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

ST777/778/779

LOW VOLTAGE INPUT, 3-3.3V/5V/ADJUSTABLE OUTPUT DC-DC CONVERTER WITH SYNCHRONOUS RECTIFIER

■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.

SCHEMATIC DIAGRAM

DIP-8

SO-8

Unlike others step-up DC-DC converters the ST777/778/779’s Sinchronous Rectifier turns 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.

VIN			22µH
		100µF
			RLIM
			2	1	5
		IN	ILIM	LX
	DELAY
	TIMER
	tOFF				ACTIVE RECTIFIER	OUT	6	VO
			SWITCH
	DELAY		DRIVER					100µF
	TIMER				RECTIFIER
	tON
					CONTROL
						PGND 4
				1 : N
		1.25V				SEL	8
		VREF
7 SHDN SHUTDOWN
		CONTROL			AGND
					3

October 2002

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ST777/778/779

ABSOLUTE MAXIMUM RATINGS

	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	0 to 85	°C

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
TYPE	DIP-8	SO-8
ST777	ST777ACN	ST777ACD
ST778	ST778ACN	ST778ACD
ST779	ST779ACN	ST779ACD

PIN CONNECTIONS

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.
7	SHDN	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).

THERMAL DATA

Symbol	Parameter	DIP-8	SO-8	Unit
Rthj-amb	Thermal Resistance Junction-ambient	100	160	°C/W

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ST777/778/779

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)

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
VO	Output Voltage ST777 779		ILOAD≤ 30mA, VIN = 1.1V to 5V or			4.8		5.0		5.2	V
	(set to 5V), (Note 3)		ILOAD≤ 80mA, VIN = 1.8V to 5V or
			ILOAD≤ 130mA, VIN = 2.4V to 5V
	Output Voltage ST778		SEL=0V			3.17		3.30		3.43	V
	(Note 3)		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
			SEL=OPEN			2.88		3.00		3.12	V
			ILOAD≤ 30mA, VIN =1.1V to 3V or
			ILOAD≤ 210mA, VIN =1.8V to 3V or
		ILOAD≤ 300mA, VIN =2.4V to 3V
	Output Voltage Range		(Note 4)			2.7				6.5	V
	ST779
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	= VIN to 5V				12		40	µA
υ	Efficiency		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							-0.3			%/°C
	Coefficient
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					1.23±2%			V
			voltage (Note 6)
IFB	FB Pin Bias Current		ST779, VFB=1.3V					50			nA
	Switch Off Leakage Current							0.1			µA
ILX
	Rectifier Off Leakage							0.1			µA
	Current

Note 1: Output in regulation, VOUT = VOUT (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.

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ST777/778/779

TYPICAL APPLICATION CIRCUIT

	1 ILIM	SEL / N.C. / FB 8
						R2
Vin		ST777
1÷6V			____
	2 V+	ST778		7	R1
			SHDN			C3
C1	L1	ST779
	22µH
22µF
	3 AGND		Vout	6		Vo
						C2
						100µF
	4 PGND		LX	5

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 (10KΩ to 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 (See Figure 16 to 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 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.

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