ST AN2627 Application note

AN2627

Application note

ST8R00 syncronous boost converter with output current cut-off function

Introduction

The ST8R00 family of synchronous step-up DC-DC converters with current output cut-off function provide up to 1 A over an input voltage range of 4 V to 6 V and an output voltage range of 6 V to 12 V.

The high switching frequency (1.2 MHz) allows the use of tiny surface-mount components. Along with the resistor divider to set the output voltage value, an inductor and two capacitors are required. A low output ripple is guaranteed by the current mode PWM topology and by the use of low ESR surface-mounted ceramic capacitors.

The device is available in two versions: burst mode (ST8R00) and continuous mode (ST8R00W).

The ST8R00 devices are thermal protected and available in the DFN8 4x4 package.

Figure 1. Simplified schematic diagram

	LX
Thermal		OUT
	Ps	Po
	Ns
INH Inhibit	PGND

		PWM control
	IN	FB
		Vref

PGND GND

AM00001v1

December 2009

Doc ID 13913 Rev 2

1/19

www.st.com

Contents	AN2627

Contents

1	ST8R00 description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .			. 4
	1.1	Inhibit function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		7
2	Selecting components for applications . . . . . . . . . . . . . . . . . . . . . . . . .			9
	2.1	Output voltage selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		9
	2.2	Input capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		10
	2.3	Inductor	. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
	2.4	Output capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		10
	2.5	Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		11
3	Thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .			12
4	Demonstration board usage recommendation . . . . . . . . . . . . . . . . . . .			13
	4.1	External component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		14
		4.1.1	Capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
		4.1.2	Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
5	BOM with most-used components . . . . . . . . . . . . . . . . . . . . . . . . . . . .			16
6	Footprint recommended data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .			17
7	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .			18

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AN2627	List of figures

List of figures

Figure 1. Simplified schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Figure 2. ST8R00 inductor current at light load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Figure 3. ST8R00W inductor current at light load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Figure 4. ST8R00W inductor current at no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Figure 5. ST8R00 cut-off block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 6. Current cut-off function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 7. Inrush current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 8. ST8R00 application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 9. Inhibit voltage vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 10. Typical application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 11. Voltage feedback vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 12. Layout considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 13. The ST8R00 demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 14. Demonstration board layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 15. Demonstration board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 16. Efficiency vs. output current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 17. Efficiency vs. output voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 18. ST8R00 efficiency vs. inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 19. ST8R00W efficiency vs. inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 20. DFN8 4x4 recommended footprint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

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ST8R00 description	AN2627

1 ST8R00 description

The ST8R00 is a family of adjustable current mode PWM synchronous step-up DC-DC converters with internal 1 A power switch. It represents a complete 1 A switching regulator with internal compensation which eliminates the need for additional components.

The two devices in the family, the ST8R00 and ST8R00W, operate at light load in two different ways. The ST8R00 works in power-save mode to achieve good efficiency, as shown in Figure 2. The ST8R00W, in order to guarantee the lowest switching ripple, operates in PWM (pulse width modulation) mode as show in Figure 3 and Figure 4.

At medium and high load current, both versions operate in PWM mode.

The thermal shutdown block turns off the regulator when the junction temperature exceeds 150 °C (typ), and the cycle-by-cycle current limiting provides protection against overcurrent sink.

Figure 2. ST8R00 inductor current at light load

IL

LX

VIN=5 V, VOUT=8 V, IOUT=60 mA, Ch1=LX, Ch4=IL

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AN2627	ST8R00 description

Figure 3. ST8R00W inductor current at light load

IL

LX

VIN=5 V, VOUT=8 V, IOUT=60 mA, Ch1=LX, Ch4=IL

Figure 4. ST8R00W inductor current at no load

IL

LX

VIN=5 V, VOUT=8 V, no load Ch1=LX, Ch4=IL

For proper functioning of the device, only a few components are required: an inductor, two capacitors and the resistor divider. The inductor chosen must not saturate at the operating peak current. Its value should be selected taking into account that a large inductor value reduces output voltage ripple, while a smaller inductor can be selected when it is important to reduce package size and the total cost of the application. Finally, the ST8R00 family has been designed to work properly with X5R or X7R SMD ceramic capacitors both at the input and at the output. These types of capacitors, thanks to their very low series resistance (ESR), minimize the output voltage ripple. Other low ESR capacitors can be used in accordance with application requirements without compromising the correct functionality of the device.

This device features an output current cut-off function. Two P-channel MOSFETs in a back- to-back configuration, as shown in Figure 5, stop the output current when the inhibit is low (Figure 6).

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ST8R00 description	AN2627
Figure 5.	ST8R00 cut-off block

LX
	OUT
Ps	Po
Ns
PGND

Figure 6. Current cut-off function

Iout

Vout

Inh

Vin

Figure 7 shows the in-rush current at start-up. Initially, the COUT capacitor is completely discharged and the current limitation is due only to the equivalent series resistor of the

inductor, the power MOSFET parasitic diode and the cut-off MOSFETs’ RDS(ON). As soon as the output voltage reaches the input voltage level, the device begins to switch and the

current is limited cycle by cycle.

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