7/13
L6920
Figure 11. No regulation. I
load
> I
load_lim
Trace1:
V
out
(100mV~/div) Trace 4: I
L
(200mA/div).
Time div.: 5 µs/div
4.1 Start-up
One of the key features of L6920 is the startup at sup-
ply voltage down to 1V (please see the diagram in
Figure 5. in case of heavy load).
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 trans-
ferred to the load through its intrinsic body diode.
The N-channel switches with a very low RDSon
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.
4.2 Shutdown
In shutdown mode (
SHDN
pulled low) all internal cir-
cuitries are turned off, minimizing the current provid-
ed by the battery (I
SHDN
< 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 para-
sitic path between power supply and output that can't
be avoided also in shutdown.
4.3 Low battery detection
The L6920 includes a low battery detector compara-
tor. Threshold is VREF voltage and a 1.3% hystere-
sis is added to avoid oscillations when input crosses
the threshold slowly. The
LBO
is an open drain out-
put so a pull up resistor is required for a proper use.
4.4 Reverse polarity
A protection circuit has been implemented to avoid
that L6920 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.
5 Application Information
5.1 Output voltage selection
Output voltage must be selected acting on FB pin.
Three choices are available: fixed 3.3V, 5V or adjust-
able output set via an external resistor divider.
Table 5. Output Voltage Selection
V
OUT
= 3.3V FB pin connected to OUT (see
application circuit)
V
OUT
= 5V FB pin connected to GND
2V ≤ V
OUT
≤ 5.2V FB pin connected to a resistive
divider
V
OUT
1.23V 1
R4
R5
------- -+
⎝⎠
⎛⎞
=
