Datasheet

ADP8863
Rev. A | Page 23 of 52
APPLICATIONS INFORMATION
The ADP8863 allows the charge pump to operate efficiently with a
minimum of external components. Specifically, the user must
select an input capacitor (C
IN
), output capacitor (C
OUT
), and two
charge pump fly capacitors (C1 and C2). C
IN
should be 1 F 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 F capacitor for C
OUT
is recommended. Larger
values are permissible, but care must be exercised to ensure that
VOUT charges above 55% (typical) of V
IN
within 4 ms (typical).
See the Short-Circuit Protection Mode section for more details.
For best practice, it is recommended that the two charge pump fly
capacitors be 1 F; 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 compo-
nents. The use of fly capacitors sized 0402 and smaller is allowed,
but the GDWN_DIS bit in Register 0x01 must be set. Minimum
voltage ratings should adhere to the guidelines in Table 7.
Table 7. Capacitor Stress in Each Charge Pump Gain State
Capacitor Gain = 1× Gain = 1.5× Gain = 2×
C
IN
V
IN
V
IN
V
IN
C
OUT
V
IN
V
IN
× 1.5 (max of 5.5 V) V
IN
× 2.0 (max of 5.5 V)
C1 None V
IN
/2 V
IN
C2 None V
IN
/2 V
IN
If one or both ambient light sensor comparator inputs (CMP_IN
and D6) are used, a small capacitor (0.1 F is recommended)
must be connected from the input to ground.
Any color LED can be used if the Vf (forward voltage) is less
than 4.1 V. However, using lower Vf LEDs reduces the input
power consumption by allowing the charge pump to operate at
lower gain states.
The equivalent circuit model for a charge pump is shown in
Figure 48.
08392-032
V
DX
C
OUT
G × V
IN
R
OUT
V
OUT
I
OUT
Figure 48. Charge Pump Equivalent Circuit Model
The input voltage is multiplied by the gain (G) and delivered to
the output through an effective resistance (R
OUT
). The output
current flows through R
OUT
and produces an IR drop to yield
V
OUT
= G ×V
IN
− I
OUT
× R
OUT
(G) (8)
The R
OUT
term is a combination of the R
DSON
resistance for the
switches used in the charge pump and a small resistance that accounts
for the effective dynamic charge pump resistance. The R
OUT
level
changes based upon the gain (the configuration of the switches).
Typical R
OUT
values are given in Table 1, Figure 13, and Figure 14.
V
OUT
is also equal to the largest Vf of the LEDs used plus the
voltage drop across the regulating current source. This gives
V
OUT
= Vf
(MAX)
+ V
DX
(9)
Combining Equation 8 and Equation 9 gives
V
IN
= (Vf
(MAX)
+ V
DX
+ I
OUT
× R
OUT
(G))/G (10)
Equation 10 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:
Vf
(MAX)
= 3.7 V
I
OUT
= 140 mA (7 LEDs at 20 mA each)
R
OUT
(G = 1.5×) = 3 Ω (obtained from Figure 13)
At the point of a gain transition, V
DX
= V
HR(UP)
. Table 1 gives the
typical value of V
HR(UP)
as 0.2 V. Therefore, the input voltage
level when the gain transitions from 1.5× to 2× is
V
IN
= (3.7 V + 0.2 V + 140 mA × 3 Ω)/1.5 = 2.88 V
LAYOUT GUIDELINES
Note the following layout guidelines:
• For optimal noise immunity, place the C
IN
and C
OUT
capaci-
tors as close as possible to their respective pins. These
capacitors should share a short ground trace. If the LEDs
are a significant distance from the VOUT pin, another capaci-
tor on VOUT, placed closer to the LEDs, is advisable.
• For optimal efficiency, place the charge pump fly capacitors
(C1 and C2) as close to the part as possible.
• The ADP8863 does not distinguish between power ground
and analog ground. Therefore, both ground pins can be
connected directly together. It is recommended that these
ground pins be connected at the ground for the input and
output capacitors.
• The LFCSP package requires the exposed pad to be
soldered at the board to the GND1 and/or GND2 pin(s).
• Unused diode pins (Pin D1 to Pin D7) can be connected to
ground or to VOUT, or remain floating. However, the
unused diode current sinks must be disabled by setting
them as independent sinks in Register 0x05 and then
disabling them in Register 0x10. If they are not disabled,
the charge pump efficiency may suffer.
• If the CMP_IN phototransistor input is not used, it can be
connected to ground or remain floating.
• 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 k series resistor.
• The ADP8863 has an integrated noise filter on the nRST
pin. Under normal conditions, it is not necessary to filter
the reset line. However, if the part is 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
IH(MIN)
level (see Table 1). Do not
allow the nRST pin to float.