ADP8863ACPZ-R7 datasheet(23/48 Pages) AD

23 / 48 page

ADP8863

Rev. 0 | Page 23 of 48

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 (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 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×

CIN

VIN

COUT

VIN

VIN × 1.5 (max of 5.5 V)

VIN × 2.0 (max of 5.5 V)

None

VIN/2

VIN

None

VIN/2

VIN

If one or both ambient light sensor comparator inputs (CMP_IN

and D6) are used, a small capacitor (0.1 μF 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 47.

VDX

COUT

G × VIN

ROUT

VOUT

IOUT

Figure 47. 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)

(8)

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, Figure 12, and Figure 13.

VOUT is also equal to the largest Vf of the LEDs used plus the

voltage drop across the regulating current source. This gives

VOUT

= Vf(MAX) + VDX

(9)

Combining Equation 8 and Equation 9 gives

VIN

= (Vf(MAX) + VDX + IOUT × ROUT(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

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

ROUT(G = 1.5×) = 3 Ω (obtained from Figure 12)

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

Note the following layout guidelines:

•

For optimal noise immunity, place the CIN and COUT 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 kΩ 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 VIH(MIN) level (see Table 1). Do not

allow the nRST pin to float.

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