ADP8866 Analog Devices, ADP8866 Datasheet - Page 20

• 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
APPLICATIONS INFORMATION
The ADP8866 allows the charge pump to operate efficiently
with a minimum of external components. Specifically, the user
must select an input capacitor (C
and two charge pump fly capacitors (C1 and C2). C
1 μF or greater. The value must be high enough to produce a
stable input voltage signal at the minimum input voltage and
maximum output load. A 1 μF capacitor for C
Larger values are permissible, but care must be exercised to ensure
that VOUT charges above 55% (typical) of VIN within 4 ms
(typical). See the Short-Circuit Protection (SCP) Mode section
for more detail.
For best practice, it is recommended that the two charge pump
fly capacitors be 1 μF; larger values are not recommended and
smaller values may reduce the ability of the charge pump to
deliver maximum current. For optimal efficiency, the charge
pump fly capacitors should have low equivalent series resistance
(ESR). Low ESR X5R or X7R capacitors are recommended for
all four components. Minimum voltage ratings should adhere to
the guidelines in Table 7:
Table 7. Capacitor Stress in Each Charge Pump Gain State
Capacitor
C
C
Capacitor)
C1 (Charge Pump
Capacitor)
C2 (Charge Pump
Capacitor)
Any color LED can be used provided that the Vf (forward
voltage) is less than 4.3 V. However, using lower Vf LEDs
reduces the input power consumption by allowing the charge
pump to operate at lower gain states.
The equivalent model for a charge pump is shown in Figure 39.
The input voltage is multiplied by the gain (G) and delivered to
the output through an effective charge pump resistance (R
The output current flows through R
drop, which yields
The R
switches used in the charge pump and a small resistance that
accounts for the effective dynamic charge pump resistance. The
R
IN
OUT
OUT
(Input Capacitor)
(Output
V
level changes based upon the gain (the configuration of the
OUT
OUT
term is a combination of the R
= G × V
Figure 39. Charge Pump Equivalent Circuit Model
G × V
R
OUT
IN
IN
V
− I
OUT
Gain = 1×
VIN
VIN
None
None
OUT
C
OUT
I
OUT
× R
V
DX
OUT
IN
(G)
), output capacitor (C
Gain = 1.5×
VIN
VIN × 1.5
(Max of 5.5 V)
VIN ÷ 2
VIN ÷ 2
OUT
and produces an IR
DSON
OUT
resistance for the
is recommended.
IN
Gain = 2×
VIN
VIN × 2.0
(Max of 5.5 V)
VIN
VIN
should be
OUT
OUT
),
Rev. 0 | Page 20 of 52
).
(6)
switches). Typical R
and Figure 16.
V
voltage drop across the regulating current source. This gives
Combining Equation 6 and Equation 7 gives
This equation is useful for calculating approximate bounds for
the charge pump design.
Determining the Transition Point of the Charge Pump
Consider the following design example where:
At the point of a gain transition, V
typical value of V
level when the gain transitions from 1.5× to 2× is
LAYOUT GUIDELINES
•
•
•
•
•
•
OUT
V
V
Vf
I
R
V
For optimal noise immunity, place the C
capacitors as close to their respective pins as possible.
These capacitors should share a short ground trace. If the
LEDs are a significant distance from the VOUT pin,
another capacitor on VOUT, placed closer to the LEDs, is
advisable.
For optimal efficiency, place the charge pump fly capacitors
as close to the part as possible.
The ground pin should be connected at the ground for the
input and output capacitors. The LFCSP exposed pad must
be soldered at the board to the GND pin.
Unused diode pins [D1:D9] can be connected to ground or
VOUT or remain floating. However, the unused diode
current sinks must be removed from the charge pump gain
calculation by setting the appropriate DxPWR bits high in
Register 0x09 and Register 0x0A.
If the interrupt pin (nINT) is not used, connect it to
ground or leave it floating. Never connect it to a voltage
supply, except through a ≥1 kΩ series resistor.
The ADP8866 has an integrated noise filter on the nRST
pin. Under normal conditions, it is not necessary to filter
the reset line. However, if exposed to an unusually noisy
signal, it is beneficial to add a small RC filter or bypass
capacitor on this pin. If the nRST pin is not used, it must
be pulled well above the V
allow the nRST pin to float.
is also equal to the largest Vf of the LEDs used plus the
OUT
OUT
OUT
IN
IN
(MAX)
= (Vf
= (3.7 V + 0.2 V + 140 mA × 3 Ω)/1.5 = 2.88 V
= 140 mA (7 LEDs at 20 mA each)
(G = 1.5×) = 3 Ω (obtained from Figure 12)
= Vf
= 3.7 V
(MAX)
(MAX)
HR(UP)
+ V
+ V
OUT
DX
DX
as 0.2 V. Therefore, the input voltage
values are given in Table 1 and Figure 14
+ I
OUT
IH(MAX)
× R
DX
OUT
= V
(G))/G
level (see Table 1). Do not
HR(UP)
IN
. Table 1 gives the
and C
OUT
(7)
(8)