Datasheet

Application information PM8903A
20/33 Doc ID 024147 Rev 1
The following suggestions may be followed in order to place the poles and zeroes of the
compensation network.
● Select a value for R
FB
in the range of some kΩ
● Select R
F
in order to obtain the desired closed loop regulator bandwidth according to
the approximate formula:
● Select C
F
in order to place F
Z1
below F
LC
(typically 0.1*F
LC
):
● Select C
P
in order to place F
P1
at 0.5*F
SW
:
● Select C
S
and R
S
in order to place F
Z2
at F
LC
and F
P2
at half of the switching
frequency:
● Check that compensation network gain is lower than open loop EA gain before F
0dB
●
Check phase margin obtained (it should be greater than 45 °)
● Repeat the whole procedure if necessary.
6.2 Output voltage setting
The PM8903A integrates a 0.6 V internal reference (V
REF
), with a total accuracy of ±1%
over line, load, and temperature variations (excluding external resistor divider tolerance,
when present).
The output voltage can be easily programmed connecting ROS and RFB resistors as follows
(see also
Figure 1 on page 4 ).
● Connect pin FB to V
OUT
through R
FB
resistor
● Connect pin FB to GND through R
OS
resistor.
Usually, the R
FB
resistor is selected in order to obtain the desired closed loop regulator
bandwidth (see
Section 6.1 for details) and it is not changed when setting the output
voltage.
Therefore, the output voltage setting is easily achieved using the following formula to select
the value of the R
OS
resistor:
R
F
F
0dB
F
LC
------------
ΔV
OSC
V
IN_MAX
--------------------- -
R
FB
⋅⋅=
C
F
1
2π R
F
0.1 F
LC
⋅⋅ ⋅
----------------------------------------------=
C
P
1
π R
F
F
SW
⋅⋅
-------------------------------=
R
S
1
π C
S
F
SW
⋅⋅
-------------------------------=
C
S
1
2π R
FB
F
LC
⋅⋅
-------------------------------------=
R
OS
R
FB
V
REF
V
OUT
V
REF
–
-----------------------------------
⋅=