ST AN2206 Application note

AN2206

Application note

Designing with L6920DB, high efficiency syncronous rectifier step-up converter

Introduction

The L6920DB is a high efficiency monolithic step-up switching converter IC especially designed for battery powered applications, thanks to its minimum start-up of 0.8 V, and a minimum operating voltage of 0.6 V.

It requires only three external components to realize the conversion from battery voltage to the selected output voltage.

It has a minimum output voltage of 1.8 V, which is enough to supply even the most advanced ASICs and microprocessors. The high switching frequency allows users to choose small inductors and output capacitors for their designs. Supervisory functions include Reference Voltage, Low Battery Detection, and Shutdown, which are provided with overcurrent protection.

March 2007

Rev 4

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Contents	AN2206

Contents

1	Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 3
	1.1	Output voltage selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	4
	1.2	Input capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	4
	1.3	Output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	4
	1.4	Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
	1.5	Switching frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
	1.6	Low battery detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
	1.7	Layout guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
2	Typical performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		9
3	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		10

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AN2206	Application information

1 Application information

Figure 1. L6920DB Example of overall application size

Figure 2. Application circuit

		LBI
VBATT=2.5V	LX	2
	7
	L1 10 H		VOUT	VOUT=3.3V
	C1
	47 F	8
	L6920D			C1
	VREF			47 F
	4		___
	C4
	100nF	3	LBO
		1	FB
	6	5
		_____
	GND	SHDN

Figure 3. Pin connections (top view)

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Application information	AN2206

1.1Output voltage selection

The output voltage must be selected by configuring the FB pin. Three choices are available:

●fixed 3.3 V

●fixed 5 V

●adjustable output set via an external resistor divider.

R4 and R5 should be selected in the range of 100 kΩ - 1 MΩ to minimize power consumption and errors due to current sunk by FB pin (a few nA). See Figure 5

Table 1.	Output voltage connection
Output voltage		Feedback connection
VOUT =3.3 V		FB Connected to OUT (See Figure 5)
VOUT =5 V		FB Connected to GND (See Figure 5)
		FB Connected to a resistor divider
1.8 V=VOUT=5.2 V		V		= 1.23V • 1	R4
					+ -------
			OUT		R5

1.2Input capacitor selection

A minimum capacitance is usually added at the input to filter the current ripple. 10 µF are enough for most of the applications.

Low-ESR Tantalum and Multilayer Ceramic Capacitors (MLCC) are good choices.

1.3Output capacitor selection

The output capacitor affects both efficiency and output ripple, so its choice has to be considered with particular care. The capacitance value should be in the range of about 10mF-100 µF. An additional, smaller, low Equivalent Series Resistance (ESR) capacitor can be used (not mandatory) in parallel for high frequency filtering. A typical value is around

1 µF.

Note:	For very high performance requirements (e.g., efficiency and output voltage ripple), a very
	low ESR capacitor has to be used (e.g. MLCC capacitors).
	Other possibilities include low-ESR tantalum capacitors, available from KEMET, and other
	sources. POSCAP capacitors from SANYO and polymeric capacitors from PANASONIC are
	also good choices (see Table 2).
Note:	The cap values and rated voltages are only suggested possibilities.
	Table 2.	Distributor's capacitor main list
	Manufacturer		Series	Cap value (µF)	Rated voltage (V)	ESR (mΩ)
	KEMET		T510/T494/ T495	10 to 47	6	30 to 100

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