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L6920DC
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 to 5.5V 3.3V and 5V fixed output voltages Low battery voltage detection Reverse battery protection 550mA minimum input current limit Switching frequency up to 1MHz 1.23V reference voltage available
MSOP8
Description
The L6920DC 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 P. 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, over voltage.
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
Application circuit
L6920DC
3.3V
June 2006
Rev 1
1/16
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Contents
L6920DC
Contents
1 Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 1.2 Pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2
Electrical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 2.2 Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 4 5
Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Typical performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Detailed description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1 5.2 5.3 5.4 5.5 5.6 5.7 Principle of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Start-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Low battery detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Low battery input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Reverse polarity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Output voltage selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6 7 8
Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Order code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
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L6920DC
Pin settings
1
1.1
Pin settings
Pin connection
Figure 1. Pin connection (top view)
1.2
Pin description
Table 1. Pin description
N 1 Type FB Description 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 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. 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 200K) has to be added for correct output setting . 1.23V reference voltage. Bypass this output to GND with a 100nF capacitor for filtering high frequency noise. No capacitor is required for stability 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. Ground pin Step-up inductor connection Power OUTPUT pin
2 3
LBI LBO
4
REF
5 6 7 8
SHDN GND LX OUT
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Electrical data
L6920DC
2
2.1
Electrical data
Maximum ratings
Table 2. Absolute maximum ratings
Symbol Vccmax Vcc to GND LBI, SHDN, FB to GND Vout max Vout to GND Parameter Value 6 6 6 Unit V V V
2.2
Thermal data
Table 3. Thermal data
Symbol RthJA TJ Parameter Maximum thermal resistance junction-ambient Maximum junction temperature Value 180 150 Unit C/W C
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L6920DC
Electrical characteristics
3
Electrical characteristics
Table 4. Electrical characteristcs (VIN = 2V, FB = GND, TJ = 25C unless otherwise specified)
Symbol VCC Section VIN Minimum Start Up Input Voltage Vout = 3.3V Il = 0 mA, FB = 1.4V, Vout = 3.3V LBI = SHDN = 2V Quiescent Current Il = 0 mA, FB = 1.4V, Vout = 5V LBI = SHDN = 2V Shut Down Current Reverse battery current Vin = 5V, Il = 0 mA Vin = -4V 11 0.1 0.1 15 1 1 A A A 0.8 9 12 V A Parameter Test condition Min Typ Max Unit
Iq
Isd Irev
Power section Ron-N Ron-P Active switch ON resistance Synchronous switch ON resistance 300 300 m m
Control section Vout Output voltage FB = OUT, Il = 0 mA FB = GND, Il = 0 mA Output voltage range VLBI VLBO Ilim Tonmax Toffmin SHDN LBI threshold
LBO logic LOW
3.2 4.9 1.8
3.3 5
3.4 5.1 5.5
V V V V
External divider
1.23 Isink = 1mA 0.55 Vout = 3.3V Vout = 3.3V 3.75 0.75 5 1 0.2 0.4 0.9 6.25 1.25 0.2 0.6 1.18 1.23 1.27
V A s s V V V
LX switch current limit Maximum on time Minimum off time
SHDN logic LOW SHDN logic HIGH
Vref
Reference Voltage
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Typical performance
L6920DC
4
Typical performance
Figure 2. Efficiency vs output current @ VOUT = 3.3V
9 5 9 0 8 5 8 0 7 5 7 0 6 5 6 0 5 5 5 0 4 5 4 0 0 .1 1 1 0 10 0 10 00 V = .2 in 1 V V = .4 in 2 V
L6920DC L90 B 62D Vu 3 V o t= .3 L1 H = 0 Cu 4 F o t= 7 C =0F in 1
Figure 3.
Efficiency vs output current @ VOUT = 5V
10 0 9 0 8 0 7 0 6 0 5 0 4 0 3 0 2 0 1 0 0 0 .1
V = .6 in 3 V V = .4 in 2 V V = .2 in 1 V
L6920DC L90 B 62D Vu 5 o t= V L1 H = 0 Cu 4 o t= 7 F C =0F in 1
1 1 0 10 0 10 00
Figure 4.
Start up voltage vs output current @ Vout = 5V & Vout = 3.3V
VStart up vs . out put cur rent
VStart up v s. output current
1600
1600
Vout =5V
1500 1400 1300
1500 1400 1300
Vout =3.3V
VStart up (mV)
1200 1100 1000 900 800 700 600 0.1 1 10 100 1000
VStart up (mV)
1200 1100 1000 900 800 700 600 0.1 1 10 100 1000
Output cu rrent (mA)
Output current (mA)
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L6920DC
Detailed description
5
Detailed description
The L6920DC 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 (10A 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. A simplified block diagram is shown here below. Figure 5. Simplified block diagram
OUT ZERO CROSSING VREF SHDN FB VOUT GND R1,R2 Y + VBG Q S R + CURRENT LIMIT LBO + VBG Ton max 5sec FB GND Y A B C OPAMP (CR) VOUT LX VIN + VBG + -+ VOUT
A B C
Toff min 1sec
LBI
D99IN1041
5.1
Principle of operation
In L6920DC 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 L6920DC directs the energy stored in the inductor to be transferred to the load. This is accomplished by alternating between two basic steps:
TON 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 the current limit or after a maximum on time set to 5s. TOFF 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 1s. 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.
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Detailed description So, in case of light load, the device works in PFM mode, as shown in Figure 8: Figure 6.
L6920DC
PFM mode condition: Vout = 5V; Vbatt =1.5V ; C2 = Vout ; C3 = Inductor Current
Figure 7 shows how the device works in case of heavy load. Figure 7. Heavy load conditions C2 = Vout; C3 = Inductor Current; Vout = 5V; Vbatt=1.5V
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L6920DC The maximum load current is defined by the following relationship: Equation 1
Detailed description
V in V out - V in I LOAD_LIM = ---------- I LIM - T OFF_MIN ------------------------ 2 L V out
Where is the efficiency and ILIM is the current limit. Of course, if ILOAD is greater than ILOAD_LIM the regulation is lost (Figure 8). Figure 8. No regulation C2 = Vout ; C3 = Inductor Current
5.2
Start-up
One of the key features of L6920DC 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.
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Detailed description
L6920DC
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 L6920DC 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
V batt-th = 1.23V 1 + R1 ------- R2
Where Vbatt-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 L6920DC 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.
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L6920DC
Detailed description
Table 5. Output voltage connection
Vout = 3.3V Vout = 5V 1.8V < Vout < 5.5V FB pin connected to OUT (see application circuit) FB pin connected to GND FB pin connected to a resistive divider V out = 1.23V 1 + R4 ------ R5
Figure 9.
Test circuit
R 1 R 2
LB I V ATT=1.5V B
L1 10H C 2 47F
_____ 2 SD HN 5 V UT O 8
C 1 R 3 47F
V ATT B 7
V UT=2.5V O
L6920D B
V EF R 4
C 4 100nF
L6920DC
___ LB O 3 FB
R 4 200K
1 6 GD N
R 5 200K
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Package mechanical data
L6920DC
6
Package mechanical data
In order to meet environmental requirements, ST offers these devices in ECOPACK(R) 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
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L6920DC
Package mechanical data
Table 6. MSOP8 Mechanical data
mm. Dim. Min A A1 A2 b c D (1) E E1 (1) e L L1 k aaa 0.400 0.050 0.750 0.250 0.130 2.900 4.650 2.900 3.000 4.900 3.000 0.650 0.550 0.950 0 (min.) 6 (max.) 0.100 0.004 0.700 0.016 0.850 Typ Max 1.10 0.150 0.950 0.400 0.230 3.100 5.150 3.100 0.002 0.03 0.010 0.005 0.114 0.183 0.114 0.118 0.193 0.118 0.026 0.022 0.037 0.028 0.033 Min Typ Max 0.043 0.006 0.037 0.016 0.009 0.122 0.20 0.122 inch
1. D and F does not include mold flash or protrusions. Mold flash or potrusions shall not exceed 0.15mm (.006inch) per side.
Figure 10. Package dimensions
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Order code
L6920DC
7
Order code
Table 7. Order code
Part number L6920DC L6920DCTR Package MSOP8 MSOP8 Packaging Tube Tape and reel
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L6920DC
Revision history
8
Revision history
Table 8. Revision history
Date 29-Jun-2006 Revision 1 First issue Changes
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L6920DC
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