ADP2441ACPZ-R2 datasheet(18/32 Pages) AD

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ADP2441

Data Sheet

Rev. A | Page 18 of 32

APPLICATIONS INFORMATION

SELECTING THE OUTPUT VOLTAGE

The output voltage is set using a resistor divider connected between

the output voltage and the FB pin (see Figure 54). The resistor

divider divides down the output voltage to the 0.6 V FB regulation

voltage. The output voltage can be set to as low as 0.6 V and as

high as 90% of the power input voltage.

The ratio of the resistive voltage divider sets the output voltage,

and the absolute value of the resistors sets the divider string

current. For lower divider string currents, the small 50 nA

(0.1 μA maximum) FB bias current should be taken into

account when calculating the resistor values. The FB bias

current can be ignored for a higher divider string current;

however, using small feedback resistors degrades efficiency at

very light loads.

To limit degradation of the output voltage accuracy due to FB

bias current to less than 0.005% (0.5% maximum), ensure that

the divider string current is greater than 20 μA. To calculate the

desired resistor values, first determine the value of the bottom

resistor, RBOTTOM, as follows:

STRING

REF

BOTTOM



(2)

where:

VREF

is the internal reference and equals 0.6 V.

ISTRING

is the resistor divider string current.

Then calculate the value of the top resistor, RTOP, as follows:



















REF

OUT

BOTTOM

TOP

(3)

ADP2441

RTOP

RFREQ

CSS

VOUT

RBOTTOM

PGOOD

EXTERNAL

SUPPLY

FREQ

SS/TRK

Figure 54. Voltage Divider

Table 5. Output Voltage Selection

Voltage (V)

RTOP (kΩ)

RBOTTOM (kΩ)

190

3.3

1.2

SETTING THE SWITCHING FREQUENCY

The choice of the switching frequency depends on the required

dc-to-dc conversion ratio and is limited by the minimum and

maximum controllable duty cycle, as shown in Figure 55. This is

due to the requirement of minimum on time and minimum off

time for current sensing and robust operation. However, the

choice is also influenced by whether there is a need for small

external components. For example, for small, area limited

power solutions, higher switching frequencies are required.

100

200

400

600

800

1000

1200

FREQUENCY (kHz)

DMAX

DMIN

Figure 55. Duty Cycle vs. Switching Frequency

Calculate the value of the frequency resistor using the following

equation:

FREQ

500



(4)

where RFREQ is in kΩ, and fSW is in kHz.

Table 6 and Figure 56 provide examples of frequency resistor

values, which are based on the switching frequency.

Table 6. Frequency Resistor Selection

RFREQ (kΩ)

Frequency

308

300 kHz

132

700 kHz

92.5

1 MHz

200

300

400

500

600

700

800

900

1000

1100

1200

100

150

200

250

300

350

RESISTANCE (kΩ)

Figure 56. Frequency vs. Resistance

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