Datasheet

ADP8860
Rev. 0 | Page 24 of 52
07967-14
APPLICATIONS INFORMATION
The ADP8860 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 components. Use voltage ratings of 10 V or greater for
these capacitors.
If one or both ambient light sensor comparator inputs (CMP_IN
and D6/CMP_IN2) are used, a small capacitor (0.1 F is
recommended) must be connected from the input to ground.
Any color of 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 43.
0
V
DX
C
OUT
G × V
IN
R
OUT
V
OUT
I
OUT
Figure 43. 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 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 and Figure 13
and Figure 14.
V
OUT
is also equal to the largest Vf of the LEDs that are 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)
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:
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
• For optimal noise immunity, place the C
IN
and C
OUT
capacitors 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
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 ADP8860 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.
• If using the LFCSP package, the exposed pad must 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, 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.