Cirrus Logic AN349 - Manual (Page 12)

AN349
12 AN349REV1
6.6 Noise Issues When Using CS1601 to replace L6562
For an existing design using the L6562, attempting to use the CS1601 with the component values of the L6562 design may lead
to instability and unpredictable performance and system behavior. Design parameters that should be modified for the CS1601
pins IAC & IFB are described in the following two sections.
For details on the ST Microelectronics L6562, refer to ST Microelectronics application note AN1895.
6.6.1 IAC – PIN 3
Based on the AN1895, recommended components used on the MULT pin on L6562 (pin 3) have the following characteristics:
- R
ac
1, R
ac
2 = 620k
 (R
ac
= 1.24M
)
- R
st
= 10k
 (Not required for CS1601.)
- C
ac
= 10nF
- The filter pole for this node is at Fp = 1
 (2  R
ac
 C
ac
 Gain) = 1.6kHz (approx.)
- The filtering cap, C
ac
, is chosen to match the pole at 1.6kHz.
- C
ac
= ~8nF. Using a standard value, C
ac
is 8.2nF or 10nF.
.
Figure 15. Input Voltage (AC) Sense Noise Decoupling
When using the CS1601, recommended components used on the IAC pin (pin 3) have the following characteristics:
- R
ac
1, R
ac
2 = 875k
 (R
ac
= 1.75M
)
- R
st
= Not required.
- C
ac
= 10nF
- The filter pole for this node is at Fp = 1
 (2  R
ac
 C
ac
 Gain) = 1.6kHz (approx.)
- The bandwidth on the IAC pin of CS1601 should be limited to approximately 1.6kHz.
- The internal impedance on pin IAC (R
int
) = ~12k
.
- The filtering cap, C
ac
, is chosen to match the pole at 1.6kHz.
- C
ac
= ~8nF. Using a standard value, C
ac
is 8.2nF or 10nF.
NOTE: For CS1601, Fp should be higher than 1.2kHz. A lower value can deteriorate the power factor of the system.
L6562
3 MULT
R
ac
2
R
ac
1
C
ac
V
rect
CS1601
3 IAC
R
st
R
ac
2
R
ac
1
C
ac
V
rect