AN901 Datasheet, PDF (22/25 Pages) STMicroelectronics – Electromagnetic compatibility

AN901
4.9. Compensation Network
The compensation network is comprised of R7 and C11. R7 is selected to match R_INT and 100 kΩ 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 obtained by multiplying the P-mode gain (Equation 33), by the frequency of the
pole created by R_LOAD and C10 (Equation 39):
f_c  ¹----⁰----^---1---0_R---³--₅--^---_--ⁿ---_R--^--₁--^R--₂---^L---^O----^A---^D--  -₂---_----_R----_L---_O---¹-_A----_D----_C----₁----₀-  43kHz
Equation 65.
To achieve good phase margin, it suggested to place the compensation zero near the pole created by R_LOAD and
C10.
C11 = -₂---_----_f--_p---₂¹----_-----_R----₇-- = -^R----^L---^O----_R^A---^D-₇---^C-----¹---⁰-- = -⁵----₁^---₀--²--₀-²--_--^--1-1--0-0--₃-^–---⁶- = 1.1nF
Equation 66.
A 1.5 nF capacitor was chosen.
4.10. Primary Snubber
Without R19 and C16 installed, V_ds of Q1 was measured to spike at 108 V and ring briefly at 30 MHz until the
energy stored in L_lkg dissipated. See Figure 10:
Figure 10. Undamped Vds Ringing
T1 was removed from the board and its primary inductance leakage was measured to be 456 nH. Applying
Equation 16 and Equation 17, R16 and C19 were calculated:
R16 = 2f_ringL_lkg = 2  3010⁶  45610^–9 = 86
Equation 67.
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