October 2006 Rev 5 1/16
16
L6920DB
Synchronous rectifier step up converter
General features
■0.8V start up input voltage
■Up to 5.5V operating input voltage
■Internal synchronous rectifier
■Adjustable output voltage from 1.8V
■3.3V and 5V fixed output voltages
■Low battery voltage detection
■Reverse battery protection
■750mA input current limit
■Switching frequency up to 1MHz
■1.23V reference voltage available
Applications
■Conversion from 1 to 3 alkaline,
■NiMH, NiCd battery cells or 1 lithium ION
■PDA and handheld instruments
■Digital cameras
■Cellular phones
■GPS
■Distributed power
Description
The L6920DB is a high efficiency monolithic step
up switching converter IC especially designed for
battery powered application.
Package is MSOP8 in order to minimize PCB
space. It requires only three external components
to realize the conversion from the battery voltage
to the selected output voltage.
The minimum output voltage is 1.8V: suitable to
supply the most advanced ASIC and µP.
High switching frequency allows for a low profile,
small sized inductor and output capacitor to be
used.
Reference voltage, low battery detection and
Shutdown are provided together with over current.
MSOP8
www.st.com
Application circuit
L6920DB 3.3V
Contents L6920DB
2/16
Contents
1 Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Electrical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4 Typical performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5 Detailed description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1 Principle of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.2 Start-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.3 Shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.4 Low battery detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.5 Low battery input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.6 Reverse polarity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.7 Output voltage selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7 Order code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
8 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
L6920DB Pin settings
3/16
1 Pin settings
1.1 Pin connection
Figure 1. Pin connection (top view)
1.2 Pin description
Table 1. Pin description
N° Type Description
1 FB Output voltage selector. Connect FB to GND for Vout=5V or to OUT for
Vout=3.3V. Connect FB to an external resistor divider for adjustable
output voltage
2 LBI Battery low voltage detector input. The internal threshold is set to 1.23V.
A resistor divider is needed to adjust the desired low battery threshold.
3LBO
Battery low voltage detector output. If the voltage at the LBI pin drops
below the internal threshold typ. 1.23V, LBO goes low.
The LBO is an open drain output and so a pull-up resistor (about
200KΩ) has to be added for correct output setting .
4 REF 1.23V reference voltage. Bypass this output to GND with a 100nF
capacitor for filtering high frequency noise. No capacitor is required for
stability
5 SHDN
Shutdown pin. When pin 5 is below 0.2V the device is in shutdown,
when pin 5 is above 0.6V the device is operating.
6 GND Ground pin
7 LX Step-up inductor connection
8 OUT Power OUTPUT pin
Electrical data L6920DB
4/16
2 Electrical data
2.1 Maximum ratings
2.2 Thermal data
Table 2. Absolute maximum ratings
Symbol Parameter Value Unit
Vccmax Vcc to GND 6 V
LBI, SHDN, FB to GND 6 V
Vout max Vout to GND 6 V
Table 3. Thermal data
Symbol Parameter Value Unit
RthJA Maximum thermal resistance junction-ambient 180 °C/W
TJMaximum junction temperature 150 °C
L6920DB Electrical characteristics
5/16
3 Electrical characteristics
Table 4. Electrical characteristcs
(VIN = 2V, FB = GND, TJ = 25°C unless otherwise specified)
Symbol Parameter Test condition Min Typ Max Unit
VCC Section
VIN Minimum Start Up Input
Voltage Vout = 3.3V 0.8 V
IqQuiescent Current
Il = 0 mA, FB = 1.4V, Vout = 3.3V
LBI = SHDN = 2V 912µA
Il = 0 mA, FB = 1.4V, Vout = 5V
LBI = SHDN = 2V 11 15 µA
Isd Shut Down Current Vin = 5V, Il = 0 mA 0.1 1 µA
Irev Reverse battery current Vin = -4V 0.1 1 µA
Power section
Ron-N Active switch ON resistance 300 mΩ
Ron-P Synchronous switch ON
resistance 300 mΩ
Control section
Vou
t
Output voltage FB = OUT, Il = 0 mA 3.2 3.3 3.4 V
FB = GND, Il = 0 mA 4.955.1V
Output voltage range External divider 1.8 5.2 V
VLBI LBI threshold 1.23 V
VLBO LBO logic LOW Isink = 1mA 0.2 0.4 V
Ilim LX switch current limit 0.7 0.8 0.9 A
Tonmax Maximum on time Vout = 3.3V 3.75 5 6.25 µs
Toffmin Minimum off time Vout = 3.3V 0.75 1 1.25 µs
SHDN SHDN logic LOW 0.2 V
SHDN logic HIGH 0.6 V
Vref Reference Voltage 1.18 1.23 1.27 V
Typical performance L6920DB
6/16
4 Typical performance
Figure 2. Efficiency vs output current @ V
OUT
= 3.3V
Figure 3. Efficiency vs output current @ V
OUT
= 5V
Figure 4. Start up voltage vs output current @ V
out
= 5V & V
out
= 3.3V
40
45
50
55
60
65
70
75
80
85
90
95
0.1 1 10 100 100
0
Vin=2.4V
Vin=1.2V
L6920DB
Vout=3. 3V
L=10µH
Cout=47 µF
Ci n=10µF
0
10
20
30
40
50
60
70
80
90
100
0.1 1 10 100 100
0
Vin=3.6V
Vin=2.4V
Vin=1. 2V
L6920DB
Vout=5V
L=10µH
Cout=47µF
Cin=10µF
VStart up vs . out put cur rent
600
700
800
900
1000
1100
1200
1300
1400
1500
1600
0.1 1 10 100 1000
Out put current (mA)
VStar t up (mV)
VStart up v s. ou tput current
600
700
800
900
1000
1100
1200
1300
1400
1500
1600
0.1 1 10 100 1000
Out put cur rent (mA)
VStart up (mV)
Vout =3.3V
Vout =5V
L6920DB Detailed description
7/16
5 Detailed description
The L6920DB is a high efficiency, low voltage step-up DC/DC converter particularly suitable
for 1 to 3 cells (Li-Ion/ polymer, NiMH respectively) battery up conversion.
These performances are achieved via a strong reduction of quiescent current (10µA only)
and adopting a synchronous rectification that implies also a reduced cost in the application
(no external diode required).
Operation is based on maximum ON time - minimum OFF time control, tailored by a current
limit set to 800mA. A simplified block diagram is shown here below.
Figure 5. Simplified block diagram
5.1 Principle of operation
In L6920DB the control is based on a comparator that continuously checks the status of
output voltage.
If the output voltage is lower than the expected value, the control function of the L6920DB
directs the energy stored in the inductor to be transferred to the load. This is accomplished
by alternating between two basic steps:
●T
ON
phase: the energy is transferred from the battery to the inductor by shorting LX
node to ground via the N-channel power switch. The switch is turned off if the current
flowing in the inductor reaches 750mA or after a maximum on time set to 5µs.
●T
OFF
phase: the energy stored in the inductor is transferred to the load through the
synchronous switch for at least a minimum off time equal to 1µs. After this, the
synchronous switch is turned off as soon as the output voltage goes lower than the
regulated voltage or the current flowing in the inductor goes down to zero.
R
Q
S
Ton max
5µsec
Toff min
1µsec
VBG
VBG
VBG
A
B
C
-
+
-
+
-
+
-
+
-
+
VOUT
ZERO CROSSING
OPAMP
(CR)
CURRENT LIMIT
OUT
FB
GND
VREF
LBI
LBO
D99IN1041
SHDN
LX VIN
VOUT
VOUT
GND
R1,R2
FB Y
Y
A
B
C
- +
Detailed description L6920DB
8/16
So, in case of light load, the device works in PFM mode, as shown in Figure 8:
Figure 6. PFM mode condition: V
out
= 5V; V
batt
=1.5V ; C2 = V
out
; C3 = Inductor
Current
Figure 7 shows how the device works in case of heavy load.
Figure 7. Heavy load conditions C2 = V
out
; C3 = Inductor Current; V
out
= 5V;
V
batt
=1.5V
L6920DB Detailed description
9/16
Considering that current in the inductor is limited to 800mA, the maximum load current is
defined by the following relationship:
Equation 1
Where η is the efficiency and ILIM = 750mA
Of course, if ILOAD is greater than ILOAD_LIM the regulation is lost (Figure 8).
Figure 8. No regulation C2 = V
out
; C3 = Inductor Current
5.2 Start-up
One of the key features of L6920DB is the startup at supply voltage down to 0.8V (please
see the diagram in Figure 4).
The device leaves the startup mode of operation as soon as Vout goes over 1.4V. During
startup, the synchronous switch is off and the energy is transferred to the load through its
intrinsic body diode.
The N-channel switches with a very low RDS(on) thanks to an internal charge pump used to
bias the power MOS gate. Because of this modified behavior, TON/TOFF times are
lengthened. Current limit and zero crossing detection are still available.
ILOAD_LIM
Vin
Vout
-----------ILIM TOFF_MIN
Vout Vin
–
2L⋅
-------------------------
⋅–
⎝⎠
⎛⎞
η⋅⋅=
Detailed description L6920DB
10/16
5.3 Shutdown
In shutdown mode (SHDN pulled low) all internal circuitries are turned off, minimizing the
current provided by the battery (ISHDN < 100 nA, in typical case).
Both switches are turned off, and the low battery comparator output is forced in high
impedance state.
The synchronous switch body diode causes a parasitic path between power supply and
output that can't be avoided also in shutdown.
5.4 Low battery detection
The L6920DB includes a low battery detector comparator.
Threshold is VREF voltage and a hysteresis is added to avoid oscillations when input crosses
the threshold slowly. The LBO is an open drain output so a pull up resistor is required for a
proper use.
5.5 Low battery input
It is possible to fix, using an external resistor divider, the LBO threshold, in order to adapt
the LBO detection at the correct input source, by the following equation:
Equation 2
Where V
batt-th
is the battery voltage at which the internal comparetor is triggered.
5.6 Reverse polarity
A protection circuit has been implemented to avoid that L6920DB and the battery are
destroyed in case of wrong battery insertion.
In addition, this circuit has been designed so that the current required by the battery is zero
also in reverse polarity.
If a battery can be inserted in reverse direction, a non polarized capacitor should be
installed in location of C2.
5.7 Output voltage selection
Output voltage must be selected acting on FB pin.
Three choices are available: fixed 3.3V, 5V or adjustable output set via an external resistor
divider.
Vbatt-th 1.23V=1R1
R2
--------+
⎝⎠
⎛⎞
⋅
L6920DB Detailed description
11/16
Figure 9. Test circuit
Table 5. Output voltage connection
Vout = 3.3V FB pin connected to OUT (see application circuit)
Vout = 5V FB pin connected to GND
1.8V < Vout < 5.5V FB pin connected to a resistive divider Vout 1.23V 1 R4
R5
--------+
⎝⎠
⎛⎞
⋅=
V
BATT
LBI
V
OUT
V
REF
GND
___
LBO
_____
SHDN
FB
1
2
3
4
5
6
7
8
L6920DB
V
BATT
=1.5V
V
OUT
=2.5
V
C4
100nF
C1
47µF
C2
47µF
L1 10µH
R4
200KΩ
R5
200KΩ
R3
R1 R2
V
BATT
LBI
V
OUT
V
REF
GND
___
LBO
_____
SHDN
FB
1
2
3
4
5
6
7
8
L6920DB
V
BATT
=1.5V
V
OUT
=2.5
V
C4
100nF
C1
47µF
C2
47µF
L1 10µH
R4
200KΩ
R5
200KΩ
R3
R1 R2
L6920DB
Package mechanical data L6920DB
12/16
6 Package mechanical data
In order to meet environmental requirements, ST offers these devices in ECOPACK®
packages. These packages have a Lead-free second level interconnect . The category of
second level interconnect is marked on the package and on the inner box label, in
compliance with JEDEC Standard JESD97. The maximum ratings related to soldering
conditions are also marked on the inner box label. ECOPACK is an ST trademark.
ECOPACK specifications are available at: www.st.com
L6920DB Package mechanical data
13/16
Table 6. MSOP8 Mechanical data
Dim.
mm. inch
Min Typ Max Min Typ Max
A 1.10 0.043
A1 0.050 0.150 0.002 0.006
A2 0.750 0.850 0.950 0.03 0.033 0.037
b 0.250 0.400 0.010 0.016
c 0.130 0.230 0.005 0.009
D (1)
1. D and F does not include mold flash or protrusions. Mold flash or potrusions shall not exceed 0.15mm
(.006inch) per side.
2.900 3.000 3.100 0.114 0.118 0.122
E 4.650 4.900 5.150 0.183 0.193 0.20
E1 (1) 2.900 3.000 3.100 0.114 0.118 0.122
e 0.650 0.026
L 0.400 0.550 0.700 0.016 0.022 0.028
L1 0.950 0.037
k 0° (min.) 6° (max.)
aaa 0.100 0.004
Figure 10. Package dimensions
Order code L6920DB
14/16
7 Order code
Table 7. Order code
Part number Package Packaging
L6920DB MSOP8 Tube
L6920DBTR MSOP8 Tape and reel
L6920DB Revision history
15/16
8 Revision history
Table 8. Revision history
Date Revision Changes
18-Mar-2005 1 First issue
03-Aug-2005 2
Changed from “Preliminary Data” to “Datasheet”.
Modified Section 1 and Table 5.
Added Figg. 3, 6 and new Section 3.
01-Jun-2006 3 New template, modified application circuit and Figure 9.
13-Oct-2006 4 Cover page description updated.
30-Oct-2006 5 Typo in Figure 7 on page 8
L6920DB
16/16
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