those values beyond which damage to the device may occur. Functional operation under these condition is not implied.

ORDERING CODES CONNECTION DIAGRAM

PIN CO NNE CTIONS

THERMAL DATA

Symbol Parameter Value Unit

DC Input Voltage to GND

-0.3 to +7 V

Switch off Pin Voltage -0.3 to +7 V Switch on Pin Voltage 30 sec short to IN or OUT

OUT, SHDN

Output, Shutdown Voltage -0.3 to +7 V

AGND to PGND Analog and Power Ground -0.3 to +0.3 V

FB FB Pin Voltage

-0.3 to (OUT+0.3) V

TOT

Continuous Power Dissipation (at TA= 85°C) DIP-8

550

Continuous Power Dissipation (at T

= 85°C) SO-8

344

STG

Storage Temperature Range

-40 to 150 °C

Operating Ambient Temperature Range

0to85 °C

TYPE DIP-8 SO-8

ST777 ST777ACN ST777ACD ST778 ST778ACN ST778ACD ST779 ST779ACN ST779ACD

Pin No. SYMBOL NAME AND FUNCTION

1 ILIM Sets switch current limit input. Connect to IN for 1A current limit. A resistor from ILIM to IN

sets lower peak inductor currents. 2 IN Input from battery 3 AGND Analog ground. Not internally connected to PGND. 4 PGND Power ground. Must be low impedance; solder directly to ground plane or star ground.

Connect to AGND, close to the device. 5 LX Collector of 1A NPN power switch and emitter of Sinchronous Rectifier PNP. 6 OUT Voltage Output. Connect filter capacitor close to pin. 7SHDN

Shutdown input disables power supply when low. Also disconnets load from input. Threshold

is set at V

/2.

8 SEL/N.C./FB - Selection pin for 3/3.3V version (778);

- Not internally connected for 5V version (777);

- Feedback pin for adjustable version (779).

Symbol Parameter DIP-8 SO-8 Unit

thj-amb

Thermal Resistance Junction-ambient

100 160 °C/W

ST777/778/779

3/11

ELECTRICAL CHARACTERISTICS (VIN=2.5V, CI= 22µF, CO=100µF, SHDN and ILIM connected to IN, AGND connected to P GND, T

=0 to 85°C, unless otherwise specified. Typical values are referred at

=25°C)

Note 1:Output in regulation, V

OUT=VOUT

(nominal) ± 4%.

Note 2:At hight V

to V

OUT

differentials, the maximum load current is limited by the maximum allowable power dissipation in the package.

Note 3:Start-up guaranteed under these load conditions. Note 4:Minimum value is production tested. Maximum value is guaranteed by design and is not production tested. Note 5:In the ST779 supply current depends on the resistor divider used to set the output voltage. Note 6:V

OUT

is set to a target value of +5V by 0.1% external feedback resistors. V

OUT

is measured to be 5V±2.5% to guarantee the error

comparatortrip point.

Symbol

Parameter

Test Conditions Min. Typ. Max. Unit

START

Start up Voltage I

LOAD

< 10mA, TA= 25°C (Note 1) 1 V

IN(MAX)

Maximum Input Voltage (Note 1,2) 6 V

Output Voltage ST777 779 (set to 5V), (Note 3)

LOAD

≤ 30mA, VIN = 1.1V to 5V or

LOAD

≤ 80mA, VIN = 1.8V to 5V or

LOAD

≤ 130mA, VIN = 2.4V to 5V

4.8 5.0 5.2 V

Output Voltage ST778 (Note 3)

SEL=0V I

LOAD

≤ 50mA,VIN =1.1V to 3.3V or

LOAD

≤ 210mA,VIN =1.8V to 3.3V or

LOAD

≤ 300mA,VIN =2.4V to 3.3V

3.17 3.30 3.43 V

SEL=OPEN I

LOAD

≤ 30mA, VIN =1.1V to 3V or

LOAD

≤ 210mA, VIN =1.8V to 3V or

LOAD

≤ 300mA, VIN =2.4V to 3V

2.88 3.00 3.12 V

Output Voltage Range ST779

(Note 4)

2.7 6.5 V

No Load Supply Current I

LOAD

= 0 mA, (Switch ON) (Note 5) 270 µA

SHDN

Shutdown Supply Current SHDN=0V, (Switch OFF) 20 35 µA

IN SHDN

Shutdown Input Current SHDN = 0 to V

15 100 nA

SHDN

=VINto 5V 12 40 µA

υ Efficiency I

LOAD

=100mA 82 %

Shutdown Input Threshold VIN=1V to 6V VIN/2 + 0.25 V

LIM

Current Limit 1.0 A

LIM TEMPCO

Current Limit Temperature Coefficient

-0.3 %/°C

OFFMIN

Minimum Switch Off Time 1.2 µs

ONMAX

Maximum Switch ON Time VIN=2.5V 4.5

=1.8V 6.5

=1V 15

CESAT NPN

Switch saturation Voltage ISW=400mA 0.25 V

=600mA 0.33

=1000mA 0.5

CESAT NPN

Rectifier Forward Drop ISW=400mA 0.18 V

=600mA 0.22

=1000mA 0.4

Error Comparator Trip Point ST779, over operating input

voltage (Note 6)

1.23±2% V

FB Pin Bias Current ST779, VFB=1.3V 50 nA

Switch Off Leakage Current 0.1 µA Rectifier Off Leakage

Current

0.1 µA

ST777/778/779

4/11

TYPICAL AP PLICATION CIRCUIT

APPLICATIONS INFORMATION

R1 and R2 must be placed only in ST779 applications to set the output voltage according to the following equation: V

OUT

= (1.23) [ (R1+R2)/R2] and to simplify the resistor selection: R1 = R2 [(V

OUT

/1.23)-1] It is possible to use a wide range of values for R2 (10KΩ to 50KΩ) with no significant loss of accuracy thanks to the very low FB input cu rrent. To have 1% error, the current through R2 must be at least 100 t im es FB ’s bias current. When larg e values are used for the feedback resistors (R1>50KΩ), stray output impedance at FB can incidentally add "lag" to the feedback response, destabilizing the regulator and creating a larger ripple at the ou tput. Lead lengths and circuit board traces at the FB node should be kept short. Compensate the loop by adding a "lead" compensation capacitor (C3, 100pF to 1nF) in parallel with R 1. The typica l value of t he L1 inductor is 22µH, enough for most applications. However, are also suitable values ranging from 10µF to 47µF with a saturation rating equal to or greater than the peak switch -current limit.

Efficiency will be reduced if the inductor works near its saturation limit, while will be maximized using an i nduc t or with a low DC resistance, preferably under 0.2Ω. Connecting ILIM to V

the maximum LX current limit (1A) is set. If this maximum value is not required is possible to reduce it connecting a resistor between ILIM and V

(SeeFigure16to choose the right value). The current limit value is misured when the switch current through the inductor begins to flatten and does ’nt coincide with the max short circuit current. Even if the device is designed to tolerate a short circuit wi thou t any dam age, it is strictly recommended to avoid a continuos and durabl e short circuit of the output to GND. To achieve the best performances f rom switching power supply topology, particular care to layout drawing is needed, in order to minimize EMI and obtain low noise. Moreover, jitter f ree operation ensures the full device functionality. Wire lengths must be minimized, filter and by-pass capacitors must be low ESR type, placed as close as possible to the integrated circuit. Solder AGND and PGND pins directly to a ground plane.

ST777 ST778 ST779

ILIM

L1 22µH

SEL / N.C. / FB

V+

Vout

SHDN

____

AGND

C1 22µF

C2 100µF

Vin

1÷6V

PGND

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