PM8903 datasheet(21/34 Pages) STMICROELECTRONICS

21 / 34 page

PM8903

Application information

Doc ID 022748 Rev 1

21/34

The following suggestions may be followed in order to place the poles and zeroes of the

compensation network.

●

Select a value for RFB in the range of some kΩ

●

Select RF in order to obtain the desired closed loop regulator bandwidth according to

the approximate formula:

●

Select CF in order to place FZ1 below FLC (typically 0.1*FLC):

●

Select CP in order to place FP1 at 0.5*FSW:

●

Select CS and RS in order to place FZ2 at FLC and FP2 at half of the switching

frequency:

●

Check that compensation network gain is lower than open loop EA gain before F0dB

●

Check phase margin obtained (it should be greater than 45 °)

●

Repeat the whole procedure if necessary.

6.2

Output voltage setting

The PM8903 integrates a 0.6 V internal reference (VREF), 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 VOUT through RFB resistor

●

Connect pin FB to GND through ROS resistor

Usually, the RFB 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 ROS resistor:

0dB

------------

ΔV

OSC

IN_MAX

---------------------- R

⋅⋅

π R

0.1 F

⋅⋅

⋅

----------------------------------------------

π R

⋅⋅

-------------------------------

π C

⋅⋅

-------------------------------

π R

⋅⋅

-------------------------------------

REF

OUT

REF

–

-----------------------------------

⋅

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