AN901 Datasheet, PDF (21/25 Pages) STMicroelectronics – Electromagnetic compatibility

AN901
4.6. C10 Selection
C10 is inversely proportional to output voltage ripple and sets the crossover frequency of control loop gain. Solving
Equation 23,
C10 = _V---^I--_OL---^O-_U---^AT---^D_---_R-^D--_I--_P-^T---_P-^s--_Lw---_E-  -¹----^-----⁰---^.--⁴-₀---_.-^-₀---₅-²---^----1---0---^–--⁶-  16F
Equation 60.
A 22 μF X7R capacitor in 1210 package was chosen.
4.7. C2 Selection
In most applications, V_IN also supplies the VDDA pin that powers the dc-dc controller and left side digital isolator
circuitry. It is recommended to minimize voltage ripple at VDDA. Solving Equation 19:
C2  -_V----_I^I-_N-^L---_^O--_R--^A--_I-^DP----_P^D---_L--^T-_E---^s--_^w-----_n--  ¹-----^-----⁰-₀--^.-_.-⁴-₀---₅-^----_-²---^-₃---1---0---^–--⁶-  5.33F
Equation 61.
A 10 μF X7R capacitor in 1210 package was chosen.
4.8. R5 and R6 Selection
The ratios of R5 and R6 are determined by the 5 V output voltage requirement. To reduce the dependence of
feedback gain on the internal error amplifier transconductance, it is recommended to have the parallel combination
resistance to be ≥10 kΩ. Higher values of R5 and R6 reduce power loss through the divider, but at the expense of
increasing output voltage error due to I_VSNS, which varies part to part. So R5 and R6 are chosen to target 10 kΩ
parallel resistance.
1010³ = _R-^R----₅₅-----+^-----^RR-----⁶⁶-
Equation 62.
5
=
1.05


RR_-----56_--
+
1_
Equation 63.
Substituting Equation 52 into Equation 53 and solving for R6,
1010³ = ³----^.-₄-⁷--_.-⁶-₇---^R6----⁶-- R6 = 12.6610³ï€¬ R5 = 48.110³
Equation 64.
The nearest 1% resistor to 12.66 kΩ is 12.7 kΩ. However, setting R5 to either 47.5 kΩ or 48.7 kΩ does not target
exactly 5 V as well as other 1% resistor pairs. A better match was found with R6 = 13.3 kΩ and R5 = 49.9 kΩ.
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