Cirrus Logic AN372 - Manual (Page 19)

AN372
AN372REV1 19
The frequency range should be as high as possible without exceeding 75kHz. This strategy will keep the
fundamental and second harmonic below the 150kHz EMI requirements.
In most low-power designs, the boost inductor peak current I
PK(BST)
is much higher than the RMS value.
Specify the boost inductor turns such that the core will reach 3000Gauss when the current equals 600mA. To
protect against runaway, set the artificial load to a constant voltage to achieve nominal
V
BST
, then adjust the
value of R
IPK
to obtain the nominal boost output current, I
BST(nominal)
.
Measure the switching frequency in the high current region in the 45° to 90° AC phase angle range. Adjust the
boost inductor value to determine the desired frequency. Adjustments to the inductor value are made by
changing the gap. Increasing the gap is always safe, but reducing the gap may saturate the core. It may be
beneficial to redesign the boost inductor if changes to the inductor gap are greater than 20%.
Step 18) Determine Boost Output Capacitor
The boost stage output capacitor is also the buck stage input capacitor. Determine the size of the boost output
capacitor using the following points:
• For a 120V line input system, capacitor C4 > 2
F/Watt of input power
• For a 230V line input system, capacitor C4 > 0.5 F/Watt of input power
Proper capacitor size is required to ensure that the following dimmer algorithms execute properly:
• Transition CCM to CRM properly
• No erroneous CCM events
• No CCM operation with trailing-edge dimmers
• No CCM operation in No-dimmer Mode
• Boost and buck stages loop stability
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
10 20 30 40 50 60 70 80 90 100 110 120 130 140
120V Min Freq
120V Max Freq
230V Min Freq
230V Max Freq
Power Multiplied by Inductance (Watts Multiplied by mH)
Sw
itching Frequency
(kHz
)
Figure 10. Switching Frequency vs Power

Inductance