ST ST1S32 User Manual

ST1S32

4 A DC step-down switching regulator

Datasheet — production data

Features

■4 A DC output current

■2.8 V to 5.5 V input voltage

■Output voltage adjustable from 0.8 V

■1.5 MHz switching frequency

■Internal soft-start and enable

■Integrated 60 mΩ and 45 mΩ Power MOSFETs

■All ceramic capacitor

■Power Good (POR)

■Cycle-by-cycle current limiting

■Current foldback short-circuit protection

■VFDFPN 8 4x4x1.0 package

Applications

■µP/ASIC/DSP/FPGA core and I/O supplies

■Point of Load for: STB, TV, DVD

■Optical storage, hard disk drives, printers, audio/graphic cards

VFDFPN 8 4x4x1.0

Description

The ST1S32 is an internally compensated 1.5 MHz fixed-frequency PWM synchronous stepdown regulator. The ST1S32 operates from 2.8 V to 5.5 V input, while it regulates an output voltage as low as 0.8 V and up to VIN.

The ST1S32 integrates a 60 mΩ high-side switch and a 45 mΩ synchronous rectifier, allowing very high efficiency with very low output voltages.

The peak current mode control with internal compensation delivers a very compact solution with a minimum component count.

The ST1S32 is available in 4 mm x 4 mm, 8-lead VFDFPN package.

Figure 1. Application circuit

VIN			L
			VOUT
	VINA
Cin_a	R3	ST1S32	R1
	EN
	Cin_sw		Cout
			R2
	GND
			AM12608V1

May 2012

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This is information on a product in full production.

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Contents	ST1S32

Contents

1	Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 3
	1.1	Pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	3
	1.2	Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	3
2	Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		4
	2.1	Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	4
3	Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		5
4	Functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		7
	4.1	Soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8
	4.2	Error amplifier and control loop stability . . . . . . . . . . . . . . . . . . . . . . . . . . .	8
	4.3	Overcurrent protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	13
	4.4	Enable function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
	4.5	Light load operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
	4.6	Hysteretic thermal shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
5	Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		15
	5.1	Input capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	15
	5.2	Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	16
	5.3	Output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	17
	5.4	Thermal dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18
	5.5	Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	20
6	Demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		22
7	Typical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		24
8	Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		26
9	Order codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		28
10	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		28

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ST1S32	Pin settings

1 Pin settings

1.1Pin connection

Figure 2. Pin connection (top view)

1.2Pin description

	Table 1.	Pin description
	No.	Type	Description
	1	VINA	Unregulated DC input voltage.
	2	EN	Enable input. With EN higher than 1.5 V the device in ON and with EN
			lower than 0.5 V the device is OFF.
			Feedback input. Connecting the output voltage directly to this pin the
	3	FB	output voltage is regulated at 0.8 V. To have higher regulated voltages an
			external resistor divider is required from VOUT to the FB pin.
	4	AGND	Ground.
			Open drain Power Good (POR) pin. It is released (open drain) when the
			output voltage is higher than 0.92 * VOUT with a delay of 170 us. If the
	5	PG	output voltage is below 0.92 * VOUT, the POR pin goes to low impedance
			immediately.
			If not used, it can be left floating or to GND.
	6	VINSW	Power input voltage.
	7	SW	Regulator output switching pin.
	8	PGND	Power Ground.

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Maximum ratings	ST1S32

2 Maximum ratings

Table 2.	Absolute maximum ratings
Symbol		Parameter	Value	Unit
	VIN	Input voltage	-0.3 to 7
	VEN	Enable voltage	-0.3 to VIN
	VSW	Output switching voltage	-1 to VIN	V
	VPG	Power-on reset voltage (Power Good)	-0.3 to VIN
	VFB	Feedback voltage	-0.3 to 1.5
	PTOT	Power dissipation at TA < 60 °C	2.25	W
	TOP	Operating junction temperature range	-40 to 150	°C
	Tstg	Storage temperature range	-55 to 150	°C

2.1Thermal data

Table 3.	Thermal data
Symbol		Parameter	Value	Unit
RthJA		Maximum thermal resistance junction-	40	°C/W
		ambient (1)

1. Package mounted on demonstration board.

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ST1S32	Electrical characteristics

3 Electrical characteristics

TJ=25 °C, VIN=5 V, unless otherwise specified.

	Table 4.	Electrical characteristics
	Symbol	Parameter	Test condition		Values		Unit
				Min.	Typ.	Max.
	VIN	Operating input voltage	(1)	2.8		5.5
		range
							V
	VINON	Turn-on VCC threshold	(1)		2.4
	VINOFF	Turn-off VCC threshold	(1)		2.0
	RDSON-P	High-side switch on-	ISW=300 mA		60		mΩ
		resistance
	RDSON-N	Low-side switch on-	ISW=300 mA		45		mΩ
		resistance
	ILIM	Maximum limiting current	(2)	5.0			A
	Oscillator
	FSW	Switching frequency		1.2	1.5	1.9	MHz
	DMAX	Maximum duty cycle	(2)	95		100	%
	Dynamic characteristics
	VFB	Feedback voltage		0.792	0.8	0.808	V
			Io=10 mA to 4 A(1)	0.776	0.8	0.824
	%VOUT/	Reference load regulation	(2)		0.2	0.6	%
	IOUT		Io=10 mA to 4 A
	%VOUT/	Reference line regulation	V = 2.8 V to 5.5 V (2)		0.2	0.3	%
	VIN		IN
	DC characteristics
	IQ	Quiescent current	Duty cycle=0, no load		630	1200	μA
			VFB=1.2 V
	IQST-BY	Total standby quiescent	OFF			10	μA
		current
	Enable
	VEN	EN threshold voltage	Device ON level	1.5			V
			Device OFF level			0.5
	IEN	EN current				0.1	μA

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Electrical characteristics						ST1S32
	Table 4.	Electrical characteristics (continued)
	Symbol	Parameter	Test condition		Values		Unit
				Min.	Typ.	Max.
	Power Good
		PG threshold			92		%VFB
	PG	PG hystereris		30	50		mV
		PG output voltage low	Isink= 6 mA open drain			400
		PG rise delay			170		μs
	Soft-start
	TSS	Soft-start duration			400		μs
	Protection
	TSHDN	Thermal shutdown			150		°C
		Hystereris			20

1.Specifications referred to TJ from -40 to +125 °C. Specifications in the -40 to +125 °C temperature range are assured by design, characterization and statistical correlation.

2.Guaranteed by design.

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ST1S32	Functional description

4 Functional description

The ST1S32 is based on a “peak current mode”, constant frequency control. The output voltage VOUT is sensed by the Feedback pin (FB) compared to an internal reference (0.8 V) providing an error signal that, compared to the output of the current sense amplifier, controls the ON and OFF time of the power switch.

The main internal blocks are shown in the block diagram in Figure 3. They are:

●A fully integrated oscillator that provides the internal clock and the ramp for the slope compensation avoiding sub-harmonic instability

●The soft-start circuitry to limit inrush current during the startup phase

●The transconductance error amplifier

●The pulse width modulator and the relative logic circuitry necessary to drive the internal power switches

●The drivers for embedded P-channel and N-channel Power MOSFET switches

●The high-side current sensing block

●The low-side current sense to implement diode emulation

●A voltage monitor circuitry (UVLO) that checks the input and internal voltages

●A thermal shutdown block, to prevent thermal run-away.

Figure 3. Block diagram

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Functional description	ST1S32

4.1Soft-start

The soft-start is essential to assure the correct and safe startup of the step-down converter. It avoids inrush current surge and makes the output voltage rise monothonically.

The soft-start is managed by ramping the reference of the error amplifier from 0 V to 0.8 V. The internal soft-start capacitor is charged with a resistor to 0.8 V, then the FB pin follows the reference so that the output voltage is regulated to rise to the set value monothonically.

4.2Error amplifier and control loop stability

The error amplifier provides the error signal to be compared with the high-side switch current through the current sense circuitry. The non-inverting input is connected with the internal 0.8 V reference, whilst the inverting input is the FB pin. The compensation network is internal and connected between the E/A output and GND.

The error amplifier of the ST1S32 is a transconductance operational amplifier, with high bandwidth and high output impedance.

The characteristics of the uncompensated error amplifier are:

Table 5. Characteristics of the uncompensated error amplifier

Description	Value
DC gain	94 dB
gm	238 μA/V
Ro	96 MΩ

The ST1S32 embeds the compensation network that assures the stability of the loop in the whole operating range. Here below are all the tools needed to check the loop stability.

In Figure 4. the simple small signal model for the peak current mode control loop is shown.

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ST1S32	Functional description

Figure 4. Block diagram of the loop for the small signal analysis

		VIN
Slope		GCO(s)
Compensation
		High side
		Switch	GDIV(s)
		L
	Current sense		VOUT
	Logic	Cout
	And	Low side
	Driver	Switch
	PWM comparator
	0.8V
			R1
VC		VFB
Rc	Error Amp
			R2
Cc
	GEA(s)
			AM12609V1

Three main terms can be identified to obtain the loop transfer function:

1.from control (output of E/A) to output, GCO(s);

2.from output (VOUT) to the FB pin, GDIV(s);

3.from the FB pin to control (output of E/A), GEA(s).

The transfer function from control to output GCO(s) results:

Equation 1

RLOAD				1
GCO(s) =	-----------------R i	----------------------------------------------------------------------------------------------------	Rout TSW	[mC (1 – D) – 0.5-]
		1 +	-----------------------------L -

	1	s
		+ ----
		ω
		z
-------------------- FH(s)
	1	s
		+ -----
		ω
		p

where RLOAD represents the load resistance, Ri the equivalent sensing resistor of the current sense circuitry (0.369 Ω), ωp the single pole introduced by the LC filter and ωz the zero given by the ESR of the output capacitor.

FH(s) accounts for the sampling effect performed by the PWM comparator on the output of the error amplifier that introduces a double pole at one half of the switching frequency.

Equation 2

ω 1

Z = ----------------------------------

ESR COUT

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