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L6920DCTR
Synchronous rectifier step up converter
June 2006 Rev 1 1/16
L6920DCSynchronous 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
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
DescriptionThe 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 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,
over voltage.
Application circuit
Contents L6920DC2/16
Contents Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.1 Pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Electrical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.1 Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Typical performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Detailed description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75.1 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
Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Order code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
L6920DC Pin settings
3/16
1 Pin settings
1.1 Pin connection
Figure 1.
1.2 Pin description
Table 1. Pin description
Electrical data L6920DC
4/16
2 Electrical data
2.1 Maximum ratings
2.2 Thermal data
Table 2. Absolute maximum ratings
Table 3. Thermal data
L6920DC Electrical characteristics
5/16
3 Electrical characteristics
Table 4. Electrical characteristcs
(VIN = 2V, FB = GND, TJ = 25°C unless otherwise specified)
Typical performance L6920DC
6/16
4 Typical performance
Figure 2. Efficiency vs output current @ VOUT = 3.3V
Figure 3. Efficiency vs output current @ V = 5V
Figure 4. Start up voltage vs output current @ Vout = 5V & Vout = 3.3V
L6920DC Detailed description
7/16
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 (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.
A simplified block diagram is shown here below.
Figure 5. Simplified block diagram
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 5µs. 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 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.
Detailed description L6920DC
8/16
So, in case of light load, the device works in PFM mode, as shown in Figure8:
Figure 6. 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
