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Data Sheet

ADP2442

Rev. B | Page 19 of 36

APPLICATIONS INFORMATION

ADIsimPOWER DESIGN TOOL

The ADP2442 is supported by the ADIsimPower design tool

set. ADIsimPower is a collection of tools that produce complete

power designs optimized to a specific design goal. These tools

allow the user to generate a full schematic, bill of materials, and

calculate performance in minutes. ADIsimPower can optimize

designs for cost, area, efficiency, and device count while taking

into consideration the operating conditions and limitations of

the IC and all real external components. Find the ADIsimPower

tool at www.analog.com/adisimpower and the user can request an

unpopulated board through the tool.

SELECTING THE OUTPUT VOLTAGE

The output voltage is set using a resistor divider connected between

the output voltage and the FB pin (see Figure 57). 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.

Figure 57. Voltage Divider

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

the absolute value of the resistors sets the divider string current.

When calculating the resistor values for lower divider string

currents, take into account the small 50 nA (0.1 μA maximum)

FB bias current. 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

(1)

where:

VREF is the internal reference and equals 0.6 V.

ISTRING is the resistor divider string current.

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













−

REF

OUT

BOTTOM

TOP

(2)

Table 6. 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 58. This

limitation 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, higher switching

frequencies are required for small, area limited power solutions.

Figure 58. Duty Cycle vs. Switching Frequency

Calculate the value of the frequency resistor by using the

following equation:

FREQ

500

(3)

where RFREQ is in kΩ and fSW is in kHz.

Table 7 and Figure 59 provide examples of frequency resistor

values that are based on the switching frequency.

Table 7. Frequency Resistor Selection

RFREQ

Frequency

308 kΩ

300 kHz

132 kΩ

700 kHz

92.5 kΩ

1 MHz

ADP2442

RTOP

RFREQ

VOUT

RBOTTOM

PGOOD

EXTERNAL

SUPPLY

FREQ

100

200

400

600

800

1000

1200

FREQUENCY (kHz)

DMAX

DMIN

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