ADP2387ACPZN-R7 datasheet(15/25 Pages) AD

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Data Sheet

ADP2387

Rev. C | Page 15 of 25

APPLICATIONS INFORMATION

INPUT CAPACITOR SELECTION

The input capacitor reduces the input voltage ripple caused by

the switch current on PVIN. Place the input capacitor as close

as possible to the PVIN pin. A ceramic capacitor in the 10 µF to

47 µF range is recommended. Keep the loop that is composed of

this input capacitor, the high-side MOSFET, and the low-side

MOSFET as small as possible.

The voltage rating of the input capacitor must be greater than the

maximum input voltage. The rms current rating of the input

capacitor must be larger than the value calculated by the

following equation:

)

(1 D

OUT

C RMS

−

OUTPUT VOLTAGE SETTING

An external resistive divider sets the output voltage of the

ADP2387. Use the following equation to calculate the resistor

values:













BOT

TOP

OUT

where:

RTOP is the top feedback resistor between VOUT and FB.

RBOT is the bottom feedback resistor between FB and GND.

To limit output voltage accuracy degradation due to the FB bias

current (0.1 µA maximum) to less than 0.5% (maximum),

ensure that RBOT < 30 kΩ.

Table 6 lists the recommended resistor divider values for various

output voltages.

Table 6. Resistor Divider Values for Various Output Voltages

VOUT (V)

RTOP ± 1% (kΩ)

RBOT ± 1% (kΩ)

1.0

1.2

1.5

1.8

2.5

47.5

3.3

2.21

5.0

VOLTAGE CONVERSION LIMITATIONS

The minimum output voltage for a given input voltage and

switching frequency is constrained by the minimum on time.

The minimum on time of the ADP2387 is typically 130 ns.

Calculate the minimum output voltage for a given input voltage

and switching frequency by using the following equation:

VOUT_MIN = VIN × tMIN_ON × fSW − (RDSON_HS − RDSON_LS) ×

IOUT_MIN × tMIN_ON × fSW − (RDSON_LS + RL) × IOUT_MIN

(1)

where:

VOUT_MIN is the minimum output voltage.

tMIN_ON is the minimum on time.

fSW is the switching frequency.

RDSON_HS is the high-side MOSFET on resistance.

RDSON_LS is the low-side MOSFET on resistance.

IOUT_MIN is the minimum output current.

RL is the series resistance of the output inductor.

The maximum output voltage for a given input voltage and

switching frequency is constrained by the minimum off time

and the maximum duty cycle. The minimum off time is typically

200 ns, and the maximum duty cycle of the ADP2387 is

typically 90%.

Calculate the maximum output voltage, limited by the minimum

off time for a given input voltage and switching frequency, by

using the following equation:

VOUT_MAX = VIN × (1 − tMIN_OFF × fSW) − (RDSON_HS − RDSON_LS) ×

IOUT_MAX × (1 − tMIN_OFF × fSW) − (RDSON_LS + RL) × IOUT_MAX (2)

where:

VOUT_MAX is the maximum output voltage.

tMIN_OFF is the minimum off time.

IOUT_MAX is the maximum output current.

Calculate the maximum output voltage, limited by the maximum

duty cycle for a given input voltage, using the following equation:

VOUT_MAX = DMAX × VIN

(3)

where DMAX is the maximum duty cycle.

As shown in Equation 1 to Equation 3, reducing the switching

frequency alleviates the minimum on time and minimum off

time limitation.

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