ADP2443ACPZN-R7 datasheet(16/25 Pages) AD

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Data Sheet

ADP2443

Rev. 0 | Page 15 of 24

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. The loop that is composed of this

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

MOSFET must be kept 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 from the

following equation:

ICIN_RMS = IOUT ×

)

(

−

OUTPUT VOLTAGE SETTING

The output voltage of the ADP2443 is set by an external resistor

divider. The resistor values are calculated using

VOUT = 0.6 ×













BOT

TOP

To limit output voltage accuracy degradation due to FB bias

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

ensure that RBOT < 30 kΩ.

Table 5 lists the recommended resistor divider values for

various output voltages.

Table 5. 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

8.0

44.2

3.57

10.0

39.2

2.49

12.0

52.3

2.74

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 ADP2443 is typically 50 ns.

Calculate the minimum output voltage at a given input voltage

and frequency 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 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.

Calculate the maximum output voltage, limited by the minimum

off time at a given input voltage and frequency, 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.

As Equation 1 and Equation 2 show, reducing the switching

frequency alleviates the minimum on time and minimum off

time limitations.

INDUCTOR SELECTION

The inductor value is determined by the operating frequency,

input voltage, output voltage, and inductor ripple current. Using

a small inductor results in a faster transient response but degrades

efficiency, due to a larger inductor ripple current; whereas using

a large inductor value results in s smaller ripple current and

better efficiency, but also results in a slower transient response.

As a guideline, the inductor ripple current, ΔIL, is typically set

to one-third of the maximum load current. Calculate the

inductor value using the following equation:

L =

OUT

∆

−

)

(

where:

VIN is the input voltage.

VOUT is the output voltage.

D is the duty cycle.

ΔIL is the inductor current ripple.

fSW is the switching frequency.

D =

OUT

Calculate the peak inductor current using

IPEAK = IOUT +

∆

The saturation current (ISAT) of the inductor must be larger than the

peak inductor current. For ferrite core inductors with a quick

saturation characteristic, the saturation current rating of the

inductor must be greater than the current limit threshold of the

switch, which prevents the inductor from reaching saturation.

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