Datasheet

ADP8866 Data Sheet
Rev. A | Page 12 of 52
POWER STAGE
Typical white LEDs require up to 4 V to drive them. Therefore,
some form of boosting is required to cover the typical Li Ion
battery voltage variation. The ADP8866 accomplishes this with
a high efficiency charge pump capable of producing a maximum
I
OUT
of 240 mA over the entire input voltage range of 2.5 V to
5.5 V. Charge pumps use the basic principle that a capacitor
stores charge based on the voltage applied to it, as shown in the
following equation:
Q = C × V (1)
By charging the capacitors in different configurations, the
charge and, therefore, the gain can be optimized to deliver the
voltage required to power the LEDs. Because a fixed charging
and discharging combination must be used, only certain
multiples of gain are available. The ADP8866 is capable of
automatically optimizing the gain (G) from 1×, 1.5×, and 2×.
These gains are accomplished with two capacitors and an
internal switching network.
In G = 1× mode, the switches are configured to pass VIN
directly to VOUT. In this mode, several switches are connected
in parallel to minimize the resistive drop from input to output.
In G = 1.5× and G = 2× modes, the switches alternatively charge
from the battery and discharge into the output. For G = 1.5×,
the capacitors are charged from VIN in series and are discharged to
VOUT in parallel. For G = 2×, the capacitors are charged from
VIN in parallel and are discharged to VOUT in parallel. In
certain fault modes, the switches are opened and the output is
physically isolated from the input.
Automatic Gain Selection
Each LED that is driven requires a current source. The voltage
on this current source must be greater than a minimum headroom
voltage (V
HR(MIN
) in Table 1) to maintain accurate current
regulation. The gain is automatically selected based on the
minimum voltage (V
DX
) at all of the current sources. At startup,
the device is placed into G = 1× mode and the output charges to
VIN. If any V
DX
level is less than the required headroom, the
gain is increased to the next step (G = 1.5×). A 100 μs delay is
allowed for the output to stabilize prior to the next gain
switching decision. If there remains insufficient current sink
headroom, the gain is increased again to 2×. Conversely, to
optimize efficiency, it is not desirable for the output voltage to be
too high. Therefore, the gain reduces when the headroom
voltage is too great. This point (labeled V
DMAX
in Figure 28) is
internally calculated to ensure that the lower gain still results in
ample headroom for all the current sinks. The entire cycle is
illustrated in Figure 28.
09478-029
NOTES
1. V
DMAX
IS THE CALCULATED GAIN DOWN TRANSITION POINT.
WAIT
100µs (TYP)
MIN (V
D1:D9
) < V
HR(UP)
0
0
1
1
1
1
0
0
STARTUP:
CHARGE
V
IN
TO V
OUT
EXIT STANDBY
VOUT > V
OUT(START)
1
WAIT
100µs (TYP)
WAIT
100µs (TYP)
MIN (V
D1:D9
) < V
HR(UP)
MIN (V
D1:D9
) > V
DMAX
MIN (V
D1:D9
) < V
DMAX
G = 2
G = 1.5
EXIT
STARTUP
G = 1
STANDBY
0
Figure 28. State Diagram for Automatic Gain Selection