Datasheet

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 A 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 A. To calculate the
desired resistor values, first determine the value of the bottom
resistor, R
BOTTOM
, as follows:
STRING
REF
BOTTOM
I
V
R 
(2)
where:
V
REF
is the internal reference and equals 0.6 V.
I
STRING
is the resistor divider string current.
Then calculate the value of the top resistor, R
TOP
, as follows:










REF
REF
OUT
BOTTOMTOP
V
VV
RR
(3)
ADP2441
FB
R
TOP
R
FREQ
C
SS
V
OUT
R
BOTTOM
PGOOD
EXTERNAL
SUPPLY
FREQ SS/TRK
10581-052
Figure 54. Voltage Divider
Table 5. 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 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.
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
10581-155
Figure 55. Duty Cycle vs. Switching Frequency
Calculate the value of the frequency resistor using the following
equation:
SW
FREQ
f
R
500,92
 (4)
where R
FREQ
is in kΩ, and f
SW
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
R
FREQ
(kΩ) Frequency
308 300 kHz
132 700 kHz
92.5 1 MHz
10581-053
200
300
400
500
600
700
800
900
1000
1100
1200
50 100 150 200 250 300 350
FREQUEN
C
Y (kHz)
RESISTANCE (k)
Figure 56. Frequency vs. Resistance