ST AN2371 APPLICATION NOTE

AN2371

Application note

ST1S06 high frequency synchronous buck converter

Introduction

The ST1S06 is an adjustable current mode pulse width modulation (PWM) synchronous, step down DC-DC converter with inhibit function. It is optimized for powering all low-voltage applications and, generally, to replace the high current linear solution when the power dissipation may cause overheating of the application environment.

It provides up to 1.5 A over an input voltage range of 2.5 V to 5.5 V. A high switching frequency (1.5 MHz) enables the use of tiny surface-mount components (SMD). In addition to the resistor divider used to set the output voltage value, only an inductor and two capacitors are required. Moreover, low output ripple is guaranteed by the current mode PWM topology and by the use of low series resistance (ESR) SMD ceramic capacitors.

The device is thermal protected and current limited to prevent damage due to accidental short circuits. It is a complete 1.5 A switching regulator with its internal compensation eliminating the need for additional components. The constant frequency, current mode, PWM architecture and stable operation with ceramic capacitors results in a low, predictable output ripple. To clamp the error amplifier reference voltage, this device includes a Soft Start control block generating a voltage ramp.

The ST1S06 is available in 6L-DFN 3x3 package

Moreover, an on-chip power on reset of 50 = 100 µs ensures correct performance when switching on the power supply. Other circuits fitted to the device protection are the Thermal Shut down block which turn-off the regulator when the junction temperature exceeds 150°C typically and the Cycle- by-cycle Current Limiting that provides protection against shorted outputs. Being the ST1S06 an adjustable regulator, the output voltage is determined by an external resistor divider. The desired value is given by the following equation:

Equation 1

VOUT = VFB		R1
	1 + ------
		R2

Due to the high switching frequency and peak current, it is important to optimize the application environment by reducing the length of the PCB traces and placing all the external component near the device. The chosen inductor must not saturate at the peak current level. Moreover, its value can be selected keeping in account that a large inductor value increases the efficiency at low output current and reduces output voltage ripple, while a smaller inductor can be chosen when it is important to reduce the package size and the total cost of the application.

Finally, the ST1S06 is 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 values can be used depending on application requirements without invalidating correct device performance.

March 2008

Rev 2

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Contents	AN2371

Contents

1	Selecting components for your application . . . . . . . . . . . . . . . . . . . . .		. 4
	1.1	Input capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	4
	1.2	Output capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
	1.3	Inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
2	Thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		6
3	Short-circuit protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		7
4	Board usage recommendation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		8
	4.1	External component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	9
	4.2	Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	9
	4.3	Capacitors selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
5	Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		11
6	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		11

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AN2371	List of figures

List of figures

Figure 1. Simplified schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Figure 2. ST1S06 board picture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 3. Board layer - top layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 4. Board layer - bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 5. ST1S06 application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 6. Efficiency vs. inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 7. Input voltage vs. output voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 8. Efficiency vs. output current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 9. Feedback voltage vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 10. Inhibit voltage vs. input voltage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 11. Switching frequency vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

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Selecting components for your application	AN2371

1 Selecting components for your application

This section provides information to help you select the best-adapted components for your application.

Figure 1. Simplified schematic

1.1Input capacitor

The input capacitor must be able to support the maximum input operating voltage and the maximum RMS input current.

Since step-down converters draw current from the input in pulses, the input current is squared and the height of each pulse is equal to the output current. The input capacitor has to absorb all this switching current that can be up to the load current divided by two (worst case, with duty cycle of 50%).

For this reason, the quality of these capacitors must be very high to minimize the power dissipation generated by the internal ESR, thus improving system reliability and efficiency.

The critical parameter is usually the RMS current rating that has to be higher than the RMS input current. The maximum RMS input current (flowing through the input capacitor) is:

Equation 2

I		= I	D –	2 D2	+	D2
				----------------η -		------η
	RMS	O

where η is the expected system efficiency, D is the duty cycle and IO the output DC current. This function reaches its maximum value at D = 0.5 and the equivalent RMS current is equal to IO divided by 2 (considering η = 1).

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