ST L4970A User Manual

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L4970A

10A SWITCHING REGULATOR

10A OUTPUT CURRENT

5.1V TO 40V OUTPUT VOLTAGE RANGE

0 TO 90% DUTY CYCLE RANGE

INTERNAL FEED-FORWARD LINE REGULATION

INTERNAL CURRENT LIMITING

PRECISE 5.1V ± 2% ON CHIP REFERENCE RESET AND POWER FAIL FUNCTIONS SOFT START

INPUT/OUTPUT SYNC PIN

UNDER VOLTAGE LOCK OUT WITH HYSTERETIC TURN-ON

PWM LATCH FOR SINGLE PULSE PER PERIOD

VERY HIGH EFFICIENCY

SWITCHING FREQUENCY UP TO 500KHz THERMAL SHUTDOWN

CONTINUOUS MODE OPERATION

DESCRIPTION

The L4970A is a stepdown monolithic power switching regulator delivering 10A at a voltage variable from 5.1 to 40V.

MULTIPOWER BCD TECHNOLOGY

Multiwatt15V

ORDERING NUMBER: L4970A

Realized with BCD mixed technology, the device uses a DMOS output transistor to obtain very high efficiency and very fast switching times. Features of the L4970A include reset and power fail for microprocessors, feed forward line regulation, soft start, limiting current and thermal protection. The device is mounted in a 15-lead multiwatt plastic power package and requires few external components. Efficient operation at switching frequencies up to 500KHz allows reduction in the size and cost of external filter components.

BLOCK DIAGRAM

June 2000

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L4970A

ABSOLUTE MAXIMUM RATINGS

Symbol	Parameter	Value	Unit
V9	Input Voltage	55	V
V9	Input Operating Voltage	50	V
V7	Output DC Voltage	-1	V
	Output Peak Voltage at t = 0.1μs f = 200KHz	-7	V
I7	Maximum Output Current	Internally Limited
V6	Bootstrap Voltage	65	V
	Bootstrap Operating Voltage	V9 + 15	V
V3, V12	Input Voltage at Pins 3, 12	12	V
V4	Reset Output Voltage	50	V
I4	Reset Output Sink Current	50	mA
V5, V10, V11, V13	Input Voltage at Pin 5, 10, 11, 13	7	V
I5	Reset Delay Sink Current	30	mA
I10	Error Amplifier Output Sink Current	1	A
I12	Soft Start Sink Current	30	mA
Ptot	Total Power Dissipation at Tcase < 120°C	30	W
Tj, Tstg	Junction and Storage Temperature	-40 to 150	°C

PIN CONNECTION (Top view)

THERMAL DATA

Symbol	Parameter		Value	Unit
Rth j-case	Thermal Resistance Junction-case	max	1	°C/W
Rth j-amb	Thermal Resistance Junction-ambient	max	35	°C/W

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		L4970A
PIN FUNCTIONS
No	Name	Function
1	OSCILLATOR	Rosc. External resistor connected to ground determines the constant charging
		current of Cosc.
2	OSCILLATOR	Cosc. External capacitor connected to ground determines (with Rosc) the
		switching frequency.
3	RESET INPUT	Input of Power Fail Circuit. The threshold is 5.1V. It may be connected via a
		divider to the input for power fail function. It must be connected to the pin 14 an
		external 30KΩ resistor when power fail signal not required.
4	RESET OUT	Open Collector Reset/power Fail Signal Output. This output is high when the
		supply and the output voltages are safe.
5	RESET DELAY	A Cd capacitor connected between this terminal and ground determines the
		reset signal delay time.
6	BOOTSTRAP	A Cboot capacitor connected between this terminal and the output allows to
		drive properly the internal D-MOS transistor.
7	OUTPUT	Regulator Output.
8	GROUND	Common Ground Terminal
9	SUPPLY VOLTAGE	Unregulated Input Voltage.
10	FREQUENCY	A series RC network connected between this terminal and ground determines
	COMPENSATION	the regulation loop gain characteristics.
11	FEEDBACK INPUT	The Feedback Terminal of the Regulation Loop. The output is connected
		directly to this terminal for 5.1V operation; It is connected via a divider for higher
		voltages.
12	SOFT START	Soft Start Time Constant. A capacitor is connected between thi sterminal and
		ground to define the soft start time constant.
13	SYNC INPUT	Multiple L4970A are synchronized by connecting pin 13 inputs together or via
		an external syncr. pulse.
14	Vref	5.1V Vref Device Reference Voltage.
15	Vstart	Internal Start-up Circuit to Drive the Power Stage.

CIRCUIT OPERATION (refer to the block diagram)

The L4970A is a 10A monolithic stepdown switching regulator working in continuous mode realized in the new BCD Technology. This technology allows the integration of isolated vertical DMOS power transistors plus mixed CMOS/Bipolar transistors.

The device can deliver 10A at an output voltage adjustable from 5.1V to 40V, and contains diagnostic and control functions that make it particularly suitable for microprocessor based systems.

BLOCK DIAGRAM

The block diagram shows the DMOS power transistor and the PWM control loop. Integrated functions include a reference voltage trimmed to 5.1V

± 2%, soft start, undervoltage lockout, oscillator with feedforward control, pulse by pulse current limit, thermal shutdown and finally the reset and power fail circuit. The reset and power fail circuit provides an output signal for a microprocessor indicating the status of the system.

Device turn on is around 11V with a typical 1V hysteresis, this threshold provides a correct voltage for the driving stage of the DMOS gate and the hysteresis prevents instabilities.

An external bootstrap capacitor charged to 12V by an internal voltage reference is needed to provide correct gate drive to the power DMOS. The driving circuit is able to source and sink peak currents of around 0.5A to the gate of the DMOS transistor. A typical switching time of the current in the DMOS transistor is 50ns. Due to the fast commutation switching frequencies up to 500kHz are possible.

The PWM control loop consists of a sawtooth oscillator, error amplifier, comparator, latch and the output stage. An error signal is produced by comparing the output voltage with the precise 5.1V ± 2% on chip reference. This error signal is then compared with the sawtooth oscillator, in order to generate a fixed frequency pulse width modulated drive for the output stage. A PWM latch is included to eliminate multiple pulsing within a period even in noisy environments. The gain and

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L4970A

Figure 1: Feedforward Waveform

Figure 2: Soft Start Function

Figure 3: Limiting Current Function

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L4970A

stability of the loop can be adjusted by an external RC network connected to the output of the error amplifier. A voltage feedforward control has been added to the oscillator, this maintains superior line regulation over a wide input voltage range. Closing the loop directly gives an output voltage of 5.1V, higher voltages are obtained by inserting a voltage divider.

At turn on output overcurrents are prevented by the soft start function (fig. 2). The error amplifier is initially clamped by an external capacitor Css and allowed to rise linearly under the charge of an internal constant current source.

Output overload protection is provided by a current limit circuit (fig. 3). The load current is sensed by an internal metal resistor connected to a comparator. When the load current exceeds a preset threshold the output of the comparator sets a flip flop which turns off the power DMOS. The next clock pulse, from an internal 40kHz oscillator will reset the flip flop and the power DMOS will again conduct. This current protection method, ensures

Figure 4: Reset and Power Fail Functions.

a constant current output when the system is overloaded or short circuited and limits the switching frequency, in this condition, to 40kHz.

The Reset and Power fail circuitry (fig 4) generates an output signal when the supply voltage exceeds a threshold programmed by an external voltage divider. The reset signal, is generated with a delay time programmed by an external capacitor on the delay pin. When the supply voltage falls below the threshold or the output voltage goes below 5V the reset output goes low immediately. The reset output is an open collector-drain.

Fig 4A shows the case when the supply voltage is higher than the threshold, but the output voltage is not yet 5V.

Fig 4B shows the case when the output is 5.1V but the supply voltage is not yet higher than the fixed threshold.

The thermal protection disables circuit operation when the junction temperature reaches about 150°C and has an hysterysis to prevent unstable conditions.

A

B

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L4970A

ELECTRICAL CHARACTERISTICS (Refer to the test circuit, Tj = 25°C, Vi = 35V, R4 = 16KΩ, C9 = 2.2nF, fSW = 200KHz typ, unless otherwise specified)

DYNAMIC CHARACTERISTICS

Symbol			Parameter	Test Condition	Min.	Typ.	Max.	Unit	Fig.
	Vi		input Voltage Range (pin 9)	Vo = Vref to 40V	15		50	V	5
				Io = 10A
	Vo		Output Votage	Vi = 15V to 50V	5	5.1	5.2	V	5
				Io = 5A; Vo = Vref
	Vo		Line Regulation	Vi = 15V to 50V		12	30	mV	5
				Io = 5A; Vo = Vref
	Vo		Load Regulation	Vo = Vref					5
				Io = 3A to 6A		10	30	mV
				Io = 2A to 10A		20	50	mV
	Vd		Dropout Voltage Between	Io = 5A		0.55	0.8	V	5
			Pin 9 and 7	Io = 10A		1.1	1.6	V
	I7L		Max. Limiting Current	Vi = 15 to 50V	11	13	15	A	5
	η		Efficiency	Io = 5A					5
				Vo = Vref	80	85		%
				Vo = 12V		92		%
				Io = 10A					5
				Vo = Vref	75	80		%
				Vo = 12V		87		%
SVR			Supply Voltage Ripple	Vi = 2VRMS; Io = 5A	56	60		dB	5
			Reject.	f = 100Hz; Vo = Vref
	f		Switching Frequency		180	200	220	KHz	5
	f		Voltage Stability of	Vi = 15V to 45V		2	6	%	5
	Vi		Swiching Frequency
	f		Temperature Stability of	°		1		%	5
	Tj			Tj = 0 to 125 C
			Swiching Frequency
	fmax		Maximum Operating	Vo = Vref; R4 = 10KΩ	500			KHz	5
			Switching Frequency	Io = 10A; C9 = 1nF
Vref SECTION (pin 14)

Symbol	Parameter	Test Condition		Min.	Typ.	Max.	Unit		Fig.
V14	Reference Voltage			5	5.1	5.2	V		7
V14	Line Regulation	Vi = 15V to 50V			10	25	mV		7
V14	Load Regulation	I14 = 0 to 1mA			20	40	mV		7
V14	Average Temperature	°	°		0.4		°	C	7
T		Tj = 0 C to 125 C					mV/
	Coefficient Reference
	Voltage
I14 short	Short Circuit Current Limit	V14 = 0			70		mA		7
VSTART SECTION (pin 15)

Symbol	Parameter	Test Condition	Min.	Typ.	Max.	Unit	Fig.
V15	Reference Voltage		11.4	12	12.6	V	7
V15	Line Regulation	Vi = 15 to 50V		0.6	1.4	V	7
V15	Load Regulation	I15 = 0 to 1mA		50	200	mV	7
I15 short	Short Circuit Current Limit	V15 = 0V		80		mA	7

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L4970A

ELECTRICAL CHARACTERISTICS (continued)

DC CHARACTERISTICS

Symbol	Parameter	Test Condition			Min.	Typ.	Max.	Unit	Fig.
V9on	Turn-on Threshold				10	11	12	V	7A
V9 Hyst	Turn-off Hysteresys					1		V	7A
I9Q	Quiescent Current	V12 = 0;	S1	= D		13	19	mA	7A
I9OQ	Operating Supply Current	V12 = 0;	S1	= C; S2 = B		16	23	mA	7A
I7L	Out Leak Current	Vi = 55V; S3 = A; V12 = 0					2	mA	7A
SOFT START

Symbol	Parameter		Test Condition	Min.	Typ.	Max.	Unit	Fig.
I12	Soft Start Source Current	V12 = 3V; V11 = 0V		70	100	130	μA	7B
V12	Output Saturation Voltage	I12	= 20mA; V9 = 10V			1	V	7B
		I12	= 200μA; V9 = 10V			0.7	V	7B
ERROR AMPLIFIER

Symbol	Parameter	Test Condition	Min.	Typ.	Max.	Unit	Fig.
V10H	High Level Out Voltage	I10 = -100μA; S1 = C	6			V	7C
		V11 = 4.7V
V10L	Low Level Out Voltage	I10 = +100μA; S1 = C			1.2	V	7C
		V11 = 5.3V;
I10H	Source Output Current	V10 = 1V; S1 = E	100	150		μA	7C
		V11 = 4.7V
I10L	Sink Output Current	V10 = 6V; S1 = D	100	150		μA	7C
		V11 = 5.3V
I11	Input Bias Current	RS = 10KΩ		0.4	3	μA	–
GV	DC Open Loop Gain	VVCM = 4V;	60			dB	–
		RS = 10Ω
SVR	Supply Voltage Rejection	15 < Vi < 50V;	60	80		dB	–
		RS = 10Ω
VOS	Input Offset Voltage	RS = 50Ω		2	10	mV	–
RAMP GENERATOR (pin 2)

Symbol	Parameter		Test Condition				Min.	Typ.	Max.	Unit	Fig.
V2	Ramp Valley	S1	= C; S2 = B				1.2	1.5		V	7A
V2	Ramp Peak	S1	= C;			Vi = 15V		2.5		V	7A
		S2	= B;			Vi = 45V		5.5		V	7A
I2	Min. Ramp Current	S1	= A; I1 = 100μA					270	300	μA	7A
I2	Max. Ramp Current	S1	= A; I1 = 1mA				2.4	2.7		mA	7A
SYNC FUNCTION (pin 13)

Symbol	Parameter		Test Condition	Min.	Typ.	Max.	Unit	Fig.
V13	Low Input Voltage	Vi = 15V to 50V; V12 = 0;		–0.3		0.9	V	7A
		S1	= C; S2 = B; S4 = B
V13	High Input voltage	V12 = 0;		3.5		5.5	V	7A
		S1	= C; S2 = B; S4 = B
I13L	Sync Input Current with	V13 = V2 = 0.9V; S4 = A;				0.4	mA	7A
	Low Input Voltage	S1	= C; S2 = B
I13H	Input Current with High	V13 = 3.5V; S4 = A;				2	mA	7A
	Input Voltage	S1	= C; S2 = B
V13	Output Amplitude			4	5		V	–
tW	Output Pulse Width	Vthr = 2.5V		0.3	0.5	0.8	μs	–

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