AN892 Datasheet, PDF (14/16 Pages) Silicon Laboratories – DESIGN GUIDE FOR ISOLATED DC/DC USING THE

AN892
A 10uF capacitor was chosen.
4.5. 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 20:
C2  -^I-^L---^O----^A----^D-_V---^D-_I-_N--^--_-¹-_R---–-_I--_P--^D-_P---_L-^--^T_E----^S---^W-----^N---  ⁰----^.-⁴-----^-----⁰---^.--²---⁵-----^-----⁰-₀--^.--_.7-₁--⁵--₅---^-----⁴----^x---¹---⁰----^–---⁶----^----⁴--  8F
Equation 47.
A 10F capacitor was chosen.
4.6. R5 and R6
The ratio of R5 and R6 is 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 k. Higher values of R5 + R6 reduce power loss through the divider, but at the expense of
increasing output voltage error due to l_VSNS which varies part to part. So R5 and R6 are chosen to target 10k
parallel resistance.
10x10³ = _R-^R----₅₅-----+^-----^RR-----⁶⁶-
Equation 48.
5
=
1.05


RR_-----56_--
+
1_
Equation 49.
Substituting Equation 48 into Equation 49 and solving for R6,
10x10³ = ³----^.-₄₇---_.-⁶-₇---^R6----⁶-- R6 = 12.66x10³ï€¬ R5 = 48.1x10³
Equation 50.
The nearest 1% resistor to 12.66 k is 12.7 k. However, setting R5 to either 47.5k or 48.7k does not target
exactly 5V as well as other 1% resistor pairs. A better match was found with R6=13.3 k and R5=49.9 k.
4.7. Compensation Network
The compensation network is comprised of R7 and C11. R7 is fixed to match R_INT and 49.9 k is the nearest 1%
resistor value. The C11 places the compensation zero in relationship to the crossover frequency. The equation for
crossover frequency can be had by multiplying the P-mode gain (Equation 31) by the frequency of the pole created
by R_LOAD and C10 (Equation 36):
f_c  -⁵---⁰----^x---¹---⁰-_R---³--₅--^---_--³---_NR-----^L---^O----^A---^D--  ₂----_----_R----_L---_O---¹-_A----_D----_C----₁----₀-  12.1kHZ
Equation 51.
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