Datasheet

Data Sheet ADP8861
Rev. B | Page 19 of 40
APPLICATIONS INFORMATION
The ADP8861 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 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
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 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. 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 6.
Table 6. 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
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 39.
V
DX
C
OUT
G × V
IN
R
OUT
VOUT
I
OUT
08391-140
Figure 39. 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) (5)
The R
OUT
term is a combination of the R
DSON
resistance for the
switches used in the charge pump and a small resistance, which
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 Tabl e 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
(6)
Combining Equation 5 and Equation 6 gives
V
IN
= (Vf
(MAX)
+ V
DX
+ I
OUT
× R
OUT
(G))/G (7)
Equation 7 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)
. Tabl e 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
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
(C1 and C2) as close to the part as possible.
• The ADP8861 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 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 ADP8861 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.