ST L6728AH User Manual

L6728AH

High frequency single phase PWM controller with Power Good

Features

■Flexible power supply from 5 V to 12 V

■Power conversion input as low as 1.5 V

■0.8 V internal reference

■0.8% output voltage accuracy

■High-current integrated drivers

■Power Good output

■Sensorless and programmable OCP across low-side RDS(on)

■OV / UV protections

■VSEN disconnection protection

■Oscillator internally fixed at 600 kHz

■LS-LESS to manage pre-bias start-up

■Adjustable output voltage

■Disable function

■Internal soft-start

■VFDFPN 10 package

Applications

■Memory and termination supply

■Subsystem power supply (MCH, IOCH, PCI)

■CPU and DSP power supply

■Distributed power supply

■General DC / DC converters

Table 1. Device summary

VFQFPN 10

Description

L6728AH is a single-phase step-down controller with integrated high-current drivers that provides complete control logic and protection to realize in a simple way general DC-DC converters by using a compact VFDFPN 10 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.

L6728AH provides simple control loop with voltage mode EA. The integrated 0.8 V reference allows regulating output voltages with ±0.8% accuracy over line and temperature variations. Oscillator is internally fixed to 600 kHz.

L6728AH provides programmable dual level over current protection as well as over and under voltage protection. Current information is

monitored across the low-side MOSFET RDS(on) saving the use of expensive and space-

consuming sense resistors.

PGOOD output easily provides real-time information on output voltage status, through VSEN dedicated output monitor.

	Order codes	Package	Packing
	L6728AH	VFDFPN 10	Tube
	L6728AHTR		Tape and reel
	May 2009	Doc ID 15726 Rev 1	1/33
			www.st.com

Contents	L6728AH

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
6	Soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		11
	6.1	Low-side-less start up (LSLess) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
7	Over-current protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		12
	7.1	Over-current threshold setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	12
8	Output voltage setting and protections . . . . . . . . . . . . . . . . . . . . . . . .		13
9	Application details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		14
	9.1	Compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
	9.2	Layout guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	16
10	Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		18
	10.1	Inductor design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18
	10.2	Output capacitor(s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	19
	10.3	Input capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	19

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L6728AH		Contents
11	20 A demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. . . . 20
	11.1 Demonstration board description . . . . . . . . . . . . . . . . . . . . . . . . . . .	. . . . 23

11.1.1 Power input (VIN) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 11.1.2 Output (VOUT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 11.1.3 Signal input (VCC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 11.1.4 Test points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

11.2 Demonstration board characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

12	5 A demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	25
	12.1 Demonstration board description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	28

12.1.1 Power input (VIN) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 12.1.2 Output (VOUT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 12.1.3 Signal input (VCC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 12.1.4 Test points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

12.2 Demonstration board characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

13	Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	30
14	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	32

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Typical application circuit and block diagram	L6728AH

1 Typical application circuit and block diagram

1.1Application circuit

Figure 1. Typical application circuit

VCC = 5V to 12V								VIN = 1.5V to 12V
	CDEC			6
		RPG
				VCC
						1
PGOOD		10	PGOOD		BOOT
							CHF	CBULK
						3
		7		L6728A			HS
			COMP		UGATE
			/ DIS			2	L	Vout
	CF
					PHASE
CP
	RF		L6728AH			4
		8					LS	COUT
			FB		LGATE
					/ OC			LOAD
			VSEN	GND
ROS		RFB	9		5	ROCSET
		ROS		RFB
L6728A Reference Schematic

1.2Block diagram

Figure 2. Block diagram

	VCC
VSEN	VOUT MONITOR		OC		VOCTH
	CONTROL LOGIC
	&
PGOOD	PROTECTIONS
						BOOT
	CLOCK		CONDUCTION CROSS		HS	UGATE
				ANTI ADAPTIVE
						PHASE
		PWM			VCC
	600 kHz
	OSCILLATOR					LGATE
					LS
						/ OC
	ERROR AMPLIFIER
						GND
	L6728AH	+	0.8V
		-
					IOCSET
	COMP / DIS	FB

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L6728AH	Pins description and connection diagrams

2 Pins description and connection diagrams

Figure 3. Pins connection (top view)

	BOOT	1		10	PGOOD
	PHASE	2	L6728A	9	VSEN
	UGATE	3		8	FB
			L6728AH
	LGATE / OC	4		7	COMP / DIS
	GND	5		6	VCC

2.1Pin descriptions

	Table 2.		Pin description
	Pin #		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.
				HS driver return path, current-reading and adaptive-dead-time monitor. Connect to the LS
	2		PHASE	drain to sense RDS(on) 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.
	3		UGATE	HS driver output. Connect directly to HS MOSFET gate.
				LGATE. LS driver output. Connect directly to LS MOSFET gate.
				OC over-current threshold set. During a short period of time following VCC rising over UVLO
	4	LGATE / OC		threshold, a 10 μA current is sourced from this pin. Connect to GND with an ROCSET resistor
				greater than 5 kΩ 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 0.6 V activates an internal clamp and causes OC threshold to be set at the
				maximum value.
	5		GND	All internal references, logic and drivers are connected to this pin.
				Connect to the PCB ground plane.
	6		VCC	Device and drivers power supply.
				Operative range from 5 V to 12 V. Filter with at least 1 μF MLCC to GND.
				COMP. Error amplifier output. Connect with an RF - CF // CP to FB to compensate the device
	7	COMP / DIS		control loop.
				DIS. The device can be disabled by pushing this pin lower than 0.75 V (typ). Setting free the
				pin, the device enables again.
				Error amplifier inverting input.
	8		FB	Connect with a resistor RFB to the output regulated voltage. Output resistor divider may be
				used to regulate voltages higher than the reference.
				Regulated voltage sense pin for OVP and UVP protections and PGOOD. Connect to the
	9		VSEN	output regulated voltage, or to the output resistor divider if the regulated voltage is higher than
				the reference.
	10		PGOOD	Open drain output set free after SS has finished and pulled low when VSEN is outside the
				relative window. Pull up to a voltage equal or lower than VCC. If not used it can be left floating.

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Pins description and connection diagrams	L6728AH

2.2Thermal data

	Table 3.	Thermal data
	Symbol	Parameter	Value	Unit
	RTH(JA)	Thermal resistance junction to ambient	45	°C/W
		(Device soldered on 2s2p, 67 mm x 69 mm board)
	RTH(JC)	Thermal resistance junction to case	5	°C/W
	TMAX	Maximum junction temperature	150	°C
	TSTG	Storage temperature range	-40 to 150	°C
	TJ	Junction temperature range	-40 to 125	°C
	PTOT	Maximum power dissipation at TA = 25 °C	2.25	W

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L6728AH	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
		to PHASE	15
VBOOT, VUGATE		to GND	33	V
		to GND; t < 200 ns	45
VPHASE		to GND	-5 to 18	V
		to GND; t < 200 ns	-8 to 30
VLGATE		to GND	-0.3 to VCC+0.3	V
		FB, COMP, VSEN to GND	-0.3 to 3.6	V
		PGOOD to GND	-0.3 to VCC+0.3	V

3.2Electrical characteristics

	VCC = 5 V to 12 V; TJ = 0 °C to 70 °C unless otherwise specified
Table 5.	Electrical characteristics
Symbol		Parameter		Test conditions	Min.	Typ.	Max.	Unit
Supply current and power-on
ICC		VCC supply current		UGATE and LGATE = OPEN		6		mA
IBOOT		BOOT supply current		UGATE = OPEN; PHASE to GND		0.7		mA
UVLO		VCC turn-ON		VCC rising			4.1	V
		Hysteresis				0.2		V
Oscillator
FSW		Main oscillator accuracy			540	600	660	kHz
VOSC		PWM ramp amplitude				1.4		V
dMAX		Maximum duty cycle			67			%
Reference and error amplifier
		Output voltage accuracy			-0.8	-	0.8	%
A0		DC gain (1)				120		dB
GBWP		Gain-bandwidth product (1)				15		MHz
SR		Slew-rate (1)				8		V/μs
DIS		Disable threshold		COMP falling	0.70		0.85	V

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Electrical specifications							L6728AH
Table 5.	Electrical characteristics			(continued)
Symbol		Parameter		Test conditions	Min.	Typ.	Max.	Unit
Gate drivers
IUGATE		HS source current		BOOT - PHASE = 5 V		1.5		A
RUGATE		HS sink resistance		BOOT - PHASE = 5 V		1.1		Ω
ILGATE		LS source current		VCC = 5 V		1.5		A
RLGATE		LS sink resistance		VCC = 5 V		0.65		Ω
Over-current protection
IOCSET		OCSET current source		Sourced from LGATE pin, during OC	9	10	11	μA
				setting phase
VOC_SW		OC switch-over threshold		VLGATE/OC rising		600		mV
Over and under-voltage protections
OVP		OVP threshold		VSEN rising	0.90	1.00	1.10	V
				unlatch, VSEN falling	0.35	0.40	0.45	V
UVP		UVP threshold		VSEN falling	0.50	0.60	0.70	V
VSEN		VSEN bias current		Sourced from VSEN		100		nA
PGOOD
PGOOD		Upper threshold		VSEN rising	0.860	0.890	0.920	V
		Lower threshold		VSEN falling	0.680	0.710	0.740	V
VPGOODL		PGOOD voltage low		IPGOOD = -4 mA			0.4	V

1. Guaranteed by design, not subject to test.

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L6728AH	Device description

4 Device description

L6728AH 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 DC-DC step-down converter. Designed to drive N-channel MOSFETs in a synchronous buck topology, with its high level of integration this 10-pin device allows reducing cost and size of the power supply solution also providing real-time PGOOD in a compact VFQFPN10 3x3 mm.

L6728AH is designed to operate from a 5 V or 12 V supply. The output voltage can be precisely regulated to as low as 0.8 V with ±1% accuracy over line and temperature variations. The switching frequency is internally set to 600 kHz.

This device provides a simple control loop with a voltage-mode error-amplifier. The erroramplifier features a 15 MHz gain-bandwidth product and 8 V/µs slew rate, allowing high regulator bandwidth for fast transient response.

To avoid load damages, L6728AH provides over-current protection as well as overvoltage, under voltage and feedback disconnection protection. The over-current trip threshold is programmable by a simple resistor connected from Lgate to GND. Output current is

monitored across low-side MOSFET RDS(on), saving the use of expensive and spaceconsuming sense resistor. Output voltage is monitored through dedicated VSEN pin.

L6728AH implements soft-start increasing the internal reference in closed loop regulation. low-side-less feature allows the device to perform soft-start over pre-biased output avoiding high current return through the output inductor and dangerous negative spike at the load side.

L6728AH is available in a compact VFDFN10 3 x 3 mm package with exposed pad.

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Driver section	L6728AH

5 Driver section

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

The driver for the 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 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 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 limitations) can be chosen freely.

5.1Power dissipation

L6728AH 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 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 + QgLS VCC)

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.

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