ADP8860ACPZ-R7 datasheet(24/52 Pages) AD

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ADP8860

Rev. 0 | Page 24 of 52

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 (CIN), output capacitor (COUT),

and two charge pump fly capacitors (C1 and C2). CIN 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 COUT is recommended.

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 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. 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 μF 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.

VDX

COUT

G × VIN

ROUT

VOUT

IOUT

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 (ROUT). The output

current flows through ROUT and produces an IR drop to yield

VOUT = G ×VIN − IOUT × ROUT(G)

(5)

The ROUT term is a combination of the RDSON resistance for the

switches used in the charge pump and a small resistance that

accounts for the effective dynamic charge pump resistance. The

ROUT level changes based upon the gain (the configuration of the

switches). Typical ROUT values are given in Table 1 and Figure 13

and Figure 14.

VOUT is also equal to the largest Vf of the LEDs that are used

plus the voltage drop across the regulating current source. This

gives

VOUT = Vf(MAX) + VDx

(6)

Combining Equation 5 and Equation 6 gives

VIN = (Vf(MAX) + VDx + IOUT × ROUT(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

IOUT = 140 mA (7 LEDs at 20 mA each)

ROUT (G = 1.5×) = 3 Ω (obtained from Figure 13)

At the point of a gain transition, VDx = VHR(UP), Table 1 gives the

typical value of VHR(UP) as 0.2 V. Therefore, the input voltage

level when the gain transitions from 1.5× to 2× is

VIN = (3.7 V + 0.2 V + 140 mA × 3 Ω)/1.5 = 2.88 V

LAYOUT GUIDELINES

•

For optimal noise immunity, place the CIN and COUT

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 kΩ series resistor.

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