FDMS7578 datasheet(20/32 Pages) AD

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ADP2325

Data Sheet

Rev. 0 | Page 20 of 32

APPLICATIONS INFORMATION

INPUT CAPACITOR SELECTION

The input decoupling capacitor attenuates high frequency noise

on the input and acts as an energy reservoir. This capacitor should

be a ceramic capacitor in the range of 10 µF to 47 µF and must

be placed close to the PVINx pin. The loop composed of this

input capacitor, high-side NFET, and low-side NFET must be

kept as small as possible. The voltage rating of the input capacitor

must be greater than the maximum input voltage. Ensure that the

rms current rating of the input capacitor is larger than that

expressed in following equation:

(

)

OUT

_rms

−

OUTPUT VOLTAGE SETTING

The output voltage of the ADP2325 can be set by an external

resistor divider using the following equation:













BOT

TOP

OUT

To limit output voltage accuracy degradation due to FBx pin

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

ensure that RBOT is less than 30 kΩ. Table 8 provides the recom-

mended resistor divider for various output voltage options.

Table 8. Resistor Divider 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 limited by the minimum on time. The

minimum on time of the ADP2325 is typically 130 ns. The

minimum output voltage in CCM mode at a given input voltage

and frequency can be calculated using the following equation:

VOUT_MIN = VIN × tMIN_ON × fSW − (RDSON1 − RDSON2) × IOUT_MIN ×

tMIN_ON × fSW − (RDSON2 + RL) × IOUT_MIN

where:

VOUT_MIN is the minimum output voltage.

tMIN_ON is the minimum on time.

IOUT_MIN is the minimum output current.

fSW is the switching frequency.

RDSON1 is the high-side MOSFET on resistance.

RDSON2 is the low-side MOSFET on resistance.

RL is the series resistance of the output inductor.

The maximum output voltage for a given input voltage and

switching frequency is also limited by the minimum off time

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

150 ns and the maximum duty is typically 90% in the ADP2325.

The maximum output voltage that is limited by the minimum off

time at a given input voltage and frequency can be calculated

using the following equation:

VOUT_MAX = VIN × (1 − tMIN_OFF × fSW) − (RDSON1 − RDSON2) ×

IOUT_MAX × (1 − tMIN_OFF × fSW) − (RDSON2 + RL) × IOUT_MAX

where:

VOUT_MAX is the maximum output voltage.

tMIN_OFF is the minimum off time.

IOUT_MAX is the maximum output current.

The maximum output voltage that is limited by the maximum

duty cycle at a given input voltage can be calculated using the

following equation:

VOUT_MAX = DMAX × VIN

where DMAX is the maximum duty cycle.

As the previous equations demonstrate, reducing the switching

frequency alleviates the minimum on time and minimum off time

limitation.

CURRENT-LIMIT SETTING

The ADP2325 has two selectable current-limit thresholds. Make

sure that the selected current-limit value is larger than the peak

current of the inductor, IPEAK.

INDUCTOR SELECTION

The inductor value is determined by the operating frequency,

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

a small inductor provides faster transient response but degrades

efficiency due to larger inductor ripple current, whereas a large

inductor value provides smaller ripple current and better effi-

ciency but results in a slower transient response. Thus, there is a

trade-off between the transient response and efficiency. As a

guideline, the inductor ripple current, ΔIL, is typically set to

one-third of the maximum load current. The inductor value can

be calculated by using the following equation:

(

)

OUT

∆

−

where:

VIN is the input voltage.

VOUT is the output voltage.

ΔIL is the inductor ripple current.

fSW is the switching frequency.

D is the duty cycle.

OUT

D =

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