Datasheet
Data Sheet ADP2442
Rev. 0 | 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 parts count while taking
into consideration the operating conditions and limitations of
the IC and all real external components. The ADIsimPower tool
can be found 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 maxi-
mum) 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, R
BOTTOM
, as follows:
STRING
REF
BOTTOM
I
V
R =
(1)
where:
V
REF
is the internal reference and equals 0.6 V.
I
STRING
is the resistor divider string current.
Next, calculate the value of the top resistor, R
TOP
, as follows:
−
×=
REF
REF
OUT
BOTTOMTOP
V
VV
RR
(2)
Table 6. Output Voltage Selection
Voltage (V) R
TOP
(kΩ) R
BOTTOM
(kΩ)
12 190 10
5
73
10
3.3 45 10
1.2 10 10
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:
SW
FREQ
f
R
500,92
=
(3)
where R
FREQ
is in kΩ and f
SW
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
R
FREQ
Frequency
308 kΩ 300 kHz
132 kΩ 700 kHz
92.5 kΩ 1 MHz
ADP2442
FB
R
TOP
R
FREQ
V
OUT
R
BOTTOM
PGOOD
EXTERNAL
SUPPLY
FREQ
10667-052
0
10
20
30
40
50
60
70
80
90
100
0 200 400 600 800 1000 1200
DUTY CYCLE (%)
FREQUENCY (kHz)
D
MAX
D
MIN
10667-155