ST777ACN - STMicroelectronics

1/11October 2002

■1V TO 6V INPUT GUARANTEES START-UP

UNDER LOAD

■MAXIMUM OUTPUT CURRENT OF 300mA

(778 OR 779 ADJUSTED TO 3V)

■LOAD FULLY DISCONNECTED IN

SHUTDOWN

■TYPICAL EFFICIENCY OF 82%

■INTERNAL 1A POWER 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 include a

Sinchronous Rectifier that eliminates 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 shutdown mode, fully disconnecting the output

from the source. This eliminates the current drain

associated with conventional step-up converters

when off or in shutdown. Supply current is 270µA

under no load and only 20µA in stand by mode.

ST777/778/779

LOW VOLTAGE INPUT, 3-3.3V/5V/ADJUSTABLE OUTPUT

DC-DC CONVERTER WITH SYNCHRONOUS RECTIFIER

VIN

22µH

100µF

RLIM

ILIM LX

OUT

PGND

SEL

AGND

SHDN

1.25V

VREF

SHUTDOWN

CONTROL

DELAY

TIMER

DELAY

TIMER

tOFF

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 CONNECTIONS

THERMAL DATA

Symbol Parameter Value Unit

VCC DC Input Voltage to GND -0.3 to +7 V

LX 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

PTOT Continuous Power Dissipation (at TA= 85°C) DIP-8 550 mW

Continuous Power Dissipation (at TA= 85°C) SO-8 344

TSTG Storage Temperature Range -40 to 150 °C

TOP 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 VIN/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

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

TA=25°C)

Note 1: Output in regulation, VOUT =V

OUT (nominal) ± 4%.

Note 2: At hight VIN to VOUT 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: VOUT is set to a target value of +5V by 0.1% external feedback resistors. VOUT is measured to be 5V±2.5% to guarantee the error

comparator trip point.

Symbol Parameter Test Conditions Min. Typ. Max. Unit

VSTART Start up Voltage ILOAD < 10mA, TA= 25°C (Note 1) 1 V

VIN(MAX) Maximum Input Voltage (Note 1,2) 6 V

VOOutput Voltage ST777 779

(set to 5V), (Note 3) ILOAD≤ 30mA, VIN = 1.1V to 5V or

ILOAD≤ 80mA, VIN = 1.8V to 5V or

ILOAD≤ 130mA, VIN = 2.4V to 5V

4.8 5.0 5.2 V

Output Voltage ST778

(Note 3) SEL=0V

ILOAD≤ 50mA,VIN =1.1V to 3.3V or

ILOAD≤ 210mA,VIN =1.8V to 3.3V or

ILOAD≤ 300mA,VIN =2.4V to 3.3V

3.17 3.30 3.43 V

SEL=OPEN

ILOAD≤ 30mA, VIN =1.1V to 3V or

ILOAD≤ 210mA, VIN =1.8V to 3V or

ILOAD≤ 300mA, VIN =2.4V to 3V

2.88 3.00 3.12 V

Output Voltage Range

ST779 (Note 4) 2.7 6.5 V

IIN No Load Supply Current ILOAD = 0 mA, (Switch ON) (Note 5) 270 µA

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

IIN SHDN Shutdown Input Current SHDN = 0 to VIN 15 100 nA

SHDN =V

IN to 5V 12 40 µA

υEfficiency ILOAD=100mA 82 %

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

ILIM Current Limit 1.0 A

ILIM TEMPCO Current Limit Temperature

Coefficient -0.3 %/°C

tOFFMIN Minimum Switch Off Time 1.2 µs

tONMAX Maximum Switch ON Time VIN=2.5V 4.5

VIN=1.8V 6.5

VIN=1V 15

VCESAT NPN Switch saturation Voltage ISW=400mA 0.25 V

ISW=600mA 0.33

ISW=1000mA 0.5

VCESAT NPN Rectifier Forward Drop ISW=400mA 0.18 V

ISW=600mA 0.22

ISW=1000mA 0.4

VFB Error Comparator Trip Point ST779, over operating input

voltage (Note 6) 1.23±2% V

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

ILX

Switch Off Leakage Current 0.1 µA

Rectifier Off Leakage

Current 0.1 µA

ST777/778/779

4/11

TYPICAL APPLICATION CIRCUIT

APPLICATIONS INFORMATION

R1 and R2 must be placed only in ST779

applications to set the output voltage according to

the following equation:

VOUT = (1.23) [(R1+R2)/R2]

and to simplify the resistor selection:

R1 = R2 [(VOUT/1.23)-1]

It is possible to use a wide range of values for R2

(10KΩto 50KΩ) with no significant loss of

accuracy thanks to the very low FB input current.

To have 1% error, the current through R2 must be

at least 100 times FB’s bias current.

When large 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 output. 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 R1.

The typical value of the 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 inductor with a low DC resistance,

preferably under 0.2Ω.

Connecting ILIM to VIN 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 VIN (SeeFigure16to

choose the right value). The current limit value is

misured when the switch current through the

inductorbegins to flatten and does’nt coincide with

the max short circuit current.

Even if the device is designed to tolerate a short

circuit without any damage, it is strictly

recommended to avoid a continuos and durable

short circuit of the output to GND.

To achieve the best performances from switching

power supply topology, particular care to layout

drawing is needed, in order to minimize EMI and

obtain low noise. Moreover, jitter free 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

22µH

SEL / N.C. / FB

V+

Vout

SHDN

____

AGND

22µF

100µF

Vin

1÷6V

PGND

ST777/778/779

5/11

TYPICAL CHARACTERISTICS

(unless otherwise specified T

= 25°C, C

=22µF, C

=100

Figure 1 : Output Voltage vs Temperature

Figure 2 : Output Voltage vs Temperature

Figure 3 : Efficiency vs Temperature

Figure 4 : Efficiency vs Input Voltage

Figure 5 : Efficiency vs Output Current

Figure 6 : Efficiency vs Low Output Current

ST777/778/779

6/11

Figure 7 : No Load Supply Current vs Input

Voltage

Figure 8 : No Load Supply Current vs

Temperature

Figure 9 : Shutdown Input Threshold vs Input

Voltage

Figure 10 : Minimum Switch Off Time vs

Temperature

Figure 11 : Maximum Switch ON Time vs

Temperature

Figure 12 : FB Pin Bias Current vs Temperature

ST777/778/779

7/11

Figure 13 : Error Comparator Trip Point vs

Temperature

Figure 14 : Maximum Output Current vs Input

Voltage

Figure 15 : Maximum Output Current vs Input

Voltage

Figure 16 : Peak Inductor Current vs

Current-Limit Resistor

ST777/778/779

8/11

Figure 17 : Line Transient

Figure 18 : Load Transient

Figure 19 : Switching Waveform

Figure 20 : Switching Waveform

PRINTED DEMOBOARD (Not in scale)

Vout

Vin

Vout (5V)

Iout

ST777/779

Vin=2.5V

Iout=10mA to 130mA

Vout

Isw

Vin=1.1V

Iout=30mA

Vout

Isw

Vin=2.5V

Iout=30mA

ST777/778/779

9/11

DIM. mm. inch

MIN. TYP MAX. MIN. TYP. MAX.

A 3.3 0.130

a1 0.7 0.028

B 1.39 1.65 0.055 0.065

B1 0.91 1.04 0.036 0.041

b 0.5 0.020

b1 0.38 0.5 0.015 0.020

D 9.8 0.386

E 8.8 0.346

e 2.54 0.100

e3 7.62 0.300

e4 7.62 0.300

F 7.1 0.280

I 4.8 0.189

L 3.3 0.130

Z 0.44 1.6 0.017 0.063

Plastic DIP-8 MECHANICAL DATA

P001F

ST777/778/779

10/11

DIM. mm. inch

MIN. TYP MAX. MIN. TYP. MAX.

A 1.75 0.068

a1 0.1 0.25 0.003 0.009

a2 1.65 0.064

a3 0.65 0.85 0.025 0.033

b 0.35 0.48 0.013 0.018

b1 0.19 0.25 0.007 0.010

C 0.25 0.5 0.010 0.019

c1 45˚ (typ. )

D 4.8 5.0 0.189 0.196

E 5.8 6.2 0.228 0.244

e 1.27 0.050

e3 3.81 0.150

F 3.8 4.0 0.149 0.157

L 0.4 1.27 0.015 0.050

M 0.6 0.023

˚ (max.)

SO-8 MECHANICAL DATA

0016023

ST777/778/779

11/11

Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsib ility for the

consequences of u se of su ch informat ion nor for any infri ngement of p atents or other rights of third parties w hich may result from

its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications

mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information

previously supplied. S TMicroelectro nics products are not authorized f or use as critical components in life support devices or

systems without express written approval of STMicroelectronics.

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