ADP8866 Analog Devices, ADP8866 Datasheet - Page 12

• Manufacturer Part Number: ADP8866
• Description: Charge Pump Driven 9-Channel LED Driver with Automated LED Lighting Effects
• Manufacturer: Analog Devices

Specifications of ADP8866

• Vin Range: 2.5 to 5.5V
• Vout (v): 4.3 to 5.5
• Synchronous: No
• Package: 20-Lead LFCSP
• Led Configuration: Parallel
• Topology: Capacitive
• I2c Support: Yes
• Max Iout (ma): 25mA
• Brightness Control: I2C
• Peak Efficiency (%): 92%
• Switching Frequency: 1MHz

ADP8866
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
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:
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×,
OUT
of 240 mA over the entire input voltage range of 2.5 V to
Q = C × V
NOTES
1. V
DMAX
IS THE CALCULATED GAIN DOWN TRANSITION POINT.
STANDBY
STARTUP
G = 1.5
G = 1
G = 2
EXIT
EXIT STANDBY
1
100µs (TYP)
Figure 28. State Diagram for Automatic Gain Selection
100µs (TYP)
100µs (TYP)
WAIT
VOU T > V
WAIT
WAIT
V
STARTUP:
0
IN
CHARGE
TO V
OUT(START)
OUT
Rev. 0 | Page 12 of 52
(1)
MIN (V
MIN (V
MIN (V
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
regulation. The gain is automatically selected based on the
minimum voltage (V
the device is placed into G = 1× mode and the output charges to
VIN. If any V
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
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.
D1:D9
0
D1:D9
) < V
D1:D9
0
HR(UP)
) < V
) < V
HR(MIN
HR(UP)
DMAX
DX
) in Table 1) to maintain accurate current
level is less than the required headroom, the
DX
0
0
) at all of the current sources. At startup,
MIN (V
D1:D9
) > V
DMAX
DMAX
in Figure 28) is