ST L6726A User Manual

L6726A

Single phase PWM controller

Feature

■Flexible power supply from 5 V to 12 V

■Power conversion input as low as 1.5 V

■1% output voltage accuracy

■High-current integrated drivers

■Adjustable output voltage

■0.8 V internal reference

■Sensorless and programmable OCP across Low-side RdsON

■Oscillator internally fixed at 270 kHz

■Programmable soft-start

■Ls-less start up

■Disable function

■FB disconnection protection

■SO-8 package

Applications

■Subsystem power supply (MCH, IOCH, PCI...)

■Memory and termination supply

■CPU and DSP power supply

■Distributed power supply

■General DC / DC converters

SO-8

Description

L6726A is a single-phase step-down controller with integrated high-current drivers that provides complete control logic, protections and reference voltage to realize in an easy and simple way general DC-DC converters by using a compact SO-8 package.

Device flexibility allows managing conversions with power input VIN as low as 1.5 V and device supply voltage ranging from 5 V to 12 V.

L6726A provides simple control loop with transconductance error amplifier. The integrated 0.8 V reference allows regulating output voltage with ±1% accuracy over line and temperature variations. Oscillator is internally fixed to 270 kHz.

L6726A provides programmable over current protection. Current information is monitored

across the low-side MOSFET RdsON saving the use of expensive and space-consuming sense

resistors.

FB disconnection protection prevents excessive and dangerous output voltages in case of floating FB pin.

Table 1.	Device summary
	Order codes	Package	Packaging
	L6726A	SO-8	Tube
	L6726ATR	SO-8	Tape and reel
March 2010		Doc ID 12754 Rev 4	1/35
			www.st.com

Contents	L6726A

Contents

1	Typical application circuit and block diagram . . . . . . . . . . . . . . . . . . .			. 4
	1.1	Application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		4
	1.2	Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		4
2	Pins description and connection diagrams . . . . . . . . . . . . . . . . . . . . . .			5
	2.1	Pin descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		5
	2.2	Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		6
3	Electrical specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .			7
	3.1	Absolute maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		7
	3.2	Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		7
4	Device description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .			9
5	Driver section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .			10
	5.1	Power dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		11
6	Soft start and disable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .			12
	6.1	Low-side-less start up (LSLess) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		12
	6.2	Enable / disable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		12
7	Protections .		. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	13
	7.1	Overcurrent protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		13
		7.1.1	Overcurrent threshold setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	13

7.2 Feedback disconnection protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 7.3 Undervoltage lock out . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

8	Application details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		15
	8.1	Output voltage selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	15
	8.2	Compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	15
	8.3	Soft-start time calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18
	8.4	Layout guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18
	8.5	Embedding L6726A-based VRs… . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	20

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L6726A Contents

9	Application Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		21
	9.1	Output inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	21
	9.2	Output capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	22
	9.3	Input capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	22

10	20 A demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		23
	10.1 Board description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		27
	10.1.1	Power input (VIN) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	27
	10.1.2	Power output (VOUT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	27
	10.1.3 IC additional supply (VCC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		27
	10.1.4	Test points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	27
	10.1.5	Demonstration board efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	27

11	5 A demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		28
	11.1 Board description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		30
	11.1.1	Power input (VIN) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	30
	11.1.2	Power output (VOUT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	31
	11.1.3 IC additional supply (VCC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		31
	11.1.4	Test points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	31
	11.1.5	Demonstration board efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	31
12	Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		32
13	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		34

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Typical application circuit and block diagram	L6726A

1 Typical application circuit and block diagram

1.1Application circuit

Figure 1. Typical application circuit

VCC = 5V to 12V										VIN = 1.5V to 19V (1)
		CDEC				5		D
				6		VCC	1	RD
				FB			BOOT	CBOOT
									CHF	CBULK
				7			2
						L6726A			HS
				COMP			UGATE
				/	DIS			RgHS
			CF				8		L	Vout
RFB	ROS	CP					PHASE
			RF
				3			4	RgLS	RSN
										COUT
				GND			LGATE		LS
							/ OC			LOAD
									CSN
								ROCSET
L6726A Reference Schematic
(1) Up to 12V with Vcc > 5V

1.2Block diagram

Figure 2. Block diagram

VCC
			OCP		VOCTH
CONTROL LOGIC
& PROTECTIONS
SS						BOOT
I	DISABLE
						UGATE
					HS
CLOCK	S Q		PWM
			CONDUCTIONCROSS	ANTI ADAPTIVE		PHASE
	R				VCC
OSCILLATOR
TRANSCONDUCTANCE					LS	LGATE
ERROR AMPLIFIER						/ OC
						GND
		+	0.8V
L6726A		-
					IOCSET
COMP / DIS	FB

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L6726A	Pins description and connection diagrams

2 Pins description and connection diagrams

Figure 3. Pins connection (top view)

BOOT							PHASE
			1		8
UGATE			2	L6726A	7		COMP / DIS
GND			3		6		FB
LGATE / OC			4		5		VCC

2.1Pin descriptions

	Table 2.	Pins descriptions
	Pin n	Name	Function
			HS driver supply.
	1	BOOT	Connect through a capacitor (100 nF) to the floating node (LS-drain) pin
			and provide necessary bootstrap diode from VCC.
	2	UGATE	HS driver output. Connect to HS MOSFET gate.
	3	GND	All internal references, logic and drivers are connected to this pin.
			Connect to the PCB ground plane.
			LGATE. LS driver output. Connect to LS MOSFET gate.
			OC. Over current threshold set. During a short period of time following VCC
			rising over UVLO threshold, a 10μA current is sourced from this pin.
	4	LGATE / OC	Connect to GND with an ROCSET resistor greater than 5kΩ to program OC
			Threshold. The resulting voltage at this pin is sampled and held internally
			as the OC set point. Maximum programmable OC threshold is 0.55 V. A
			voltage greater than 0.75V (max) activates an internal clamp and causes
			OC threshold to be set at 400 mV. ROCSET not connected sets the 400 mV
			default threshold.
	5	VCC	Device and LS driver power supply.
			Operative range from 4.1 V to 13.2 V. Filter with at least 1μF MLCC to GND.
			Error amplifier inverting input.
	6	FB	Connect with a resistor RFB to the output regulated voltage. Additional
			resistor ROS to GND may be used to regulate voltages higher than the
			reference.
			COMP. Error amplifier output. Connect with an RF - CF // CP to GND to
			compensate the device control loop in conjunction to the FB pin.
	7	COMP / DIS	During the soft-start phase, a 10 μA current is sourced from this pin so the
			compensation capacitors also act to program the SS time.
			DIS. The device can be disabled by pulling this pin lower than 0.4 V (min).
			Setting free the pin, the device enables again.
			HS driver return path, current-reading and adaptive-dead-time monitor.
	8	PHASE	Connect to the LS drain to sense RdsON drop to measure the output current.
			This pin is also used by the adaptive-dead-time control circuitry to monitor
			when HS MOSFET is OFF.

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Pins description and connection diagrams	L6726A

2.2Thermal data

Table 3.	Thermal data
Symbol	Parameter	Value	Unit
RthJA	Thermal resistance junction to ambient(1)	85	°C/W
TMAX	Maximum junction temperature	150	°C
TSTG	Storage temperature range	-40 to 150	°C
TJ	Junction temperature range	-20 to 150	°C

1.Measured with the component mounted on a 2S2P board in free air (6.7 cm x 6.7 cm, 35 μm (P) and 17.5 μm (S) copper thickness).

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L6726A	Electrical specifications

3 Electrical specifications

3.1Absolute maximum ratings

Table 4.	Absolute maximum ratings
Symbol		Parameter	Value	Unit
VCC		to GND	-0.3 to 15	V
VBOOT		to PHASE	15	V
		to GND	45
VUGATE		to PHASE	-0.3 to (VBOOT - VPHASE) + 0.3
		to PHASE; t < 50ns	-1	V
		to GND	VBOOT + 0.3
VPHASE		to GND	-8 to 30	V
VLGATE		to GND	-0.3 to VCC + 0.3	V
		to GND; t < 50ns	-2.5
		FB, COMP to GND	-0.3 to 3.6	V

3.2Electrical characteristics

	VCC = 12 V; TA = -20 °C to +85 °C unless otherwise specified.
Table 5.	Electrical characteristics
Symbol		Parameter		Test conditions		Min.	Typ.	Max.	Unit
Recommended operating conditions
VCC		Device supply voltage		See Figure 1		4.1		13.2	V
VIN		Conversion input voltage						13.2	V
				VCC < 7.0 V				19.0	V
Supply current and power-ON
ICC		VCC supply current		UGATE and LGATE = OPEN			6		mA
IBOOT		BOOT supply current		UGATE = OPEN; PHASE to GND			0.5		mA
UVLO		VCC Turn-ON		VCC rising				4.1	V
		Hysteresis					0.2		V
Oscillator
FSW		Main oscillator accuracy		TA = 0 °C to +70 °C		243	270	297	kHz
						225	270	315
VOSC		PWM ramp amplitude					1.1		V
dMAX		Maximum duty cycle				80			%

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Electrical specifications								L6726A
Table 5.	Electrical characteristics			(continued)
Symbol		Parameter		Test conditions	Min.	Typ.	Max.		Unit
Reference
		Output voltage accuracy		VOUT = 0.8 V, TA = 0 °C to 70 °C	-1	-	1		%
				VOUT = 0.8 V	-1.5	-	1.5
Transconductance error amplifier
gm		Transconductance(1)					5		mS
IFB		Input bias current		Sourced from FB		100			nA
A0		Open loop gain(1)				70			dB
F		Unity gain(1)				4			MHz
0
ICOMP		Current capability		Source current		360			μA
				Sink current		-360			μA
Soft-Start and disable
ISS		Soft-start current		From COMP pin		10			μA
DIS		Disable threshold		COMP falling	0.4	0.5			V
Gate drivers
IUGATE		HS source current		BOOT - PHASE = 5 V to 12 V		1.5			A
RUGATE		HS sink resistance		BOOT - PHASE = 5 V to 12 V		1.1			Ω
ILGATE		LS source current		VCC = 5 V to 12 V		1.5			A
RLGATE		LS sink resistance		VCC = 5 V to 12 V		0.65			Ω
Over-current protection
IOCSET		OCSET current source		Sourced from LGATE pin.		10			μA
				See Section 7.1.1
VOC_SW		OC switch-over threshold		VLGATE/OC rising			780		mV
VOCTH_FIXED		Fixed OC threshold		VPHASE to GND		-400			mV

1. Guaranteed by design, not subject to test.

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L6726A	Device description

4 Device description

L6726A is a single-phase PWM controller with embedded high-current drivers that provides complete control logic and protections to realize in an easy and simple way a general DCDC step-down converter. Designed to drive N-channel MOSFETs in a synchronous buck topology, with its high level of integration this 8-pin device allows reducing cost and size of the power supply solution.

L6726A is designed to operate from a 5 V or 12 V supply bus. Thanks to the high precision 0.8V internal reference, the output voltage can be precisely regulated to as low as 0.8 V with ±1% accuracy over line and temperature variations (between 0 °C and +70 °C). The switching frequency is internally set to 270 kHz.

This device provides a simple control loop with externally compensated transconductance error-amplifier and programmable soft start. Low-side-less feature allows the device to perform soft-start over pre-charged output avoiding negative spikes at the load side.

In order to avoid load damages, L6726A provides programmable threshold over current protection. Output current is monitored across low-side MOSFET RdsON, saving the use of expensive and space-consuming sense resistor. L6726A also features FB disconnection protection, preventing dangerous uncontrolled output voltages in case of floating FB pin.

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Driver section	L6726A

5 Driver section

The integrated high-current drivers allow using different types of power MOSFET (also multiple MOSFETs to reduce the equivalent RdsON), maintaining fast switching transition.

The driver for high-side MOSFET uses BOOT pin for supply and PHASE pin for return. The driver for low-side MOSFET uses the VCC pin for supply and GND pin for return.

The controller embodies an anti-shoot-through and adaptive dead-time control to minimize low side body diode conduction time, maintaining good efficiency while saving the use of Schottky diode:

●to check for high-side MOSFET turn off, PHASE pin is sensed. When the voltage at PHASE pin drops down, the low-side MOSFET gate drive is suddenly applied;

●to check for low-side MOSFET turn off, LGATE pin is sensed. When the voltage at LGATE has fallen, the high-side MOSFET gate drive is suddenly applied.

If the current flowing in the inductor is negative, voltage on PHASE pin will never drop. To allow the low-side MOSFET to turn-on even in this case, a watchdog controller is enabled: if the source of the high-side MOSFET doesn't drop, the low side MOSFET is switched on so allowing the negative current of the inductor to recirculate. This mechanism allows the system to regulate even if the current is negative.

Power conversion input is flexible: 5 V, 12 V bus or any bus that allows the conversion (See maximum duty cycle limitation and recommended operating conditions) can be chosen freely.

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L6726A	Driver section

5.1Power dissipation

L6726A embeds high current MOSFET drivers for both high side and low side MOSFETs: it is then important to consider the power that the device is going to dissipate in driving them in order to avoid overcoming the maximum junction operative temperature.

Two main terms contribute in the device power dissipation: bias power and drivers power.

●Device bias power (PDC) depends on the static consumption of the device through the supply pins and it is simply quantifiable as follow (assuming to supply HS and LS drivers with the same VCC of the device):

PDC = VCC (ICC + IBOOT)

●Drivers power is the power needed by the driver to continuously switch on and off the external MOSFETs; it is a function of the switching frequency, the voltage supply of the driver and total gate charge of the selected MOSFETs. It can be quantified considering

that the total power PSW dissipated to switch the MOSFETs (easy calculable) is dissipated by three main factors: external gate resistance (when present), intrinsic MOSFET resistance and intrinsic driver resistance. This last term is the important one to be determined to calculate the device power dissipation. The total power dissipated to switch the MOSFETs results:

PSW = FSW [QgHS (VBOOT – VPHASE) + QgLS VCC]

where VBOOT - VPHASE is the voltage across the bootstrap capacitor.

External gate resistors helps the device to dissipate the switching power since the same power PSW will be shared between the internal driver impedance and the external resistor resulting in a general cooling of the device.

Figure 4. Soft start (left) and disable (right)

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