ST AN2816 Application note

AN2816

Application note

High-efficiency step-down controller with embedded 2 A LDO regulator

Introduction

The PM6675S device consists of a single, high-efficiency step-down controller and an independent low dropout (LDO) linear regulator.

The constant on-time (COT) architecture assures fast transient response supporting both electrolytic and ceramic output capacitors. An embedded integrator control loop compensates the DC voltage error due to the output ripple.

Selectable low-consumption mode allows the highest efficiency over a wide range of load conditions. The low-noise mode sets the minimum switching frequency to 33 kHz for audiosensitive applications. The LDO linear regulator can sink and source up to 2 Apk. Two fixed current limits (±1 A and ±2 A) can be chosen.

An active soft-end is independently performed on both the switching and the linear regulators outputs when disabled.

Figure 1. PM6675S demonstration board

AM01000v1

August 2008

Rev 1

1/36

www.st.com

Contents	AN2816

Contents

1	Main features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 5
	1.1	Switching section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
	1.2	LDO section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
2	Evaluation kit schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		6
3	Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		7
4	Component assembly and layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		9
5	I/O interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		12
6	Recommended equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		13
7	Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		14
	7.1	JP3 fixed or adjustable output voltage (VSEL pin) . . . . . . . . . . . . . . . . . .	14
	7.2	JP1 power-saving mode (NOSKIP pin) . . . . . . . . . . . . . . . . . . . . . . . . . .	14
	7.3	JP2 current limit (LILIM pin) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	15
	7.4	JP5 compensation network (COMP pin) . . . . . . . . . . . . . . . . . . . . . . . . .	16
8	Test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		17
9	Getting started . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		18
10	PM6675S demonstration tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		19
	10.1	VOUT and LOUT turn-on (soft-start) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	19
	10.2	VOUT working mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	21
	10.3	VOUT and LOUT load regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	24
	10.4	VOUT and LOUT load transient responses . . . . . . . . . . . . . . . . . . . . . . . . .	25
	10.5	VOUT efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	26
	10.6	VOUT gate drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	27
	10.7	VOUT and LOUT turn-off (soft-end) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	28
	10.8	UV, OV and thermal protections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	30

2/36

AN2816			Contents
	10.9	VOUT current limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. . . . . 32
	10.10	LOUT current limit (foldback) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. . . . 33
	10.11	Switching frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. . . . 34
11	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. . . . 35

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List of figures	AN2816

List of figures

Figure 1.	PM6675S demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 1
Figure 2.	PM6675S demonstration board schematic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 6
Figure 3.	Top side component placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 9
Figure 4.	Top view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	9
Figure 5.	Layer 2 view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
Figure 6.	Layer 3 view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
Figure 7.	Bottom view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
Figure 8.	Bottom side component placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
Figure 9.	JP3 (VSEL) setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
Figure 10.	JP1 (NOSKIP) setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	15
Figure 11.	JP2 (LILIM) setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	15
Figure 12.	JP5 (COMP) setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	16
Figure 13.	PM6675S demonstration board test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	17
Figure 14.	VOUT soft-start at 270 mW load, pulse-skip mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	19
Figure 15.	LOUT turn-on, VOUT in pulse-skip mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	20
Figure 16.	VOUT = 1.5 V, VIN = 12 V, IVOUT = 0 A, forced-PWM mode . . . . . . . . . . . . . . . . . . . . . . . .	21
Figure 17.	VOUT = 1.5 V, VIN = 12 V, IVOUT = 0 A, pulse-skip mode . . . . . . . . . . . . . . . . . . . . . . . . . .	22
Figure 18.	VOUT = 1.5 V, VIN = 12 V, no load, non-audible pulse-skip mode (33 kHz) . . . . . . . . . . . .	23
Figure 19.	VOUT load regulation - VIN = 12 V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	24
Figure 20.	LOUT load regulation - LDOIN = 1.5 (VOUT), he dropout voltage (0.35 V) limits the
	maximum sourced current to about 1.8 A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	24
Figure 21.	VOUT load transient (VIN = 12 V, LOAD = 0 A to 8 A at 2.5 A/µs), pulse-skip mode. . . . . .	25
Figure 22.	VOUT load transient (VIN = 12 V, LOAD = 8 A to 0 A at 2.5 A/µs), pulse-skip mode. . . . . .	25
Figure 23.	LOUT load transient (VIN = 12 V, LOAD = 1.8 A to 1.85 A at 2.5 A/µs), pulse-skip mode . .	25
Figure 24.	Forced PWM (blue), non-audible pulse-skip (green), pulse-skip (red), efficiency vs.
	output current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	26
Figure 25.	External MOSFET gate signals (VIN = 12 V, LOAD = 0), pulse-skip mode . . . . . . . . . . . .	27
Figure 26.	External MOSFET gate signals (VIN = 12 V, LOAD = 7 A), pulse-skip mode . . . . . . . . . .	27
Figure 27.	VOUT and LOUT output voltages VOUT soft-end . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	28
Figure 28.	VOUT and LOUT output voltages, LOUT soft-end . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	29
Figure 29.	UV protection, pulse-skip mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	30
Figure 30.	OV protection, pulse-skip mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	31
Figure 31.	VOUT and LOUT rails, thermal shutdown, pulse-skip mode, LOUT powered by VOUT . . . . . 31
Figure 32.	VOUT current limit protection during a load transient (0 A to 9 A at 2.5 A/µs, valley current
	limit programmed at about 9 A (RILIM = 1 kW) pulse-skip mode. . . . . . . . . . . . . . . . . . . . .	32
Figure 33.	LOUT current limit during an output short . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	33
Figure 34.	Switching frequency vs. input voltage, VOUT = 1.5 V, IVOUT = 7 A, forced PWM mode . . .	34
Figure 35.	Switching frequency vs. output current, VOUT = 1.5 V, VIN = 12 V . . . . . . . . . . . . . . . . . . .	34

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AN2816	Main features

1 Main features

1.1Switching section

●4.5 V to 28 V input voltage range

●0.6 V, ±1% voltage reference

●1.5 V fixed output voltage

●0.6 V to 3.3 V adjustable output voltage

●1.237 V ±1% reference voltage available

●Very fast load transient response constant on-time loop control

●No-RSENSE current sensing using low-side MOSFETs' RDS(on)

●Negative current limit

●Latched OVP, UVP and thermal shutdown

●Fixed 3 ms soft-start

●Selectable pulse-skipping at light load

●Selectable non-audible (33 kHz) pulse-skip mode

●All ceramic output capacitor applications supported

●Output voltage ripple compensation

●Output soft-end

1.2LDO section

●0.6 V to 3.3 V adjustable output voltage

●Selectable ±1 A or ±2 A current limit

●Dedicated Power Good signal

●Ceramic output capacitors supported

●Output soft-end

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Evaluation kit schematic	AN2816

2 Evaluation kit schematic

Figure 2. PM6675S demonstration board schematic

J1	1 VIN									J2	1 VOUT				D3	STPS1L30A														J3	1 PGND	J4	1 SPG
														1			220u		2
										4TPE220MF			C3 C4
															1			2
																	220u
										2x					1			2
						R6 0
									MLC1240-901ML						6.8k							2	6.8n
2x UMK325BJ106KM-T				C1 C2	10u					L1 0.9u	1 2				R14 7.5k R15							1	C15
			1		2
			1		2															R16	4R7		C18	1n			TP1	GND TP					JP5		680p INT-VESR
					10u			S12NH3LL															1		2
																			STPS1L30M
																1					D2	2
						8								8																		0		2
						00								7
						7			3											3
						6			2					6						2		S12NH3LL												1	C16
						5			1					5						1
						Q1		4						Q2			4																			0
																																			1	2
	2	R1 330k					3R3	2								R17 0							VCC				1		R13	100k	2				C17	N.M.
	1		C14	100n	2	0	R4	1				C13	100n		0								R11 100k	1 2	R12 100k	1 2	SW1				LEN-SWEN		0
			1																								3				2
						0						2															4				1
		1		18k	2
			R2									1														1	2
																										C22	100p			0
											1.2k													1		2
										2															C21	100p
											R3
												VCC															6				5
							100n			1						18	17		16	15	14	13					4				3
																											2				1
				D1																							JP2				LILIM
	VCC	2			1	1	2							CSNS		VCC		LGATE PGND SPG LEN SWEN					LILIM
					BAT54J	C12																			12
						R10 0						19		PHASE							PM6675S		COMP
												20													11
														HGATE									VSEL
												21													10
														BOOT									VSNS									VSEL
												22													9		JP3
														LIN									VOSC
												23													8
														LOUT				LFB NOSKIP LPG SGND AVCC					VREF
		C11	10u									24													7
															THPD	LGND
						0						U1
		1	2											25		1	2 3 4 5 6										R9 13k					R8 12k	0
										C20		N.M.
										1		2			0											1
										C19		N.M.											C10		100n		C9
										1		2														2
			1		2																		1		2						NOSKIP
VCC				R7	3R9					C7		10u															JP1						0
										1		2			0												6				5
										C6		10u
																											4				3
										1		2															2				1
				C5 1u		0
			1		2						R19	7.5k					R20		10k				0
																																			0
																VCC				R18 100k
		1			1		1		1														1									1	1
	J5				J10	J6			J7													J8										J11	J9
		VCC			VCCGND		LIN		LOUT														LPG									LGND	PGND
																																				AM01001v1

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AN2816	Bill of material

3 Bill of material

	Table 1.		PM6675S demonstration board bill of material
	Qty	Component		Description	Package	Part number	Manufacturer	Value
	2		C1, C2	Ceramic, 50 V, X5R, 20%	SMD	UMK325BJ106K	Taiyo Yuden	10 µF
					1210	M-T
	2		C3, C4	POSCAP, 4 V,15 mΩ, 20%	SMD	4TPE220MF	Sanyo	220 µF
					7343 (D)
	1		C5	Ceramic, 6.3 V, X5R, 10%	SMD		Standard	1 µF
					3216-12
	3	C6, C7, C11		Ceramic, 6.3 V, X5R, 10%	SMD	JMK212BJ106K	Taiyo Yuden	10 µF
					0805	G-T
	4	C9, C10,		Ceramic, 50 V, X7R, 20%	SMD		Standard	100 nF
		C13, C14			0603
	1		C12	Ceramic, 50 V, X7R, 10%	SMD		Standard	100 nF
					0805
	1		C15	Ceramic, 50 V, X7R, 10%	SMD		Standard	6n8
					0603
	1		C16	Ceramic, 50 V, X7R, 10%	SMD		Standard	680 p
					0603
	1		C17	Ceramic, 20%	SMD		Standard	N.M
					0603
	1		C18	Ceramic, 50 V , X7R, 10%	SMD		Standard	1 nF
					0805
	2	C19, C20		Ceramic, 6.3 V, X5R, 10%	SMD	JMK212BJ106K	Taiyo Yuden	N.M
					0805	G-T
	2	C21, C22		Ceramic, 50 V, X7R, 10%	SMD		Standard	100 pF
					0603
	1		R1	Chip resistor, 0.1 W, 1%	SMD		Standard	300 kΩ
					0603
	1		R2	Chip resistor, 0.1 W, 1%	SMD		Standard	18 kΩ
					0603
	1		R3	Chip resistor, 0.1 W, 1%	SMD		Standard	1.2 kΩ
					0603
	1		R4	Chip resistor, 0.1 W, 1%	SMD		Standard	3R3
					0603
	1		R6	Chip resistor, 0.1 W, 1%	SMD		Standard	0
					0805
	1		R7	Chip resistor, 0.1 W, 1%	SMD		Standard	3R9
					0603
	1		R8	Chip resistor, 0.1 W, 1%	SMD		Standard	12 kΩ
					0603

7/36

Bill of material							AN2816
Table 1.		PM6675S demonstration board bill of material (continued)
Qty	Component		Description	Package	Part number	Manufacturer	Value
1		R9	Chip resistor, 0.1 W, 1%	SMD		Standard	13 kΩ
				0603
1		R10	Chip resistor, 0.1 W, 1%	SMD		Standard	0
				0603
4	R11, R12,		Chip resistor, 0.1 W, 1%	SMD		Standard	100 kΩ
	R13,R18			0603
2	R14, R15		Chip resistor, 0.1 W, 1%	SMD		Standard	N.M
				0805
1		R16	Chip resistor, 0.1 W, 1%	SMD		Standard	4R7
				0805
1		R17	Chip resistor, 0.1 W, 1%	SMD		Standard	0
				0603
1		R19	Chip resistor, 0.1 W, 1%	SMD		Standard	7k5
				0603
1		R20	Chip resistor, 0.1 W, 1%	SMD		Standard	10 kΩ
				0603
1		L1	SMT, 13.4 Arms, 2.57 mΩ	10.5x10.5	MLC1240-	Coilcraft	0.9 µH
				mm	901MX
2		Q1, Q2	N-Channel, 30 V	SO-8	STS12NH3LL	STMicroelectronics	STS12NH3LL
1		D1	Schottky, 30 V, 0.3 A	SOD-323	BAT54J	STMicroelectronics	BAT54J
1		D2	Schottky, 30 V, 1 A	DO216-	STPS1L30M	STMicroelectronics	STPS1L30M
				AA
1		D3	Schottky, 30 V, 1 A	DO216-	STPS1L30M	STMicroelectronics	N.M.
				AA
1		U1	Controller	VFQFPN-	PM6675S	STMicroelectronics	PM6675S
				24
	J1, J2, J3,
11	J4, J5, J6,		Header, single pin
	J7, J8, J9,
	J10,J11
3	JP1, JP2,		Jumper, 2x3, 100 mils
		JP3
1		JP5	PCB pads selector
1		TP6	Test point
1		SW1	Dip switch 2	DIP-2		Standard

8/36

AN2816	Component assembly and layout

4 Component assembly and layout

Figure 3. Top side component placement

AM01002v1

Figure 4. Top view

AM01003v1

9/36

Component assembly and layout	AN2816
Figure 5. Layer 2 view

AM01004v1

Figure 6. Layer 3 view

AM01005v1

10/36

AN2816	Component assembly and layout

Figure 7. Bottom view

AM01006v1

Figure 8. Bottom side component placement

AM01007v1

11/36