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 1/11

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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

V

=2.5V

BATT

L1 10µH

C1

47µF

100nF

LX

V

REF

C4

4

GND

Figure 3. Pin connections (top view)

7

L6920D

6

2

5

8

3

1

_____
SHDN

V

___

LBO

FB

OUT

C1

47µF

=3.3V

V

OUT

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Application information AN2206

1.1 Output 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

=3.3 V FB Connected to OUT (See Figure 5)

V

OUT

V

=5 V FB Connected to GND (See Figure 5)

OUT

FB Connected to a resistor divider

R4

1.8 V=V

OUT

=5.2 V

V

OUT

1.23V 1

⎛⎞

------- -+

•=
⎝⎠

R5

1.2 Input 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.

1.3 Output 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 Ta b l e 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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